Optimizing Thermal Efficiency of Building Envelopes with Sustainable Composite Materials

Abstract

The growing global energy demand, particularly in India, calls for innovative strategies to improve building energy efficiency. With buildings contributing significantly to energy consumption, especially in cooling-dominated climates, sustainable insulation materials are essential in minimizing energy usage. This study explores the potential of bamboo biochar, fly ash, and lime as sustainable insulation materials for building envelopes. This study also addresses the critical issue of energy efficiency in building construction, specifically focusing on the comparative analysis of three materials for their thermal performance, environmental impact, and economic viability. This research aims to identify the most sustainable material choice by assessing each material’s life cycle energy consumption, thermal resistance, and associated costs. The research methodology involves an extensive review of 125 relevant studies to assess the thermal performance of these materials. U-values were computed from the reported thermal conductivity data and systematically arranged in chronological order to evaluate and compare their insulation effectiveness over time. Additionally, these materials were analyzed under sustainability criteria, incorporating life cycle analysis and a carbon footprint assessment. This study identifies existing research gaps and offers recommendations for future research, creating structure for the development of sustainable insulation system.

1. Introduction

1.1. Background

The industrial revolution significantly accelerated environmental degradation, with cement production and energy consumption driving greenhouse gas emissions and resource depletion [1,2]. Global CO₂ emissions peaked at 33 gigatons in 2021, worsening climate change and biodiversity loss [3]. India, as the third-largest CO₂ emitter, faces severe pollution and health impacts, primarily due to fossil fuel dependence for over 70% of electricity production. To address this, India aims to reduce emissions intensity by 33–35% by 2030 under its National Action Plan on Climate Change [4,5]. See Figure 1a,b.

1.2. Environmental Issues Connected to Energy Generation

Air pollution, driven by energy production and fossil fuel combustion, is a leading health hazard, causing 7 million premature deaths globally each year, including 1.67 million in India in 2019 [4,5,6]. Climate change, linked to rising temperatures and severe weather, poses risks to ecosystems and economies, with significant impacts on agriculture and biodiversity in India [7]. Energy security is threatened by reliance on fossil fuels, requiring diversification to ensure sustainability [8,9]. The economic burden of energy-related issues, including air pollution, costs billions globally and 3% of India’s GDP annually [10,11]. Reducing energy consumption is crucial to addressing these challenges. See Figure 2.

1.3. Energy Scenario

The graphs indicate the distribution of energy consumption across different sectors globally and in India, with a focus on the building sector (residential and commercial). The building sector’s energy consumption is highlighted in orange to emphasize its significance. See Figure 3.

The building sector, encompassing residential and commercial structures, accounts for 40% of global energy use and 33% of India’s electricity consumption, contributing significantly to climate-altering emissions [12,13,14,15,16]. Coal power plants release about 0.82 kg of CO₂ per kWh, translating to 246 million metric tons of CO₂ annually from India’s building sector [17]. Enhancing energy efficiency and adopting sustainable practices, such as optimized thermal insulation, are critical strategies to reduce energy consumption and CO₂ emissions.

1.4. Role of Thermal Insulation

Thermal insulation is a cost-effective and environmentally beneficial strategy to optimize energy efficiency, reducing electricity consumption and emissions by up to 50% in residential and commercial buildings [3,6,18,19]. It enhances comfort, durability, and economic impact while minimizing energy demand. See Figure 4.

The graph above indicates that the use of insulation minimizes significant energy consumption for the lifetime of the building [20].

1.5. Building Envelope

The building envelope, particularly roofs and walls, is critical in controlling heat gain, accounting for up to 60% of total heat transfer in hot climates [21]. Strategies such as high-performance insulation, reflective coatings, and thermal retrofitting can reduce cooling loads and energy use by up to 40% [22,23]. Fenestration protection and infiltration management further enhance energy efficiency, with measures like double-glazing, shading devices, and proper sealing reducing energy demands by up to 20% [24]. See components of building envelope in Figure 5.

1.6. Thermal Comfort

Reduces heat transfer through the building envelope, maintaining stable indoor temperatures [25]. Thermal conductivity, humidity control, reflectance, and thermal mass act as key factors in controlling thermal comfort, where air tightness, solar control, high performance glazing, and air movement can be control through standards construction practices. As a component of the building envelope, the roof and walls receive direct heat, which is transferred inside through conductivity. However, to minimize the heat gain through conductivity, study should be focused on the roof and walls.

1.7. Selective Overview

A broader literature review suggests the following keywords. The “Open Knowledge Map” tool can help identify general research focus areas. See

Table 1. Sustainability parameters. Source: Author, based on data from [26,27,28,29].

Category	Criteria	Description
1. Environmental impact	Embodied energy	The total energy needed to make the material, including obtaining raw materials, making the product, and transporting it.
	Carbon footprint	The total amount of climate-altering gases discharged throughout the material’s life.
	Resource efficiency	The deployment of replenish able or recycled resources in the material’s production.
	Toxicity	The presence of harmful chemicals or pollutants in the material.
2. Economic viability	Cost	Initial cost, maintenance, and potential savings over the building’s life cycle.
	Availability	The ease of sourcing the material locally to reduce transportation emissions and costs.
	Durability	The material’s longevity and resistance to wear and environmental conditions, reducing the need for frequent replacements.
3. Social impact	Health and safety	The material’s impact on the wellness and quality of life of inhabitants including air quality within buildings and exposure to harmful substances.
	Aesthetics and comfort	The material’s contribution to the visual and thermal comfort of the building occupants.
4. Performance	Thermal insulation	The material’s ability to insulate the building, contributing to energy efficiency.
	Structural integrity	The durability and weight-bearing ability of the material.
	Moisture resistance	The material’s strength to keep out moisture and stop mold from growing.
	Fire resistance	How well the material can stop and handle fire.
5. Compliance	Certification and standards	Adherence to industry standards and certifications, such as ECBC, LEED, BREEAM, or other green building standards.
6. Life cycle assessment	Renewability	The material’s potential for renewal and its impact on future resource availability.
	Recyclability	The ability to reuse the material after it is no longer needed.
	End-of-life disposal	The effect on the environment when disposing of the material.

below.

Keywords—Broad overview of selected sustainable materials

Keyword

A.

Building envelope thermal comfort.

B.

Sustainable materials.

“Open knowledge map”—Building envelope thermal performance and sustainable materials. See Figure 6.

Analysis of the Open Knowledge Map:

The map highlights key themes such as “Building Envelope”, “Thermal Insulation”, “Energy Efficiency”, and “Sustainable Materials”, central to energy-efficient building design. Related concepts like “Building Energy Performance”, “Embodied Carbon Footprint”, and “Thermal Inertia” show interconnections, emphasizing the interdisciplinary nature of research. Overlapping areas suggest links between environmental factors, material selection, and climate adaptation. The global scope of this research underscores the importance of sustainability and regional considerations in building science.

The map highlights the connection between thermal performance and sustainability in buildings, emphasizing eco-friendly materials and energy efficiency in hot climates. It also focuses on reducing the environmental impact of building materials, addressing climate change and sustainability. This comprehensive overview of interconnected research topics aids in exploring the literature on building envelope thermal efficiency and sustainable materials.

1.8. Sustainable Materials

Sustainable materials enhance energy efficiency, environmental performance, and long-term cost savings in construction due to durability and minimal maintenance [26,30]. Key parameters for evaluation include environmental, economic, and social benefits, guided by standards like ISO 21930, USGBC, and BREEAM.

The selection of sustainable materials in building construction does not merely minimize total electricity requirement but also leads towards the Sustainable Development Goals (SDGs). The expansion of bamboo industries has consistently been perceived as being in harmony with the objectives of the standards laid down by United Nations (SDGs) [12].

There are 1400 types of bamboo found around the world. Woody bamboos from cooler northern areas are identified in the North Temperate Zone and in fewer quantities in high areas of Africa, India, Madagascar, and Sri Lanka. Woody bamboo from tropical areas is distributed in tropical and subtropical landscapes (Figure 7a,b). The Bambuseae tribe has near about 1290 type of breeds around the world and is made up of three main groups [31]. India stands out having the second largest variety of bamboos after China, which is number one [32].

Why Bamboo: Bamboo has some unique characteristics that make it a sustainable resource for the building construction domain. Bamboo, known for its rapid growth compared to all other plants globally, can reach heights ranging from 15 to 30 m in just 2 to 4 months, with daily growth rates ranging from 20 to 100 cm [34]. It possible to collect bamboo after 3 to 5 years, a significantly shorter time frame compared to timber, which typically takes 20–40 years to mature [35]. The heat conductivity of engineered bamboo and bamboo-based products, at the same density, is found to be similar to or aligned with that of wood [15]. Bamboo has anisotropic properties where bamboo fibers in the transverse direction possess low conductivity [36]. Bamboo biochar produced from novel bamboo possesses excellent insulation and other important properties that are required for buildings.

The exploration of sustainable resources such as bamboo biochar, fly ash, and lime offers a positive approach to improving the insulation performance of building shells while aligning with broader sustainability goals. These materials offer unique advantages in terms of electricity optimization, ecological influence, and robustness.

Bamboo Biochar: Bamboo biochar is produced by pyrolysis, a process that converts bamboo into a carbon-rich material by heating it in the absence of oxygen. Bamboo grows rapidly, reaching maturity in 3 to 5 years, making it a renewable alternative to timber, which takes much longer to mature [35].

Fly Ash: Residue from coal burning in power plants is an extensively available material with excellent pozzolanic properties. It improves the robustness and resilience of concrete while minimizing its thermal conductivity [37]. The inclusion of fly ash in construction helps repurpose industrial byproducts, lowers the demand for raw materials, and lowers the embodied energy of building materials.

Lime: Lime is a traditional building material with excellent thermal insulation and moisture-regulating properties. It has been used for centuries in construction due to its ability to stabilize indoor temperatures and prevent heat transfer. Lime-based plasters and mortars are breathable, allowing for moisture to escape while preventing water ingress, thereby enhancing the energy efficiency and longevity of buildings [38]. Sustainable insulation material shown in Figure 8.

Sustainable properties of the materials illustrated in the

Table 2. Sustainability of bamboo biochar, fly ash, and lime. Source: Author.

Material	Thermal Properties	Environmental Impact	Other Properties
Bamboo biochar	Low thermal conductivity	Carbon sequestration, renewable	Fast-growing, renewable, low energy
Fly ash	Reduces thermal transfer	Repurposes waste, low CO2	Improves concrete strength
Lime	Excellent thermal insulation	Low embodied energy, breathable	Fire-resistant, moisture regulation

.

1.9. Unique Contributions

This research is unique because bamboo biochar has not been explored much as a thermal insulation material, especially for a full insulation system in buildings. While other plant-based biochars have been used in concrete and sometimes for insulation, they have not been used in complete building systems. Also, the benefits of mixing bamboo biochar with fly ash and lime for better insulation have not been studied enough.

Objectives

The main aim of this research is to assess the viability of bamboo biochar, fly ash, and lime as eco-friendly insulation materials for building envelopes. This study has the following objectives:

• Assess the heat conductivity and insulation value of bamboo biochar, fly ash, and lime to determine their insulation effectiveness.
• Analyze the sustainability performance of these materials through life cycle analysis and carbon footprint assessment.
• Identify research gaps and offer recommendations for future research in the development of sustainable insulation systems.

By improving the insulation effectiveness of building envelopes through the use of eco-friendly resources, this research intends to aid global endeavors to decrease energy use and address climate shift.

Limitation: This study performs a comprehensive literature review based on previous researches and available data but does not include any experiments conducted by the author. Further research involving extensive experiments is needed to determine the optimal mixing proportions.

2. Literature Review

This section provides a comprehensive review of previous research on sustainable insulation materials, specifically bamboo biochar, fly ash, and lime, and is organized into three key categories: thermal performance, environmental impact, and cost. This approach underscores the relevance of these materials and supports the innovation and necessity of the present study.

2.1. Thermal Performance

In terms of thermal efficiency, sustainable materials like bamboo biochar, fly ash, and lime have shown promising potential. Each material’s thermal properties are critical to enhancing energy efficiency in building envelopes. See flow diagram in Figure 9.

A thorough literature review was conducted to understand the overall research landscape. The main goal of this review was to find sustainable insulation materials for building envelopes, with a particular focus on bamboo as the starting point. However, this review revealed that bamboo alone is not enough to create a sustainable insulation system. Instead, it needs to be combined with other sustainable materials to be effective.

The criteria for selecting these additional materials were that they should have good insulation properties, be compatible with cement, and be sustainable. In the exploration of bamboo, bamboo biochar emerged as a promising material that meets these criteria. Additionally, fly ash and lime were identified as materials with great potential to form a cement-based combination that can function as an effective insulation system.

Creating a search phrase as shown in

Table 3. Main words and related terms. Source: Author.

Keywords	Associated Terms
Building	Building envelope, building, structure, building outside, building skin, building façade, building exterior, building casing, structural envelope, building enclosure
Insulation	W/mK, U-value, insulation, thermal insulation, insulating material, heat insulation, insulation layer, thermal resistance, thermal resistivity, thermal reduction, thermal improvement, low conductivity, reduce thermal conduction, poor conductivity, poor thermal conductivity, low conductance
Bamboo biochar	Bamboo biochar, bamboo pyrolysis, bamboo charcoal, bamboo carbon, bamboo thermal decomposition, bamboo carbonization
Fly ash	Fly ash, pulverized fuel ash, fuel ash, coal ash, ash residue, combustion ash, fossil fuel ash, coal combustion, fly dust, cenosphere, bottom ash
Lime	Different variety of lime include lime, calcium oxide, quicklime, calcium lime, lime powder, dry lime, liquid lime, lime hydrate, calcium hydroxide, slaked lime and lime putty.

.

Literature search and screening: Google Scholar was used as the search platform, focusing on well-known and reputable published articles for the literature review. Initially, the search covered the last 10 years, but it generated a large quantity of data. To make this study more manageable, the focus was narrowed down to articles from the last five years.

Selection criteria: As shown in Figure 10, a total of 125 research papers were selected through a systematic review process. The selection criteria for including research prioritized the literature that provided data on the thermal conductivity or resistance of materials. Secondary criteria included studies that, in addition to insulation properties, emphasized the integration of other sustainable materials, life cycle analysis, carbon sequestration, mitigation of environmental issues, and economic aspects such as contributions to the circular economy.

The search was conducted using databases like Google Scholar and Scopus, focusing on keywords related to bamboo biochar, fly ash, lime, and sustainable building materials. After removing duplicates and irrelevant studies, the final papers were chosen based on their relevance to thermal performance, U-values, and sustainability in construction. This selection process is summarized in the PRIZMA diagram.

2.1.1. Research Review for Bamboo Biochar, Ash, and Lime

This research review focused on using bamboo biochar as an insulation material. The original goal was to include only papers on bamboo biochar, but since there are few studies on this topic, this review also covered other types of biomass biochar. Additionally, the insulation properties of ash and lime were reviewed.

Bamboo biochar

Bamboo biochar is a type of charcoal made by heating bamboo at low temperatures with little oxygen. It is a material full of carbon that is becoming popular for uses like improving soil, filtering water, and being added to building materials. Bamboo biochar has a large surface area and tiny holes, which makes it good at absorbing pollutants, storing carbon, and improving the strength and heat properties of mixed materials. Process of Bamboo Biochar illustrated in Figure 11.

Bamboo biochar: A previous study looks at how bamboo biochar is made and its many uses, like in farming, cleaning water, and building materials. It shows how bamboo biochar can help improve soil and lower greenhouse gas emissions, but it does not focus on its heat properties [40]. Another study examines the impact of zeolite and bamboo biochar as carbon dioxide absorbents in concrete. It was found that adding bamboo biochar to concrete improves its CO₂ absorption capacity while slightly compromising its strength. The study reports that the effective mixing proportion absorbs 1.2 g of carbon dioxide per day and improves the overall sustainability of the concrete [41]. A paper discusses the relatively underexplored potential of bamboo-based biochar as a valuable material, often referred to as “black gold”. It emphasizes the need for more research on bamboo biochar, particularly in its applications in energy storage and environmental management [42]. Another study focuses on the synthesis of a gold nanoparticle-fortified bamboo biochar nanocomposite. This nanocomposite is shown to have enhanced electrical and catalytic properties, which could be applied in sensors and other advanced technological applications [43]. Research examines the utilization of bamboo biochar in cement mortar. Including bamboo biochar in cement mortar can improve the material’s durability and reduce its thermal conductivity, making it more suitable for insulation purposes [44]. Bamboo biochar yield is 24–74%; however, the maximum yield possible (80%) can be obtained using D Giganteus bamboo species [40]. See Figure 12a,b. The addition of bamboo biochar at 1% enhances both compressive and tensile strengths by 7.48% and 15%, respectively, when compared to conventional concrete [41]. Bamboo biochar is an eco-friendly material, produced with lower energy requirements, making it a sustainable option to traditional construction materials [43]. Due to the limited research on “Bamboo biochar”, some important properties like thermal performance, environmental impact, economic viability, and life cycle assessment have not been discussed in bamboo biochar-related research work. Biochar yield from other biomasses showcases great possibility in terms of thermal performance, CO₂ reduction, and VOC adsorption. A review of all biochar-related research was conducted to find its construction-related properties and applications. Results indicated a potential reduction in carbon footprint, a heat conductivity range of 0.08–0.2 W/mK, and that for every 1 ton cement replaced, 1351–1505 kg of carbon dioxide throughout the total life cycle is saved by using biochar in concrete, resulting in a CO₂ saving of 59 to 65 kg per ton [45].

Figure 13 illustrates the high porosity of biochar produced at 400 °C and 500 °C.

Biomass biochar: A previous literature review focuses on bamboo biochar, but it found that studies on its use as insulation in building construction are limited. To better understand its thermal properties, biochar made from other types of biomass was also examined. Biochar concrete blocks had about 41% lower heat transfer compared to regular concrete [46]. Partial substitution of cement with biochar, shows that the percentage of biochar in the concrete mix outperforms in mechanical properties, especially improving flexural strength [47]. The combination of zeolite and bamboo biochar absorbs 1.2 g of carbon dioxide every day. It was also observed that carbon dioxide infiltrates 15 mm on the surface, and that the combination increases compressive strength by 7.48% [41]. A research study delves into different applications of biochar and its properties like CO₂ absorption, improvements in mechanical strength, flammability, and low conductivity. It concludes by presenting its application as an insulating material in the form of clay–lime mixed with biochar plaster, bricks, concrete, and roof tiles [48]. Four types of biochar were tested at different pyrolysis temperatures and examined with infrared spectroscopy and X-ray diffraction. The results shows a low thermal conductivity value of 0.13 W/mk [49]. Research demonstrates biochar as a cement admixture to obtain thermal and acoustical properties. A 2% biochar mix shows 0.192 W/mk low conductivity and a sound absorption coefficient from 200 to 2000 Hz [50]. A study shows that biochar has many benefits like keeping heat and sound in, blocking electromagnetic waves, making eco-friendly concrete, making strong concrete, storing carbon, supporting a circular economy, and saving money [51]. In previous research, biochar flammability properties were explored to determine their connection with the heating process. It was observed that slow pyrolysis reduced the flammability of biochar prepared at 450 °C more than that produced at 350 °C [52]. Research examines the flammability characteristics of 3D-printed polymers and composites. The findings indicate that biochar, when used as a filler, enhances the fire resistance of polymers [53]. Within a VOC sorption range of 5.58–91.2 mgg⁻¹, the increase in feedstock temperature deceases the VOC removal %. Sorption is a passive technique for the removal of volatile organic compounds [54]. The surface area and non-organized organic carbon content of biochar have a strong impact on VOC removal [54]. Biochar can used as a plastering material for building. Results show that carbon dioxide adsorption capacity increased fourfold [55]. Another research outcome shows that it reduces internal shrinkage by 16.3%, increases relative humidity by 5.5%, and also increases compressive strength by 6% [56].

U-value calculation: Most of the literature conducted on this sustainable material has been provided; however, to compare its insulation property, the U-value calculation for the wall assembly has been considered. Wall assembly: 230 mm autoclaved aerated concrete (AAC) block wall (0.18 W/mK), 15 mm internal plaster, and 25 mm external plaster. The U-value calculation is determined as per the formula mentioned below:

U = 1/R T

where

RT = total thermal resistance, m²K/W;

H1 = inside air heat transfer coefficient, W/(m² K) (Default value = 0.12);

Ho = outside air heat transfer coefficient, W/(m² K) (Default value = 0.06);

R1 = thermal resistance of material 1—Internal plaster (15 mm thick), m² K/W;

R2 = thermal resistance of material 2—Brick work (230 mm thick), m² K/W;

R3 = thermal resistance of material 3—External plaster (25 mm thick), m² K/W.

Source: BEE, Part 1 [57]

Verify that the thermal resistance formula accurately reflects the total resistance, RT = R1 + R2 + R3 + H1 + Ho, where H1 and Ho represent the heat transfer coefficients for inside and outside air, respectively. See Figure 14.

The Energy Conservation Building Code (ECBC) 2017 is an Indian standard created by the Bureau of Energy Efficiency (BEE) under the guidelines of Indian Ministry of Power. It provides guidelines and minimum energy performance standards for new commercial buildings or those undergoing major renovations. ECBC sets the energy efficiency standards for design and construction, aiming to reduce energy consumption in buildings while maintaining comfort. The ECBC draws upon international standards, including ASHRAE standards, to set its own performance criteria. For instance, ASHRAE Standard 90.1, which is a widely accepted benchmark for energy efficiency in building design, has been referred to in the formulation of ECBC 2017 guidelines.

Location: Extreme climatic conditions are good for testing the results of building insulation material; thus, Jaipur city was chosen as a study location. According to ASHRAE, Jaipur is classified as having a hot semi-arid climate under the Köppen climate classification system. Meanwhile, the ECBC categorizes Jaipur as falling under a composite climate, indicating that it exhibits characteristics of both hot–dry and warm–humid climates.

The allowable maximum U-value for an opaque assembly, such as a wall, in the hot and dry region for ECBC compliance is 0.4, serving as a benchmark for future U-values. However, to meet higher efficiency standards, ECBC+ and Super ECBC set the maximum U-values at 0.34 and 0.22, respectively. Similarly, for roofs, the U-values for ECBC compliance, ECBC+, and Super ECBC are 0.33, 0.2, and 0.2, respectively. This is illustrated in

Table 4. Guidelines for wall assembly. Source: Author, based on data from ECBC 2017.

	Composite Climate
ECBC Standards Parameter	Wall U-Value	Roof U-Value
ECBC compliance	0.4	0.33
ECBC+	0.33	0.20
Super ECBC	0.22	0.20

.

U-values, calculated using conductivity data obtained from the literature review, are organized in descending order from highest to lowest. Additionally, some research papers indicate the percentage reduction in insulation, which is presented in a separate section at the end of the table. The same approach has been applied for biochar, fly ash, and lime.

Table 5. Insulation properties of biochar in chronological order. Source: Author.

Source	Process	Thermal Conductivity (W/mK)	U-Value (W/m2K)
Biochar
[50]	Biochar as cement admixture	0.192	0.6
[58]	Biochar cement brick	0.18	0.6
[49]	Biochar–geopolymer materials	0.13	0.57
[59]	Biochar foam	0.092	0.53
[45]	Biochar	0.08–0.2	0.51
			Percentage
[46]	Masonry concrete (41% reduce)	Reduced	41%
[60]	Biochar and clay mix.	Reduced	67.21%

illustrates the insulation properties of biochar.

Table 6. Research related to biochar. Source: Author.

Source	Statement
[59]	Reduces initial setting time, as well as water penetration and sorptivity.
[61,62]	A 30% biochar addition improves cement hydration and microstructure development, reduces CO2 emissions, sequesters 59 kg CO2 per ton, and generates an overall profit.
[39]	Bamboo biochar yields range from 24 to 74%, with a maximum yield of 80% (D. Giganteus bamboo).
[63]	Improvements in both mechanical and physical properties.
[64]	Biochar and other vegetable fibers shows improvement in tensile strength and water absorbency.
[65]	Can also improve insulation, provide electromagnetic protection, and enhance moisture control.
[66]	Enhances porosity and surface area modification.
[67]	High porosity and low thermal conductivity.
[68]	Saves up to 33% of energy and reduces CO2 emissions by 63%.
[69]	Biochar produced at 350 degrees Celsius typically has an average porosity of less than or equal to ten micrometers.
[70]	Self-healing in cement—reduces cracks up to 700 μm and regulates humidity.
[59]	Bio-carbon enhances thermal insulation, flame retardancy, and mechanical properties.
[71]	Biochar-modified polyurethane foam—thermal conductivity of 0.025 W/mK.
[60]	A 67.21% decrease in thermal conductivity and 22.58% increase in water vapor resistance.
[58]	Biochar cement brick exhibits low thermal conductivity of 0.18 W/mK.
[72]	Flexural strength improved 26% (with 20% biochar); low thermal conductivity at optimum 10% biochar
[73]	Optimum CO2 uptake was achieved at 4–6% biochar content
[74]	Improved compressive strength by 18–20%; reduced water capillary absorption by 50–60%
[75]	40–50% high compressive strength, internal curing with biochar.
[76]	Density increases with the pyrolysis temperature.
[77]	At 700 °C thermal conductivity lower by 2% cement replacement.
[60]	67.21% reduced thermal conductivity; 22.58% increased water resistance.
[78]	5% biochar improved the thermal conductivity of paraffin wax.
[79]	Biochar + lime plaster wall (25 × 25 m) can remove 63.5 kg CO2 annually.
[80]	Rice husk biochar: strength—increased by 34%; compressive strength and water tightness—improved by 17 and 23%; capillary absorption—lowered by 23%.
[74]	Capillary absorption—lowered by 50%.

highlights other significant studies on biochar.

2.1.2. Fly Ash

It is a delicate powder produced by the combustion of pulverized coal in power stations. It mainly contains silica, alumina, and iron, and its special properties make it useful for making concrete. Research has reviewed the insulation properties of fly ash, exploring its potential as a sustainable material for improving thermal efficiency in building applications. A decline in the thermal performance of cement concrete when a large proportion of fly ash, accounting for 50 percent of the weight, is incorporated. Results indicate that due to the high volume of fly ash (class C), thermal conductivity was reduced by up to 45% [81]. Substituting sand with fly ash and bottom ash can achieve reduced thermal conductivity in mortar and plaster. The study shows that 100% fly ash sand replacement-reduced thermal conductivity by 82%; similarly, 100% bottom ash sand replacement reduced thermal conductivity by 75%. There was a 15.58% reduction in U-value for 50% fly ash replacement mortar in a brick wall panel [37]. Cenospheres have a thermal conductivity 0.065 W/mk. The thermal conductivity tested in a research paper spanned 0.096–0.109 W/mk [82]. Research compared different U-values of sustainable materials and checked their alignment with the ECBC. Fly ash brick, being a sustainable material, showcases U-values closer to the ECBC-prescribed values [83]. Adding expanded vermiculite (lightweight aggregate) to fly ash-based geopolymer mortars enhanced thermal insulation, with 0.094 W/mK thermal conductivity. Hence, expanded vermiculite can be a good option instead of other lightweight materials like pumice and expanded clay for making geopolymer insulation [84]. Prior research focuses on replacement of high volume (50% or more) fly ash in cement concrete and its influence on thermal conductivity. The result shows that there is significant reduction in thermal conductivity (0.06 W/mk) [81]. In another study, the use of snow or crushed ice was used with fly ash to determine whether better insulation properties could be achieved. The findings of the research showed that 20% addition of crushed ice or snow reduces thermal conductivity to 0.225 W/mK. It also indicated that addition of a greater percentage of ice increases the thermal conductivity value [85]. There was an 8% decrease in the thermal conduction value after mixing fly ash into concrete. It is recommended to add 20% or more fly ash into the mix. Also, after fly ash mixing, a reduction in thermal diffusivity and thermal expansion and increase in specific heat capacity was observed [86]. There is reduction in heat conductivity with a value of 0.505 W/mK (lime 10%, gypsum 5%, fly ash 20–25%, and polystyrene 2.5%) [87]. Research looks into the possibility of using rice husk ash (RHA) as an ingredient to achieve thermal comfort without significantly affecting mechanical properties. The outcome of the research shows the least heat transfer of 0.213 W/mK at 100% rice husk ash (RHA) (which is 67.42% lower than for a standard mix); similarly, it increases the porosity in the mix. After 50% RHA it shows near about similar thermal conductivity. It also shows a 32.7% decrease in density after 50% RHA [88]. Prior research focuses on the combination of carbon nanotubes and fly ash and its effect on mechanical, chemical, and thermal properties. Carbon nanotubes increase porosity in the concrete mix. Further research should be conducted to study the effect of carbon nanotubes and fly ash on thermal conductivity [89].

Table 7. Insulation properties of fly ash in chronological order. Source: Author.

Source	Process	Thermal Conductivity (W/mK)	U-Value (W/m2K)
Fly Ash
[90]	Fly ash cement mortar.	0.816	0.66
[91]	Fly ash-based raw composition.	0.0872	0.52
[82]	Cenospheres.	0.065	0.48
[92]	Class C fly ash with polypropylene.	0.0564	0.46
[93]	Wood ash has very low thermal conductivity.	0.002	0.05
			Percentage
[94]	Coal bottom ash in concrete.	Reduce	15%
[89]	High volume (50%/more) fly ash (Class C and F).	Reduce	45%
[37]	Sand replacement 100%.	Reduce	82%

illustrates the insulation properties of fly ash.

Table 8. Research related fly ash. Source: Author.

Source	Statement
[95]	Cenosphere addition: Decreases thermal conductivity, increases porosity. Improves compressive strength at a later stage; however, to improvise initial setting time, a finer cementitious material can used as an additive.
[96]	Mine tailing and ash increase thermal insulation and porosity.
[97]	The higher the water/cement ratio, the lower the heat conductivity and compressive resistance. Replacement of fly ash can lower the heat transfer in concrete by up to 80%.
[91]	Lightweight aggregates based on fly ash were obtained by heating at 1150 degrees, showing lower thermal conductivity of up to 0.0872 W/mk.
[93]	Heat conductivity of wood ash 0.002 W/mK. Wood ash has very low thermal conductivity.
[90]	Fly ash alone exhibits a heat conductivity of 0.816 W/mK, a compactness of 2.02 kg/m3, and a particle size of 70 μm.
[94]	There is reduction of 15–20% in the thermal conductivity of coal bottom ash (CBA) concrete.
[92]	Fly ash-based geopolymer has thermal conductivity of 0.0564 W/mk.
[98]	Incorporating fly ash into multilayer packaging reduces the composite’s thermal conductivity.
[99]	Fly ash concrete after exposure to a high temp. of 550 °C reduces thermal conductivity by 26%.
[100]	30% by weight of fly ash into bricks reduces their thermal conductivity.
[101]	Lightweight aggregate (vermiculate 15–30% by weight) and foam resulted in geopolymer fly ash concrete, showing good mechanical strength and excellent thermal insulating properties.
[102]	Incorporating 20% by weight, rubber wood ash in cement fiber board reduces thermal conductivity to 0.62 W/mK without affecting the mechanical properties.
[103]	Higher fly ash content results in reduced early hydration rate and reduce adiabatic temperature rise (14th day). It shows high influence on thermal insulation properties due to pore formation.
[104]	Result indicates that fly ash thermal conductivity decrease with the increase in content of biomass, pores, carbon content and specific surface. Fly ash conductivity is same or lower than natural soil.
[105]	Fly ash and polypropylene mix reduce down thermal conductivity to 0.248 W/mK (225 °C).

highlights other significant studies on fly ash.

2.1.3. Lime

Lime is a natural and sustainable binder known for its excellent heat insulation qualities. It has been used in building for hundreds of years due to its ability to improve energy efficiency and maintain a comfortable indoor climate. Recent research has focused on exploring and enhancing the insulation properties of lime.

Lime–cement plaster with perlite as an additive and coated with silicon oil exhibits properties like porosity, low water transport rate, and high transmission of water vapor. Hence, this plaster can be considered a high-insulation plaster with high durability [38]. Incorporating expanded glass granulate into lime-based plaster significantly enhances its thermal properties [106]. It reduces the heat conductivity of perlite lime plaster to a value of 0.12 W/mK [51]. Research shows that jute, which is natural fiber, can be added with lime plaster to reduce its thermal conductivity. Though the insulation property is not good compared to hemp shives, it is significantly lower than 0.162 W/mK and can be considered a low-cost insulation material [107]. A study investigated the addition of phase-changing material (PCM) in hydrated and hydraulic lime plaster, showing improvements in thermal insulation as well physical and mechanical properties. The research concluded that while hydraulic applications showed better thermal insulation, mechanical properties remained unchanged, with improvements in physical properties, increased porosity, decreased density, and reduced capillary action [108]. Another study shows that such plaster can reduce heat flux through historic walls by 20–40%, making it a viable option for energy-efficient refurbishment [109]. A study explores the growth and characterization of an advanced biocomposite material made from aggregates of waste paper and lime, highlighting its potential for use in building thermal insulation. The study demonstrates that the biocomposite has favorable thermal conductivity (0.061 W/mK) and compressive strength, making it a viable option for eco-friendly construction, though it highlights the need for further research on moisture management properties (biocomposite material made from aggregates of waste paper and lime) [110]. Clay and lime plasters demonstrate significant potential for use in fire-resistant timber structures, highlighting the need for further studies and standardization [111]. Straw fibers reinforced with lime and cement show potential for building insulation with acceptable compressive strength and low thermal conductivity but require further optimization [112]. Research explores the application of building information modeling (BIM) integrated with LCA for designing sustainable building envelopes using lime-based insulation materials, demonstrating a reduction in energy consumption and environmental impact [113].

Table 9. Insulation properties of lime in chronological order. Source: Author.

Source	Process	Thermal Conductivity (W/mK)	U-Value (W/m2K)
Lime
[114]	Cement–lime with glass spheres as aggregate.	0.3	0.63
[107]	Lime with jute.	0.162	0.59
[115]	Admixtures like perlite and pumice as aggregate, as well as boron minerals and steel fibers with cement and lime.	0.13	0.57
[106]	Perlite lime plaster.	0.12	0.56
[110]	Waste paper and lime.	0.061	0.47
[116]	Lime-based plaster with aerogel (very high cost).	0.028	0.35
			Percentage
[117]	Olive stone in cement–lime mortar.	Reduced	76%

illustrates the insulation properties of lime.

Table 10. Research related lime. Source: Author.

Source	Statement
[116]	Cenosphere addition: Decreases thermal conductivity, increases porosity. Improves compressive strength at later stage; however, to improvise initial setting time, a finer cementitious material can be used as an additive.
[118]	Lime plaster with 50% by volume sand is replaced with expanded perlite. Provides lightweight plaster with the required mechanical properties, enhanced thermal insulation, high porosity, and resistance to salt crystallization.
[119]	Lime–pozzolan plaster with phase-changing material (PCM) addition significantly enhances thermal capacity while maintaining reasonable mechanical strength and improving moisture resistance.
[120]	Incorporating hydrophobic and pozzolanic admixtures into lime plasters effectively enhances their durability and thermal capacity.
[121]	The external layers of lime–sand plasters were found to have higher density and lower total porosity compared to the backing layers.
[122]	Lime plaster provides better fire protection than clay plaster.
[123]	Lime–pozzolana (fly ash) plasters can effectively replace traditional lime plasters in historical buildings, offering enhanced strength without compromising essential properties
[124]	PCM-modified lime–cement plasters are effective for moderating indoor climates and reducing energy consumption, despite a trade-off in mechanical properties.
[125]	Lime-based insulating renders can effectively enhance the comfortable thermal conditions and reduce the electricity usage of heritage buildings, though their moisture retention characteristics in winter need careful consideration to prevent potential damage.
[126]	The lime test cell was 3–5 °C cooler, providing comfortable indoor conditions for 40% longer.
[127]	Lime–metakaolin plasters offer a cost-effective and durable alternative for historical building renovations, potentially replacing current commercial renovation plasters.

highlights other significant studies on lime.

In addition, lime also demonstrates fire protection and the moisture control properties. These properties are elaborated below.

Impact of Lime Plaster on Fire Protection:

Previous sections highlighted lime plaster’s ability to resist high temperatures due to its thermal mass and non-combustible nature. A lower temperature increase signifies superior fire protection. Lime plaster protects against fire better than clay plaster [122].

Influence of Moisture Content on Thermal Conductivity:

Lime plaster’s thermal conductivity is significantly influenced by its moisture content, as noted earlier in the research. Higher moisture levels can temporarily increase thermal conductivity, but lime plaster’s porous nature facilitates drying, restoring optimal insulation performance.

There is a linear relationship between moisture content and conductivity. The commercial plasters had the lowest thermal conductivity across all moisture levels, likely due to hydrophobization. This process prevents water from contacting pore walls directly, reducing its impact on heat transfer [127].

These insights emphasize the importance of aligning material properties with specific application requirements. Lime plaster’s demonstrated fire resistance and thermal efficiency make it a valuable component in sustainable and safe building designs, guiding material selection to achieve long-term performance and environmental benefits.

• Bamboo Biochar: Bamboo biochar exhibits low thermal conductivity due to its unique structure and carbon content. Studies report a U-value range of 0.6–0.51 W/m²K, making it effective for insulation. Additionally, its capacity for CO₂ absorption supports its use in building materials where thermal resistance and carbon reduction are priorities.
• Fly Ash: Fly ash demonstrates moderate thermal insulation properties, with U-values ranging between 0.66 and 0.05 W/m²K, depending on composition and treatment. Its inclusion in concrete and mortar improves insulation while also enhancing structural durability, though optimization of mix ratios is necessary to maximize insulation without compromising mechanical strength.
• Lime: Lime-based materials provide notable thermal insulation (U-value 0.63–0.35 W/m²K) and humidity regulation. Their natural insulating properties can stabilize indoor temperatures by reducing heat flux through walls. Lime’s moisture-regulating capabilities further contribute to building comfort, particularly in humid climates.
• The literature review indicated that all three sustainable materials have significant potential for use as building insulation. The Table 11 demonstrates that the U-value ranges of these materials are very close to the standard value. However, fly ash and lime exhibit lower values that exceed the standard compliance limit of 0.40, as per ECBC guidelines. This suggests room for improvement to meet higher standards such as ECBC and ECBC Super standards. Therefore, an optimal combination of these three materials could achieve superior ECBC compliance.

Table 11. Comparison of U-value ranges for all three materials with ECBC standards. Source: Author.

Material	ECBC Standards	U-Value Range (W/m2K)	Important Properties
Bamboo biochar	0.40	0.60–0.51	Insulation, CO2 sequestration
Fly ash		0.66–0.05	Insulation, improves mechanical properties
Lime		0.63–0.35	Insulation, humidity control, fire resistance

Table 11. Comparison of U-value ranges for all three materials with ECBC standards. Source: Author.

Material	ECBC Standards	U-Value Range (W/m2K)	Important Properties
Bamboo biochar	0.40	0.60–0.51	Insulation, CO2 sequestration
Fly ash		0.66–0.05	Insulation, improves mechanical properties
Lime		0.63–0.35	Insulation, humidity control, fire resistance

• Chronological order—The chronological arrangement of literature data offers clear insights for selecting the most effective material typology with the highest proven performance. It also facilitates informed decision-making by identifying the optimal form of the material while presenting alternative options with comparable values.
• Bamboo biochar—A chronological analysis of the literature results demonstrated that biochar alone offers superior insulation properties. However, bamboo biochar has not yet been explored for its potential as a building insulation material. Other options in the sequence, such as combinations of biochar with foam, geopolymer materials, biochar bricks, and cement admixtures, have shown limited effectiveness. This analysis provides a clear, informed basis for decision-making, confirming that biochar alone can be incorporated into the proposed mix. Nonetheless, there is significant potential for further research into the insulation properties of bamboo biochar.
• Fly ash—A chronological analysis of the literature data revealed that wood ash exhibits the lowest insulation value, while 100% sand replacement yields the best performance among all options. However, wood ash was not considered due to its exclusion from the research scope. C-type fly ash, cenospheres, and high-volume (50%) fly ash are identified as viable alternatives for incorporation into the proposed mix. The remaining options in chronological order serve as references, indicating the minimum achievable results with the inclusion of fly ash.
• Lime—The chronological analysis suggests that lime, when combined with other sustainable materials, presents significant opportunities for achieving improved results. Starting with the lowest insulation value, the combination of lime and aerosol material is an initial option. However, due to the high cost of aerosol, this combination may not be practical. Aggregates like olive stone, perlite, and pumice offer a more feasible next step when paired with lime. Waste paper emerges as the most promising option, providing the highest potential for performance while also reducing costs through the reuse of waste materials.
• Comparative Analysis—Among the three materials, biochar alone offers better thermal conductivity (0.8–0.02 W/mK), positioning it as a strong candidate for insulation. Fly ash and lime also contribute significantly to thermal regulation, with fly ash enhancing concrete’s durability and lime providing moisture control as well as fire resistance.

A comparative analysis has been illustrated with other insulation materials in the below

Table 12. Comparison with other insulation materials. Source: Author.

Insulation Material	Thermal Conductivity (W/m·K)	Approximate Cost (INR/kg)
Glass wool	0.035–0.040	100–200
Rock wool	0.035–0.040	120–250
Expanded polystyrene (EPS)	0.032–0.038	100–150
Extruded polystyrene (XPS)	0.029–0.033	150–200
Polyurethane foam (PUF)	0.022–0.028	200–300
Bamboo biochar	0.200–0.080	220–250
Fly ash	0.816–0.056	2–5
Lime	0.300–0.061	5–10

.

2.2. Environmental Impact

The ISO 14040 standards provide a comprehensive framework for life cycle analysis (LCA); however, the following criteria have been specifically adhered to:

• Goal and Scope Definition: ISO 14040 highlights the importance of clearly defining the goals and boundaries of a life cycle assessment, focusing on aspects such as raw material extraction, production, use, and disposal stages for embodied energy and carbon footprint assessment [128].
• Life Cycle Inventory (LCI): This phase involves quantifying inputs (raw materials, energy) and outputs (emissions, waste) across each stage, providing the data foundation for embodied energy and carbon footprint analysis.
• Life Cycle Impact Assessment (LCIA): The impact assessment translates LCI data into meaningful metrics such as global warming potential (carbon footprint) and cumulative energy demand (embodied energy).
• Interpretation: This final phase involves interpreting results to identify critical environmental impacts and improvement opportunities.

Sustainability is a core aspect of utilizing bamboo biochar, fly ash, and lime, as each material has a relatively low carbon footprint and aligns with circular economy principles.

• Bamboo Biochar: Biochar sequesters carbon, offsetting emissions generated during production. Fast-growing bamboo, which reaches maturity in 3–5 years, presents a renewable resource option. Additionally, biochar production involves pyrolysis, a process that reduces waste by converting organic matter to a carbon-rich material.
• Fly Ash: As a byproduct of coal combustion, fly ash reduces landfill waste and repurposes industrial residue. By using fly ash in construction, overall CO₂ emissions are minimized, as it replaces energy-intensive cement components. Studies suggest fly ash incorporation can decrease concrete’s carbon footprint by up to 65%.
• Lime: Lime has a moderate carbon footprint, primarily due to CO₂ emissions during calcination. However, lime’s recyclability and ability to be produced locally in many regions help lower its embodied energy. When used in plasters and mortars, lime also supports indoor air quality by permitting natural ventilation and moisture control.
• Comparative Analysis: Bamboo biochar has the lowest carbon footprint due to carbon sequestration, followed by fly ash as an effective reuse of industrial waste. Lime, while emitting CO₂ during production, remains eco-friendly due to its longevity and recyclability.

A comparative analysis of the environmental impact of sustainable materials versus other insulation materials has been conducted to evaluate the efficacy of sustainable materials. See

Table 13. Environmental impact analysis: A comparative study. Source: Author.

Material	Environmental Impact
	CO2 Sequestration	Renewable Resource	Reduces Waste	VOC	Humidity Control	Recycle	Carbon Footprint
Glass wool	No	No	Limited	Low	No	Yes	High
Rock wool	No	No	Limited	Low	No	Limited	High
Expanded polystyrene (EPS)	No	No	Limited	High	No	Limited	High
Extruded polystyrene (XPS)	No	No	Limited	High	No	No	High
Polyurethane Foam (PUF)	No	No	Limited	High	No	No	Very high
Bamboo biochar	Yes	Yes	Yes	Low	Yes	Limited	Low
Fly ash	No	No	Yes	Low	No	Limited	Low
Lime	No	Yes	Limited	Low	Yes	Yes	Moderate

.

Explanation.

CO₂ Sequestration: Bamboo biochar has the ability to sequester carbon, making it a sustainable choice.

Renewable Resource: Bamboo biochar and lime are renewable or derived from renewable sources.

Reduces Waste: Fly ash is a byproduct of coal plants, repurposing industrial waste, while bamboo biochar also utilizes waste biomass.

VOC Emission: Synthetic materials like EPS, XPS, and PUF emit higher volatile organic compounds (VOCs).

Humidity Control: Bamboo biochar and lime help regulate humidity naturally.

Recyclability: Glass wool, rock wool, and lime are recyclable; biochar has limited recyclability due to its use in soil amendments.

Carbon Footprint: Synthetic materials like PUF, EPS, and XPS have a high carbon footprint, while bamboo biochar and fly ash have low environmental impacts.

Embodied Energy

• A previous research paper quantifies embodied energy by examining the total energy required for material production, from raw material extraction to final application. For example, the embodied energy of bamboo biochar, fly ash, and lime is calculated by including energy used in processing (e.g., pyrolysis for bamboo biochar) and transportation.
• The emphasis on renewable materials like bamboo biochar aligns with reducing embodied energy, as bamboo grows rapidly with minimal energy input, reducing its life cycle energy footprint.

Carbon Footprint

• Carbon sequestration potential of bamboo biochar, which can offset emissions by storing carbon during its life cycle.
• Emissions from raw material extraction, processing, and end-of-life disposal (e.g., CO₂ released during lime production is noted but offset by its ability to reabsorb CO₂ over time).
• Specific metrics, such as CO₂ savings per ton of biochar-added concrete, are referenced, showcasing quantitative alignment with LCA carbon footprint methodologies.

Alignment with ISO 14040

• The research incorporates life cycle thinking as prescribed by ISO 14040 by evaluating the materials’ environmental impact at every stage—from production to disposal.
• The study’s comparative approach to bamboo biochar, fly ash, and lime reflects the LCIA principle of identifying critical impact categories (e.g., global warming potential and energy demand).

2.3. Cost

Affordability is essential for sustainable building materials to gain broader acceptance. Each material’s cost is influenced by its availability, production requirements, and life cycle savings.

• Bamboo Biochar: Although slightly more costly due to specialized production (pyrolysis), biochar’s longevity and insulation efficiency can lead to long-term savings. Studies indicate that biochar’s application in insulation reduces energy costs by approximately 20–30% over a building’s life cycle [68].
• Fly Ash: As an industrial byproduct, fly ash is low-cost and widely available. Its integration into building materials reduces reliance on cement, providing both initial savings and reduced life cycle costs. Fly ash’s economic benefits are particularly evident in regions where coal power plants are prevalent, as transportation costs are minimized. The distribution of coal power plants across India, as highlighted in the material availability section of the results and discussion, indicates that fly ash is readily available throughout India, with the exception of the extreme northern and eastern regions [129].
• Lime: Lime is moderately priced and available globally. Its low-maintenance requirements and durability make it cost-effective over time. Lime-based materials reduce energy demand for heating and cooling, offsetting initial expenses through lower utility costs.
• Comparative Analysis: Fly ash is the most cost-effective due to its status as an industrial byproduct. Bamboo biochar, while more expensive initially, offers long-term energy savings. Lime’s cost is balanced by its availability and durability, making it economically viable for insulation applications Table 14.

Table 14. Comparison and analysis. Source: Author.

Material	Initial Cost (INR/kg or INR/sq.m)	Durability	Maintenance Frequency	Long-Term Maintenance Cost	Environmental Impact
Bamboo biochar	220–250 per kg	High	Minimal	Low	Eco-friendly, carbon sequestration
Fly ash	2–5 per kg	High	Minimal	Negligible	Sustainable, repurposes waste
Lime	5–10 per kg	High	Moderate	Low	Sustainable, low carbon footprint
Fiberglass	150–300 per sq.m	Moderate	Frequent inspections	Moderate to high	Moderate environmental impact
Polystyrene (EPS)	1500–2600 per sq.m	High	Periodic checks	Moderate	High carbon footprint
Polystyrene (XPS)	450–700 per sq.m	Moderate	Periodic checks	Moderate	High carbon footprint

Table 14. Comparison and analysis. Source: Author.

Material	Initial Cost (INR/kg or INR/sq.m)	Durability	Maintenance Frequency	Long-Term Maintenance Cost	Environmental Impact
Bamboo biochar	220–250 per kg	High	Minimal	Low	Eco-friendly, carbon sequestration
Fly ash	2–5 per kg	High	Minimal	Negligible	Sustainable, repurposes waste
Lime	5–10 per kg	High	Moderate	Low	Sustainable, low carbon footprint
Fiberglass	150–300 per sq.m	Moderate	Frequent inspections	Moderate to high	Moderate environmental impact
Polystyrene (EPS)	1500–2600 per sq.m	High	Periodic checks	Moderate	High carbon footprint
Polystyrene (XPS)	450–700 per sq.m	Moderate	Periodic checks	Moderate	High carbon footprint

2.3.1. Economic Analysis

• Initial Costs:

Bamboo Biochar: Higher cost due to production scalability and eco-friendly processes.

Fly Ash: The most economical option, as it repurposes waste materials.

Lime: Affordable and widely used, offering a sustainable balance.

Conventional Materials: Fiberglass and polystyrene have significantly higher costs.

• Durability and Maintenance:

Bamboo Biochar: Exceptional durability and low maintenance, making it a cost-effective choice in the long term.

Fly Ash and Lime: Both are durable with minimal maintenance needs; lime may require occasional reapplication.

Conventional Materials: Require frequent checks and maintenance, particularly fiberglass, which is prone to moisture damage.

• Environmental Impact:

Sustainable Materials: Bamboo biochar, fly ash, and lime have low environmental footprints and support sustainability goals.

Conventional Materials: Polystyrene and fiberglass contribute to pollution and have high embodied carbon, making them less sustainable.

Sustainable materials like bamboo biochar, fly ash, and lime offer significant advantages in terms of cost, durability, and environmental benefits over conventional insulation materials. Table 14 presents a comparative analysis of sustainable materials versus current insulation materials.

2.3.2. Savings Calculation

Based on a baseline energy cost of INR 1000 per sq. m annually, 30% energy savings translate to INR 300 per sq. m. Over a 50-year building life cycle, this results in INR 15,000 per sq. m in total cost savings.

Material Contributions:

• Bamboo Biochar: Offers the highest insulation efficiency, reducing both energy costs and emissions significantly over the building’s lifespan.
• Fly Ash: Provides moderate insulation performance and is the most economical option upfront, suitable for large-scale applications.
• Lime: Balances cost and performance, providing medium insulation efficiency and durability.

Environmental Impact: Each kWh of saved energy reduces CO₂ emissions by 0.82 kg, contributing to a substantial environmental benefit over time (PNNL-20405) [130].

2.3.3. Life Cycle Cost Assessment (LCCA)

LCCA evaluates all costs associated with materials over their life cycle, from production to disposal, while incorporating potential energy savings and maintenance.

Table 15. A detailed analysis of Life Cycle Cost Assessment. Source: Author.

Parameter	Bamboo Biochar	Fly Ash	Lime	Combined Impact
Initial production cost	INR 220–INR 250 per kg	INR 2–INR 5 per kg	INR 5–INR 10 per kg	Moderate due to integration of affordable materials like fly ash and lime.
Installation cost	High (due to biochar)	Low	Moderate	Averaged due to balancing biochar with fly ash and lime.
Energy savings	30%			Substantial reductions in heating and cooling loads, saving INR 300 per sq.m annually.
Maintenance cost	Negligible	Negligible	Moderate	Overall maintenance remains low with periodic lime reapplications.
Disposal cost	Low (biodegradable)	Negligible (waste reuse)	Low	Eco-friendly, with fly ash reducing landfill waste.
Life cycle savings (50 yrs)	INR 15,000 per sq.m			Total life cycle savings due to consistent energy cost reductions.
Environmental benefits	Carbon sequestration	Waste utilization	Low carbon footprint	High sustainability impact across all materials.

provides a detailed breakdown of a combination of bamboo biochar, fly ash, and lime, assuming 30% energy savings.

The combination of bamboo biochar, fly ash, and lime provides substantial economic and environmental benefits over the building’s life cycle. By reducing energy costs, minimizing maintenance, and leveraging sustainable disposal practices, this approach achieves both cost efficiency and ecological sustainability.

3. Methodology

This research employs a systematic methodology to evaluate the potential of sustainable materials such as bamboo biochar, fly ash, and lime for enhancing thermal insulation in building envelopes. The revised methodology addresses selection criteria, U-value calculations, life cycle analysis (LCA), and chronological analysis to ensure a comprehensive assessment.

3.1. Literature Review and Data Collection

A systematic review of 125 studies was conducted, prioritizing research that provided data on the following:

• Thermal conductivity or resistance of materials.
• Integration of sustainable materials.
• Life cycle analysis (LCA), carbon sequestration, and circular economy contributions. This review focused on bamboo biochar, fly ash, and lime, analyzing their thermal properties, environmental impact, and material performance. No real-time experiments were conducted; data were sourced entirely from the published literature.

3.2. U-Value Calculation

The U-value, a critical metric for thermal performance, was calculated using the following formula:

U = 1/Rt

where Rt is the sum of thermal resistances of all envelope layers (e.g., plaster, wall). Thermal conductivity values from the literature were used to estimate U-values for combinations of bamboo biochar, fly ash, and lime.

• Reason for U-value use: U-values quantify thermal transmittance, directly correlating with energy efficiency in building envelopes.
• The calculated U-values were benchmarked against building insulation standards and validated using cross-referenced studies.
• Results were organized chronologically to highlight advancements in material properties over time.

3.3. Life Cycle Analysis (LCA)

LCA was integrated to evaluate the environmental performance of bamboo biochar, fly ash, and lime across their life cycle—from production to disposal. The following parameters were analyzed:

• Embodied Energy: Total energy used in production and transportation, emphasizing the benefits of waste-based materials like fly ash.
• Carbon Footprint: Bamboo biochar’s carbon sequestration, fly ash’s emissions reduction as a byproduct, and lime’s moderate CO₂ contribution during calcination were assessed.
• End-of-Life: Recyclability and reuse potential of materials were analyzed, focusing on minimizing their environmental impact.

These parameters helped compare the materials’ ecological viability, reinforcing the sustainability argument for their use.

3.4. Chronological Analysis and Gap Identification

Derived U-values and sustainability metrics were arranged chronologically for the following purposes:

• Identify trends and technological advancements in thermal performance.
• Highlight gaps in existing research, including the following:

  ○

  Limited experimental validation.

  ○

  The need for further exploration of composite materials combining bamboo biochar, fly ash, and lime for optimal performance.

This chronological framework provided insights into research progress and areas requiring further investigation.

3.5. Future Directions

Based on findings, the following suggestions can be made:

• Real-time experimental validation of the heat efficiency of these materials is recommended.
• Composite formulations of bamboo biochar, fly ash, and lime should be explored for achieving target U-values of 0.4 for walls and 0.33 for roofs.
• LCA findings indicate opportunities to reduce carbon emissions and enhance sustainability through optimized resource use.

4. Results and Discussion

4.1. Correlation

4.1.1. Biochar and Fly Ash (A + B)

The inclusion of bio-based charcoal and fly ash into building insulation systems has shown potential to significantly improve thermal resistance, but further research is needed to optimize their mixture for different climatic conditions [131]. Combining biochar and fly ash in insulation materials could lead to enhanced energy efficiency in buildings, yet long-term studies are required to assess the durability and thermal performance over time [132]. The inclusion of biochar and fly ash can reduce thermal bridging in insulation systems, but future research should explore their combined impact on the structural integrity of building envelopes [133]. Integrating biochar and fly ash into insulation materials could significantly lower the carbon footprint of buildings; however, further studies are needed to evaluate the life cycle environmental benefits [134]. The prospect of fly ash and biochar in creating innovative insulation materials is promising, but gaps remain in understanding their performance in high-humidity environments [135].

4.1.2. Fly Ash and Lime (B + C)

The combination of fly ash and lime has shown promising effects in enhancing soil stabilization, which could be extended to enhance the heat insulation properties of earthen structural resources, yet research is needed to assess their long-term durability in varying climatic conditions [136]. Fly ash and lime can synergistically enhance the cementitious properties of building materials, potentially improving their thermal insulation characteristics, but further studies are required to optimize their use in eco-friendly insulation systems [137]. The integration of fly ash and lime in concrete can enhance thermal insulation, yet research gaps exist in understanding how these materials interact under different environmental stressors, particularly in high-moisture areas [138]. Fly ash–lime composites may dramatically enhance the thermal insulation capability of building envelopes, but there is a need for more research on their performance in retrofitting existing structures [139]. The development of sustainable insulation materials using fly ash and lime offers potential environmental benefits, though research is needed to standardize their application in building insulation to ensure consistency in thermal performance [140].

4.1.3. Lime and Biochar (C + A)

The integration of biochar with lime has demonstrated significant improvements in soil stabilization and nutrient retention, which can be leveraged to improve the heat insulation qualities of building materials, but more study is needed to evaluate their long life thermal output in various climatic conditions [141]. The combination of biochar and lime in structural resources offers enhanced insulation and carbon sequestration capabilities, yet the potential for large-scale application in different types of buildings needs further investigation to determine practical feasibility [142]. The use of biochar–lime composites in insulation resources can improve both thermal and moisture regulation; yet, future research needs to prioritize refining the mixture ratios and application methods for different building environments [143]. The synergy between biochar and lime not only enhances the thermal functionality of insulation resources but also supports sustainable building practices by reducing the carbon footprint, although there is a gap in the literature regarding their impact on indoor air quality [144]. The composite use of biochar and lime with cement shows promising potential for carbon-neutral concrete, offering high-density, durable carbon storage while reducing CO₂ emissions from clinker production [145].

4.1.4. Biochar, Fly Ash, and Lime (A + B + C)

Mixing of bio-based charcoal, fly ash, and lime shows significant improvements in the crushing strength and thermal insulation properties of compressed earth blocks (CEBs), yet further studies are required to assess the long-term durability and moisture resistance of these composites in different climates [146]. Utilizing biochar, fly ash, and lime in building insulation materials not only improves thermal insulation but also reduces the carbon footprint of construction. However, investigation is necessary to refine the mixture ratios for different environmental conditions [147]. The assimilation of biochar with fly ash and lime in building materials shows promise for sustainable construction by enhancing both mechanical and thermal properties. Nonetheless, research gaps exist in understanding the long-term impacts of these materials on indoor air quality [142]. The composite of biochar, fly ash, and lime has potential in reducing the environmental impact of building insulation systems. However, further studies are needed to evaluate their performance in large-scale applications, particularly in terms of thermal resistance and life cycle costs [141]. Although the blend of biochar, fly ash, and lime improves the insulation properties of construction materials, future research should focus on identifying the optimal proportions to maximize thermal performance while maintaining structural integrity [134].

Correlation identification:

To analyze the correlation between all three materials, the gathered information has been consolidated for a comprehensive overview. Based on the literature review and the revealed properties of the materials, the data are organized and presented in the table below. The information is categorized into combinations of materials: (A + B) biochar and fly ash, (B + C) fly ash and lime, (C + A) lime and biochar, and (A + B + C) all three materials together. Correlation demonstrated in

Table 16. Comparative relationship and identification of future research area for biochar, fly ash, and lime. The abovementioned 125 research papers comprise the data source for what follows below. Source: Author.

Research Area	(A + B)	(B + C)	(C + A)	(A + B + C)
Improve thermal resistance in insulation systems	✔	✔	✔	✔
Enhance energy efficiency in buildings	✔	✔		✔
Reduce thermal bridging in insulation systems	✔		✔	✔
Lower the carbon footprint of buildings	✔	✔	✔	✔
Innovative insulation materials’ performance in high-humidity environments
Enhance soil stabilization and nutrient retention		✔	✔
Improve cementitious properties and thermal insulation		✔
Impact on structural integrity of building envelopes	✔
Performance in retrofitting existing structures		✔
Standardize application in building insulation		✔
Long-term thermal performance in varying climates	✔		✔
Carbon sequestration capabilities			✔
Moisture regulation in insulation materials			✔
Impact on indoor air quality
Long-term resilience under varying environmental conditions			✔
Optimize mixture ratios for different climatic conditions				✔
Performance in large-scale applications				✔
Identify optimal proportions for maximum thermal performance				✔

.

In the above table, a tick shows that correlation exists; however, areas without a tick indicate a scope for further research.

4.2. Economic Viability and Cost Implications

4.2.1. Initial Cost Analysis

A comparison of the initial costs of bamboo biochar, fly ash, and lime with conventional insulation materials like fiberglass and polystyrene is necessary. While fly ash is typically low-cost due to its availability as an industrial byproduct, bamboo biochar and lime may incur moderate costs due to production and transportation. Highlighting these costs provides a clear picture of the affordability of these sustainable materials.

4.2.2. Long-Term Cost Savings

Despite potentially higher initial costs, the long-term savings from reduced energy consumption are significant. Bamboo biochar and lime offer strong insulation properties, lowering heating and cooling expenses. Additionally, the durability and low maintenance of these materials contribute to further long-term savings, making them economically viable over time.

4.2.3. Life Cycle Cost Assessment (LCCA) and Cost-Effectiveness

The aim of executing a life cycle cost assessment (LCCA) is to evaluate the total cost of these materials, from production to end-of-life disposal. Bamboo biochar’s carbon sequestration and fly ash’s recycling benefits contribute to reducing long-term environmental and financial impacts. This assessment demonstrates the cost-effectiveness of these materials over their lifespan.

4.2.4. Scalability and Market Availability

As sustainable materials like bamboo biochar and lime gain popularity, large-scale production will reduce costs. Fly ash, already widely available, offers cost advantages, especially for large projects. Emphasizing scalability and market availability will show how these materials can become more affordable with wider adoption.

4.2.5. Policy and Incentives

Government incentives and green building certifications, such as LEED and ECBC, can offset initial costs and make these materials more attractive. Discussing available subsidies and tax credits will further enhance the economic argument for using bamboo biochar, fly ash, and lime in construction.

4.2.6. Cost–Benefit Analysis

A financial impact assessment comparing these materials with conventional insulation solutions summarizes the initial costs, long-term savings, and life cycle benefits. This offers a clear cognition of the financial and ecological advantages of using bamboo biochar, fly ash, and lime in sustainable building practices.

4.3. Sustainability

The sustainability performance of construction materials like bamboo biochar, fly ash, and lime is critical in promoting environmentally responsible building practices. Each material offers unique advantages in the context of environmental impact, financial viability, and community-oriented impact, making them valuable components in sustainable construction systems.

Table 17. Sustainability performance of bamboo biochar, fly ash, and lime. Source: Author.

Category	Criteria	Bamboo Biochar	Fly Ash	Lime
1. Environmental impact	Embodied energy	Low to moderate, depending on the production process.	Low, as it is a byproduct of coal combustion.	Moderate to high, depending on the extraction and processing methods.
	Carbon footprint	Low, as it sequesters carbon during production.	Low to moderate, depending on the transportation and processing.	Moderate, due to CO2 emissions during calcination.
	Resource efficiency	High, as it uses renewable bamboo resources.	High, as it repurposes industrial waste.	Moderate, uses natural limestone resources.
	Toxicity	Low, generally free from harmful chemicals.	Low to moderate, depending on trace elements in the ash.	Low, but can be caustic in handling.
2. Economic viability	Cost	Low to moderate, depending on production scale and location.	Low, as it is a waste product with low acquisition costs.	Moderate, depending on production and transportation costs.
	Availability	High in areas where bamboo is abundant.	High, widely available as a byproduct of coal-fired power plants.	Moderate, widely available but depends on local geology.
	Durability	High, contributes to longevity and stability when used in construction materials.	It augments the toughness and lasting power of concrete.	High, known for its long-lasting properties in construction.
3. Social impact	Health and safety	Safe to use, with no significant health risks associated.	Generally safe; however, handling can release fine particulates.	Safe in use, though caustic during handling.
	Aesthetics and comfort	Contributes to thermal comfort, but not typically visible in finished construction.	Contributes to thermal mass and strength but not aesthetically significant.	Provides a traditional, smooth finish in construction, contributing to aesthetic appeal.
4. Performance	Thermal insulation	Good thermal insulation properties, reduces heat transfer.	Moderate, adds to thermal mass but not primarily used for insulation.	Moderate, contributes to thermal regulation when used in lime plaster or render.
	Structural integrity	Adds strength and stability when combined with other materials.	High, significantly enhances structural integrity in concrete.	High, particularly in lime-based mortars and plasters, providing good binding strength.
	Moisture resistance	High, helps in moisture regulation and prevents mold growth.	Moderate to high, depending on the mix and application.	High, especially in lime plaster, which is breathable and resists moisture.
	Fire resistance	High, inherently fire-resistant due to its carbon-rich composition.	High, contributes to the fire resistance of concrete.	High, particularly in lime plaster and renders, which are non-combustible.
5. Compliance	Certification and standards	Can contribute to meeting green building standards like LEED and BREEAM.	Widely recognized and accepted in various construction standards.	Compliant with many traditional and modern construction standards, including LEED and BREEAM.
6. Life cycle assessment	Renewability	High, renewable and sustainable if bamboo is sourced responsibly.	Not renewable, as it is a byproduct of non-renewable coal combustion.	Moderate, depends on the sustainability of limestone extraction.
	Recyclability	High, can be reused or integrated into other construction materials.	High, often recycled in cement and concrete production.	High, can be recycled into new construction materials or reused in different applications.
	End-of-life disposal	Low environmental impact, biodegradable and can be used as soil amendment.	It can be reused or safely thrown away in landfills.	Low environmental impact, can be reused or safely disposed of, and can even enhance soil quality.

demonstrates the sustainable performance of bamboo biochar, fly ash, and lime.

Figure 15 illustrates the vulnerability of Portland cement compared to other cement types.

The graph illustrates that Portland cement contributes to high CO₂ emissions, while incorporating sustainable materials like fly ash significantly reduces these emissions [148]. Incorporating bamboo biochar, fly ash, and lime into construction materials presents a compelling opportunity to enhance sustainability in the building industry. Bamboo biochar offers high resource efficiency and renewability, fly ash effectively repurposes industrial waste while reducing CO₂ emissions, and lime provides structural integrity with moderate environmental impact. However, optimizing the use of these materials requires careful consideration of their life cycle performance and regional availability to maximize their benefits in sustainable construction practices.

4.4. Material Availability

Bamboo biochar: Bamboo biochar has drawn interest for its prospect in sustainable farming, soil improvement, and carbon sequestration. The production of bamboo biochar supports circular economy principles by utilizing bamboo waste, thus minimizing ecological repercussion and promoting ecological balance [149]. The production and utilization of bamboo biochar can lead to social benefits such as job creation, especially in rural areas where bamboo is abundant. It can also contribute to improving soil fertility and agricultural productivity, which supports food security and livelihoods [150].

Fly ash: It is a waste product generated by coal burning in power plants and is abundantly available in regions with significant coal-based power generation. The utilization of fly ash in construction resources such as insulation plaster is well documented due to its pozzolanic properties, which play a part in to the robustness and resilience of the material. The availability of fly ash in India is depicted in Figure 16 below.

Lime: Lime is a widely available material derived from limestone and used in construction for its binding properties. It is often combined with fly ash in various proportions to produce sustainable construction materials, including insulation plaster [151].

4.5. Policy and Industrial Implications

Policy: The use of sustainable materials like bamboo biochar, fly ash, and lime in construction can significantly enhance energy efficiency and reduce carbon emissions. These materials improve insulation properties, align with energy conservation standards such as ECBC, and contribute to global sustainability efforts like the UN Sustainable Development Goals. Additionally, they support circular economy principles by reducing waste and utilizing renewable resources, making them valuable in promoting environmentally responsible building practices.

SDGs: Bamboo and bamboo biochar meaningfully support the achievement of numerous Sustainable Development Goals (SDGs). Bamboo helps reduce poverty (SDG 1) by providing income through its use in construction and crafts, especially in rural areas [152]. As a renewable biomass source, bamboo also supports clean energy production (SDG 7), offering alternatives to fossil fuels through biofuels and electricity [150]. Its use in low-cost housing aids in building sustainable cities (SDG 11) due to its great resilience and replenishable nature [152]. Bamboo biochar enhances soil fertility and carbon sequestration, promoting environmentally responsible use (SDG 12) and environmental protection measures (SDG 13) by reducing greenhouse gas emissions [141]. Additionally, bamboo supports life on land (SDG 15) by restoring degraded land and preventing soil erosion [150].

Fly ash supports SDGs by promoting resource efficiency (SDG 12) through recycling in construction, reducing the need for virgin materials and lowering carbon emissions (SDG 13) [153]. It also enhances soil health for land reclamation, supporting life on land (SDG 15) [153].

Lime supports SDG 12 by reducing cement usage and lowering carbon emissions in construction [154]. It also aids SDG 15 by improving soil health and enabling land reclamation [154].

4.6. Novelty and Unique Contributions

Bamboo biochar has not been widely used as a thermal insulation material. Other plant-based biochars have been used in concrete to partially replace cement and sand, and occasionally for insulation, but not as part of a complete insulation system for the entire building envelope. Using bamboo biochar in a full insulation system for building envelopes is a new idea, and its effectiveness could be enhanced when combined with fly ash and lime.

4.7. Real World Application

The practical implementation of bamboo biochar, fly ash, and lime is intended as plaster for walls and IPS flooring for roofs. Different ratios for internal plaster, external plaster, and IPS flooring are demonstrated in

Table 18. Composition of mortar. Source: Author.

Type of Work	Cement	Composition	Standards
Internal plaster	1:4	1 unit cement, 4 units sand	IS-1661:1972, cement and cement–lime plaster
External plaster	1:6	1 unit cement, 6 units sand	IS-1661:1972, cement and cement–lime plaster
IPS flooring	1:2:4	1 unit cement, 2 units sand, 4 units aggregate	Civil Work Specifications 2023

:

As per the literature review, replacement percentages for biochar, fly ash, and lime in mortar are shown in the table below.

Various replacement percentages, as demonstrated in the literature, are compiled in

Table 19. Replacement percentage. Source: Author.

Material	Replacement for	Replacement Percentage	Ratio	Cement Type	Sources
Bamboo biochar	Cement	15–20%	1:4 to 1:6 (internal/external plaster)	Ordinary Portland cement (OPC)	[41,44,45]
	Sand	10–12%		OPC/blended cement	[46,80]
Fly ash	Cement	25–30%	1:5 to 1:6	Blended cement, PPC	[37]
	Sand	15%		OPC/blended cement	[82,86]
Lime	Cement	20–25%	1:4 to 1:5	Hydrated lime + OPC	[106]
	Sand	10–15%		Portland pozzolana cement (PPC)	[10,110,119]

.

Bamboo Biochar: Biochar has been shown to improve thermal properties when used as a cement replacement, especially in combination with other sustainable materials. Research suggests a replacement percentage of up to 20% for cement and around 10% for sand.

Fly Ash: Well documented for reducing cement use while improving workability and strength, fly ash can replace up to 30% of cement and 15% of sand in plaster or mortar mixes.

Lime: Lime is frequently used in traditional building techniques and can replace up to 25% of cement and 15% of sand in construction. It is especially beneficial for improving moisture resistance and thermal regulation.

5. Conclusions

A comprehensive review of the existing research highlights several gaps, which are outlined below.

Bamboo biochar: Many studies on bamboo biochar have focused on its applications in agriculture, water treatment, and CO₂ absorption, with less emphasis on its thermal properties and potential as an insulation material in building construction. Other biomass biochar exhibits high porosity, low density, low thermal conductivity, VOC adsorption, CO₂ sinking, improved indoor air quality, humidity control, and reduced water capillary activity. While bamboo biochar is recognized for its low carbon footprint, comprehensive life cycle assessments that include its production, use, and end-of-life disposal are limited. Research on the scalability and practical implementation of bamboo biochar in construction is lacking, requiring studies on economic viability, supply chain logistics, and barriers to widespread adoption.

Fly ash: Expanded vermiculite, coal bottom ash, cenospheres, carbon nanotubes–fly ash, and high volume fly ash (50% or more) are grey areas in the literature that need to be explored more and incorporated in future research work.

Lime: The thermal properties of waste paper, expanded glass granulate, jute, perlite, pumice, boron minerals, and steel fibers show great potential; however, further exploration is needed to determine their optimum percentages in combination.

Cost: Current insulation building insulation materials like glass wool and rock wool are expensive; however, cost-effectiveness is a major aspect that should be thoroughly checked to make it a viable research option for social acceptance.

Compliance: There is a research gap in the standardization of eco-friendly insulation materials in the construction industry; however, adhering to established standards could help bridge this gap.

Optimum mix: Bamboo biochar has demonstrated the ability to enhance thermal properties when used as a substitute for cement, with studies indicating it can replace up to 20% of cement and 10% of sand. Fly ash, known for improving both workability and strength, can be used to replace as much as 30% of cement and 15% of sand. Lime, frequently utilized in traditional construction methods, can substitute up to 25% of cement and 15% of sand, providing benefits such as improved moisture resistance and thermal performance. The optimal replacement percentages for these materials need to be determined through extensive experimentation.

Based on this entire research study, it appears feasible to achieve a U-value of 0.4 for walls and 0.33 for roofing, provided that the optimal mix is identified.

Findings: In this section, the findings are organized into two categories: sensory components and physical components. See

Table 20. Sensory components and physical components. Source: Author.

Category	Criteria	Bamboo Biochar	Fly Ash	Lime
Sensory components	Thermal comfort	Low thermal conductivity, enhances indoor temperature stability	Reduces heat transfer in concrete mixes	Maintains stable indoor temperatures
	Humidity regulation	Minimal contribution	Limited role, depends on integration	High moisture regulation, prevents dampness and allows ventilation
	Air quality	Passive VOC absorption	Minimal impact	Improves air quality by reducing mold risks
Physical components	Thermal insulation (U-values)	U-value: 0.6–0.51 W/m2K	U-value: 0.66–0.05 W/m2K	U-value: 0.63–0.35 W/m2K
	Carbon footprint	High carbon sequestration potential	Reduces CO2 emissions by replacing cement components	Moderate emissions during calcination, partially offset by recyclability
	Durability	High structural resilience and cement compatibility	Enhances robustness and longevity of concrete structures	Long-lasting, fire-resistant, suitable for modern and heritage buildings
	Recyclability	Limited to soil amendments	Highly reusable as an industrial byproduct	Recyclable into new plasters and construction materials

.

Key findings and research gaps compiled in the

Table 21. Key finding and research gaps. Source: Author.

Category	Key Findings	Research Gaps
Bamboo biochar	- Recognized for low carbon footprint and VOC adsorption. - Provides high porosity, low density, and low thermal conductivity. - Improves indoor air quality and controls humidity.	- Limited studies on its thermal insulation properties. - Lack of life cycle assessments, including end-of-life disposal. - Need for scalability and economic viability research for construction use.
Fly ash	- Enhances workability and strength when used as a replacement material. - Known applications in concrete for improved structural properties.	- Research needed on its use with expanded vermiculite, coal bottom ash, and cenospheres. - Lack of studies on advanced combinations like carbon nanotubes–fly ash and high-volume fly ash (≥50%).
Lime	- Offers moisture resistance and thermal performance. - Frequently used in traditional construction methods. - Compatible with waste paper, jute, perlite, and other aggregates.	- Further exploration required to determine optimal mix percentages with materials like expanded glass granulate, pumice, and steel fibers. - Thermal properties in combination with novel aggregates need validation.
Cost	- Current materials like glass wool and rock wool are expensive. - Cost-effectiveness is critical for large-scale social adoption.	- Comprehensive cost analysis required for eco-friendly alternatives to ensure economic feasibility.
Compliance	- Eco-friendly materials lack standardization in the construction industry. - Adherence to existing standards could enhance adoption.	- Standardized testing and certification frameworks are needed for eco-friendly materials to gain industry acceptance.
Optimum mix	- Bamboo biochar can replace up to 20% of cement and 10% of sand. - Fly ash can substitute 30% of cement and 15% of sand. - Lime can replace 25% of cement and 15% of sand.	- Extensive experiments needed to validate optimal replacement percentages. - Studies required to achieve target U-values of 0.4 for walls and 0.33 for roofs by identifying the best material combinations (as per ECBC standards).
Thermal performance	- Demonstrates potential for reducing U-values, enhancing energy efficiency.	- Validation needed for achieving target U-values with different material combinations.

.

Further research: The roadmap for the further research on experimentation for the optimum proportion could be explored as follows: Experiments would be carried out for wall plaster (both internal and external) and IPS flooring for roof top covers. Mixing percentages would vary: 10, 30, 50, 70, 90, or 100. The table mentioned below can be used to find the optimum percentage mix.

Climate: Climate plays a crucial role in conducting experiments. If experiments are performed in extreme climatic conditions, the results can be applied to moderate and milder climates as well. Therefore, the hot and dry regions of India should be the preferred choice for initial experiments, as they represent a challenging scenario for testing.

To determine optimum mix experiment process suggested in

Table 22. Experimental variations in sustainable material percentages to determine optimal mix proportions. Source: Author.

Cement Replacement		Sand Replacement
Lime	Cement	Bamboo Biochar	Fly Ash
10%	100%	10%	100%
30%	90%	30%	90%
50%	70%	50%	70%
70%	50%	70%	50%
90%	30%	90%	30%
100%	10%	100%	10%

.

6. Recommendations

A single sustainable material may not provide a comprehensive solution; however, the combination of bamboo biochar, fly ash, lime, and other sustainable materials, as highlighted in the literature review, can be used to create an efficient building insulation system while addressing environmental challenges such as CO₂ reduction, VOC absorption, humidity control, and waste management. Future studies should focus on experimenting with bamboo biochar, fly ash, and lime as plaster for walls and roof coverings. These plaster and roof samples must be tested to ensure compliance with ECBC standards, specifically achieving the recommended U-value of 0.4 for wall assemblies and 0.33 for roof assemblies. This approach has potential for the development of a robust building envelope system that meets sustainability criteria, reduces costs, and is feasible for widespread implementation across India.