Characterization and Energy Performance of Rice Husk Fiber Insulation Applied by the Blowing Technique in an Industrialized Modular Housing System

Abstract

The construction sector plays a key role in climate change due to its high energy consumption and greenhouse gas emissions. Developing environmentally friendly building materials with low environmental impact is essential to improving energy efficiency. Insulation derived from agricultural waste is particularly promising due to its low ecological footprint, responsible resources use, and potential for integration into various construction systems. This study evaluates the potential of rice husk fiber as a thermal insulating material applied through the blowing technique in the Skylark 250 modular system. Rice husk fiber was morphologically and thermally characterized using scanning electron microscopy (SEM), while its thermal behavior was analyzed by thermogravimetric analysis (TGA) alongside a fire behavior assessment. Additionally, energy simulations were conducted to compare the thermal performance of rice husk fiber with other insulating materials when integrated into a building’s thermal envelope. The results showed an average thermal conductivity of 0.040 W/mK, a U-value of 0.17 W/m²K, and a heating demand of 9.56 kWh/m²-year when applied to the modular system. The material also exhibited good fire resistance, with a smoldering velocity of 3.40 mm/min. These findings highlight rice husk fiber’s potential as a sustainable insulation material for modular construction, contributing to energy efficiency and climate change mitigation.

1. Introduction

The construction sector has a significant environmental impact, primarily due to its extensive consumption of non-renewable resources. This consumption leads to land degradation, the generation of substantial solid waste, and increased levels of dust, gas emissions, and noise pollution. Notably, the sector is responsible for approximately 50% of global carbon dioxide emissions, 50% of solid waste production, and nearly 50% of total energy and natural resource consumption [1]. To promote sustainable development in the built environment, the use of locally sourced, renewable, and recycled materials has become increasingly important [2]. In particular, the adoption of natural materials has gained traction in recent years, driven by a growing emphasis on energy conservation, sustainable architecture, and the challenges associated with the disposal of conventional insulation materials such expanded polystyrene (EPS) [3]. Among the alternative resources, agricultural residues have emerged as a promising, renewable, and cost-effective option for sustainable construction practices [4]. These materials offer thermal insulation potential while supporting circular economy principles. Derived from recycled industrial and agricultural waste, natural fibers not only enhance thermal comfort but also help to reduce environmental impact, aligning with the fundamental principles of sustainable development and circular economy practices [5]. The use of bio-based materials contributes to material recycling and contributes positively to their environmental impact [6]. The growing shift toward these materials is driven by their potential to reduce the overall environmental footprint of construction processes.

Thermal insulation remains one of the most effective strategies for achieving energy efficiency in residential, commercial, and industrial buildings. Advanced insulation material reduces the demand for heating and cooling, improves the thermal resistance of building envelopes, and lowers both energy consumption and greenhouse gas emissions [7]. Eco-friendly insulation materials are particularly advantageous, as they are typically non-toxic, do not emit harmful chemicals or volatile organic compounds (VOCs), and tend to have a lower carbon footprint compared to traditional materials. Moreover, their production is often less energy-intensive, they generate fewer emissions, and many of them are recyclable or biodegradable, which enhances their reuse potential and reduces waste [5,8].

Rice is among the most widely cultivated crops globally, feeding nearly half of the world’s population [9]. Rice husk represents approximately 20% of global rice production, generating nearly 200 million tons of this agricultural waste annually. Additionally, the slow biodegradability of this residue poses significant environmental concerns, emphasizing the importance of valorizing this natural fiber [10]. Given these environmental challenges, recent research efforts have focused on developing innovative insulation materials derived from rice husk, with examples including insulation panels produced via the pulping method, exhibiting a thermal conductivity of 0.037 W/mK and 137 Kg/m³ [11], as well as composites combined with various binders to create panels exhibiting favorable thermal and properties in the range of 107–378 kg/m³ and 0.0045–0.108 W/mK [12,13,14,15,16,17,18]. Significant research has also been conducted on utilizing rice husk ash to improve certain concrete properties, where it has been used as a substitute for Portland cement. A hollow insulated block, where rice husk ash and rice husk insulation mixed with aluminous cement served as the binder, has been examined [19]. Rice husk is notably characterized by its high silica content compared to other agricultural residues [10]. The promising results observed in other studies advocate for further exploration of rice husk’s properties and its potential applications in energy-efficient construction [17]. Considering the potential of this agricultural residue, it is essential to investigate its integration into broader contexts, such as industrialized construction systems.

Industrialized construction represents a transformative approach that integrates technological solutions to automate processes, enhance supply chain efficiency, and promote sustainability [20]. Despite challenges related to initial invest costs, the limited integration of digital tools, and the need for specialized expertise, this construction approach has demonstrated strong potential in improving efficiency, product standardization, modularization and mass production. Within this broader framework, there is a growing need for further research into environmentally friendly and economically viable insulation materials [21]. Among these systems, modular construction stands out for its ability to reduce material waste, energy consumption, and environmental impact while also decreasing construction time and cost. Consequently, it is increasingly recognized as a promising strategy for sustainable development in the construction industry [22]. Moreover, modular systems facilitate mass customization by allowing products to be tailored to specific consumer requirements without incurring substantial additional costs or delays [23]. The Skylar 250 system, an open-source construction model and the primary system used in WikiHouse, is a modular, industrialized solution designed for rapid assembly, accessibility, and low energy consumption. Using subtractive manufacturing, this system enables efficient and precise prefabrication, which significantly reduces both construction time and material waste [24]. A notable advantage of this system is its lower environmental impact compared to conventional construction methods. It generates significantly fewer greenhouse gas emissions due to its reduced air conditioning demand, estimated at approximately 2000 kWh/year, compared to about 4200 kWh/year which is required by traditional systems [25].

Previous studies involving modular construction systems have demonstrated that the incorporation of sustainable insulation materials, such as wheat straw, can effectively meet thermal performance standards, achieving a thermal transmittance of 0.45 W/m²K for walls. In addition, energy simulations have shown that optimized modular designs using sustainable insulation can significantly reduce total energy demands, reaching values as low as 55 kWh/m² year [25]. Other research conducted in Temuco, Chile reported heating energy demands ranging from 40 to 82 kWh/m² year in buildings where thermal insulation was improved, regardless of the specific material used. These results represent substantial improvements when compared to buildings without enhanced thermal insulation [26].

These findings emphasize the importance of passive design strategies in reducing energy consumption. Furthermore, the integration of appropriate thermal insulation materials, particularly those based on natural fibers, can enhance energy efficiency and indoor thermal comfort, ultimately benefiting homeowners and contributing to the reduction in environmental impacts associated with construction [25,27].

In this context, rice husk fiber, a renewable agricultural by-product, offers a promising alternative for building insulation. Its natural, biodegradable properties make it suitable for modern construction systems, particularly for use in building components such as walls and roofs. When applied through the blowing technique, rice husk fiber enables uniform distribution, preserves internal porosity, and improves thermal performance. This technique allows for insulation to be injected without removing the internal or external cladding of a building component. It creates internal air pockets whose characteristics vary depending on the type of fiber used, enhancing the system’s overall thermal efficiency [28,29,30]. Moreover, the blowing technique has proven to be a rapid and efficient application method, significantly reducing installation time. This is especially true in modular construction, where minimizing on-site labor and assembly time is essential [25]. In addition to lowering labor costs, this technique often employs materials that are less expensive than traditional insulation options, making it an economically viable choice for sustainable housing projects [31].

The objective of this study is to evaluate the potential of rice husk fiber as a thermal insulation material in industrialized modular construction systems. To achieve this, our research investigates its technical and environmental viability as a sustainable alternative to conventional insulation materials, applying the blowing technique within modular building components. The methodology combines experimental laboratory testing to characterize the key physical and thermal properties of rice husk fiber, such as density and thermal conductivity. Additionally, the fire behavior of the material is assessed through combustion analysis. Finally, energy simulations were conducted using Desing Builder software version 5.5.2.007 to evaluate the thermal and environmental performance of modular buildings insulated with rice husk fiber, with a specific focus on the energy efficiency benefits achieved by the incorporation of this sustainable material.

2. Materials and Methods

This section presents the methods used to evaluate the physical properties of the rice husk agro-waste, including thermal conductivity, density, moisture content, and fire behavior. It details the experimental setup and parameters for determining optimal blowing conditions, such as nozzle aperture, airflow speed, and power. Additionally, it outlines the procedures for measuring energy consumption and performing simulations in the Design Builder software. This comprehensive approach ensures a thorough understanding of the performance of rice husk fiber within modular construction systems.

2.1. Sample Preparation and Material Application

The rice husk was obtained through “Arrocera Mantul”, located in a commune in the Maule region of Chile. This product was obtained after the milling phase in the process to obtain rice; tests were conducted and in the loose material, it was found that there was preliminary conductivity 0.041 W/mK, densities of 130–170 kg/m³, and moisture of around 10.68%.

For the blowing process, The X-FLOC equipment, model M99-DS, was used, which can be seen in Figure 1. This equipment has a nominal power of 3.6 kW, nominal blowing power of 440 m³/h (adjustable 2 × 1.8 (kW)), and a variable opening range from 1 to 10. “For the application of the material by means of the blowing technique, 2 × 4” pine wood specimens were used, whose dimensions were 60 × 40 × 9 cm in length, width, and thickness, respectively.

2.2. Physical Characterization

2.2.1. Thermal Stability

Thermogravimetric analysis (TGA) was performed using a TGA/DSC STA 6000, Perkin Elmer, Waltham, MA, USA, to evaluate the thermal stability of natural rice husk fibers, following a methodology like that of previous studies on lignocellulosic fibers [32,33,34]. The purge gas was nitrogen (N₂) set at a flow rate of 40 mL/min. The mass of the sample was 20.233 mg. The temperature program involved heating from 25 to 120 °C at a rate of 50 °C/min, followed by a 3 min hold at 120 °C. Subsequently, the temperature was increased from 120 to 950 °C at a rate of 100 °C/min, with cooling from 950 to 450 °C at 100 °C/min. At 450 °C, a gas switch from nitrogen to oxygen (O₂) was performed with a flow rate of 40 mL/min. The heating then continued from 450 to 800 °C at 100 °C/min, concluding with a 3 min isothermal hold at 800 °C [32].

2.2.2. Thermal Conductivity

The thermal conductivity of the prototypes was measured using the KD2 Pro instrument manufactured by Decagon Devices Inc., Pullman, WA, USA, which has been utilized in other studies analyzing fiber-based thermal insulation materials for construction [11,30,35]. The device uses an interchangeable sensor that is inserted into the material, providing a thermal conductivity value. The measurement process is based on the transient line heat source method, enabling the quick analysis of homogeneous materials, with measurement intervals of 1 s during a 90 s heating and cooling cycle. The KD2 Pro complies with IEEE 442-1981 [36] specifications and ASTM D5334-00 standards [37], with an accuracy of ±0.001 W/mK. To ensure the stabilization of the sensor, a 10 min gap was maintained between analyses. Variations in the results may have occurred depending on fiber properties such as length, diameter, density, moisture content, and arrangement.

2.2.3. Density and Moisture

The density was calculated manually by correlating the volume of the test specimens, as previously described, with the mass of material insufflated in each of the four repetitions carried out. The mass was measured in kilograms and the volume was recorded in cubic meters, resulting in density values expressed in Kg/m³.

The moisture content was measured using the Boeco moisture analyzer, Hamburg, Germany, model BMA H50, as shown in Figure 2. This device operates with halogen light and functions within a temperature range of 10–40 °C, providing results with an accuracy of 0.001%. The measurement process using this analyzer is illustrated in [28].

2.2.4. Surface Analysis

The morphological analysis of the rice husk and the prototypes was conducted using scanning electron microscopy (SEM) with a Hitachi VPSEM SU 3500 microscope, Tokyo, Japan. The analysis was carried out under the following conditions: magnifications of 40–100–200–500× for the surface view and 30–100–200–500× for the cross-sectional view. The BSE detector was used, with settings of 10 KeV, WD12 mm, and 30 Pa, as specified by the laboratory equipment guidelines [25,33].

2.3. Fire Behavior

To analyze the fire behavior of the samples of rice husk, a fire reaction test and a flameless test to assess smoldering combustion were performed.

2.3.1. Fire Reaction Test

The rice husk fiber was placed in a cylindrical metal container and the fiber was subjected to a portable butane gas torch capable of reaching temperatures up to 1200 °C, with the flame applied directly to the center of the fiber for a duration of three seconds and at a constant distance. After the flame was withdrawn, the ignition time—the interval between flame removal and the appearance of ignition—was measured, along with the subsequent extinguishment time. This process was repeated continuously for two minutes, with all ignition and extinguishment events recorded on video for later analysis. This study aimed to observe the fiber’s behavior before heating, during ignition, and after flame extinction. By analyzing the footage, the average ignition time and total number of ignitions were determined, providing insights into the flammability and combustion characteristics of rice husk fiber under repeat heat exposure. Following this fire behavior evaluation, a drip analysis based on the UNE 23-725-90 standard [38] was performed.

2.3.2. Smoldering Combustion Analysis Test

The experimental setup utilized in this study was based on prior research that examined the upward smoldering process using other combustible materials under a comparable experimental configuration [39].

The experimental setup in Figure 3 consists of an electric panel, a hot plate, a cylindrical reactor, seven thermocouples, and a data logger. The reactor was a perforated cylindrical steel structure with a diameter of 80 mm (D) and a height of 120 mm (L). During testing, temperatures along the central axis of the reactor were monitored using 5 thermocouples positioned at distances of 0.5, 3, 6, 9, and 12 cm from the hot plate, and an additional thermocouple was placed directly on the hot plate. After recording the temperature data, the smoldering velocities and the maximum temperature reached during the combustion process were determined.

Smoldering is a phenomenon of combustion without flame, which is especially crucial in combustible porous materials. During the experiments, the hot plate was heated for one hour until it reached a predetermined temperature of 360 °C, after which it was left to cool down to room temperature for another hour. The process was monitored using an infrared camera, capturing images every 5 min [39,40]. The smoldering velocity was calculated by analyzing the temperature data recorded by the thermocouples, specifically by identifying the time it took for each thermocouple position to reach a temperature of 270 °C. In order to perform the experiment, rice husk, recycled textiles, and cellulose were selected to compare their fire behavior. As shown in Figure 3, these materials exhibit similar thermal insulation properties: cellulose ranges between 0.035 and 0.040 W/mK [41], and recycled textiles are in the range of 0.33 to 0.39 [42], making them suitable reference materials for evaluating rice husk fiber’s insulation performance. Additionally, due to their physical characteristics, these materials are also suitable for application via the blowing technique within modular construction systems.

2.4. Thermal Performance of the Insulating Envelope and Energy Simulation

2.4.1. Thermal Inertia

To evaluate the impact of the rice husk fiber material on thermal inertia, a component of the Skaylark 250 system, specifically the block M version 0.1, was selected (Figure 4). The thermal inertia of the material was analyzed using the Ubakus online tool, which adheres to the procedures outlined in the DIN 4108 [43] and DIN EN ISO 6946 [44] standards. These standards define methods for assessing thermal resistance and energy efficiency in building systems.

Ubakus employs finite element methods to calculate thermal resistance and transmittance, offering a reliable approach for evaluating and optimizing the thermal performance of the construction materials. For thermal capacity calculations, the tool determines the temperature profile within the material for two internal temperatures differing by 1 °C under static conditions (t→∞). From these profiles, the average temperature difference (dT) is calculated for each layer, and the stored heat (dQ) is computed using the formula:

dQ = dT · density · specific heat capacity · thickness

(1)

The key thermal parameters considered included time lag (the delay between external and internal temperature peaks), decrement factor (the ratio of temperature amplitudes), the thermal capacity of the inner layers, and the heat storage capacity. A 24 h simulation with sinusoidal external temperature variations ranging from 15 °C to 35 °C provided temporal temperature profiles, enabling a comprehensive analysis of the material’s thermal damping properties [25,29]. For the thermal inertia assessment, rice husk fiber was analyzed and compared with recycled PUR, as both materials are suitable for application through the blowing technique.

2.4.2. Energy Simulation

DesignBuilder version 5.5.2.007 is a comprehensive building simulation software that utilizes the open-source EnergyPlus version 8.6 engine to simulate indoor microclimates and energy consumption. The software allows users to create detailed building models with ease, offering a user-friendly interface. It enables users to define every aspect of the building’s systems, including schedules, construction materials, lighting, HVAC systems, and window-to-wall ratios, providing a thorough and customizable modeling experience [45].

Scenario Setup

The simulation sequence is as follows: A modular housing unit, which features an area of 14.75 m² (Figure 5) and is located in Temuco, Chile within thermal zone F according to the Chilean building standards [46] and has a Köppen climate classification of Cfb, is built using Skylark 250. The modular housing unit will be modeled to evaluate energy consumption, indoor thermal comfort, and CO₂ emissions under three different scenarios. The first scenario uses rice husk fiber thermal insulation, the second employs a recycled PUR insulation material, and the third considers the absence of any insulation. The modular unit will have a standardized design to ensure comparability between scenarios. The parameters for energy simulation in DesignBuilder are presented in

Table 1. Parameters for energy simulation in DesignBuilder.

Parameters for Energy Simulation in DesignBuilder
Parameters	Tipe	Characteristics
Activity	Residential	0.02 (people/m2)—residential, residential occ schedule
Construction	Project template	Walls, roofs, and floors of Skylar 250 system have model infiltration of 0.7 (ac/h)
Opening	Project template	30% wall to window ratio—double glazing, clear, 6 mm/6 mm
Lighting energy	Project template	5.00 W/m2—100 lux
HVAC		ON, heating COP 0.5 and cooling COP 4.5, fuel electricity from grid

. Simulations will be carried out for the city of Temuco, using weather data derived from the EnergyPlus climate file database provided by the U.S. Department of Energy. Parameters such as outdoor Dry-Bulb Temperature, wind speed, and relative humidity will be enforced via DesignBuilder’s pre-installed weather file. Additionally, the simulations will allow for the assessment of the thermal envelope’s efficiency under each scenario, providing a comparative analysis of energy performance and carbon footprint.

3. Results and Discussion

3.1. Material Application Results

The rice husk was injected into the test panel by means of the blowing technique (Figure 6A) using an X-Floc M99-DS blowing machine X-Floc Dämmtechnik-Maschinen GmbH, Renningen, Germany. The correct application was achieved using the following configuration: 80% blowing power, corresponding to 352 m³/h, with an opening at the second level and the hopper operating at 80% capacity. It was necessary to maintain a consistent feed of the rice husk fiber to prevent blockages in the machine. Experimental testing confirmed that, under these conditions, the material could be efficiently blown without causing blockages in the hose or nozzle. This configuration allowed for uniform fiber distribution and density to be reliably achieved in each required application, contributing to consistently low thermal conductivity. Additionally, this method ensured that the thermal insulation material filled the entire empty space, as shown in Figure 6B. As demonstrated in the studies [28,30], this technique ensures a uniform distribution of fibers without the need for chemical additives, thereby reducing the material’s environmental impact. Additionally, the blowing method offers flexibility in application, making it suitable for both new construction and renovation work [30].

3.2. Physical Characterization

3.2.1. Thermal Stability Analysis

The proximal analysis carried out on rice husk in its natural state is detailed in Figure 7. According to the TGA, the fiber began to lose weight at 120 °C. This was observed, corresponding to 7.88% being mainly related to moisture, and subsequently, a more significant mass loss of 55.13% was observed between 120 °C and 759 °C, associated with the volatile compounds hemicellulose and cellulose. Once the environmental change to oxygen took place, carbon losses of 17.07% were observed between 758 and 798°, corresponding to lignin. Finally, the percentage of ashes was 19.92%, and this was displayed at 800 °C. The rice husk fiber presented stability until 120 °C, indicating that the temperature used for applying or processing should have been lower than this value to avoid thermal degradation. Similar results are shown for other natural fibers [30,32].

The differential curve TGA(DTGA) shows in its shoulders the three stages mentioned. The first one corresponds to moisture with a peak at 85.32 °C, the second one with a peak at 371 °C corresponds to cellulose, and the last one with a peak at 553 °C corresponds to lignin. As can be seen in Figure 7, the rice husk stays thermally stable up to 120 °C; above this temperature, thermal degradation starts, but the rice husk shows a temperature peak for maximum degradation at 371, which is similar to that reported in other studies of rice husk TGA [12,13]. In addition, based on this analysis, the rice husk fiber was found to remain stable up to 120 °C, indicating that its processing temperature should remain below this threshold to avoid thermal degradation. In this regard, it was noted that the blowing application process did not cause a rise in the material’s temperature, ensuring that no degradation occurred during its application.

3.2.2. Thermal Conductivity Results

The thermal conductivity is measured [M], as shown in

Table 2. Results of thermal conductivity measurements.

Thermal Conductivity [W/mK]					AVG	SD
Measurement (M)	M1	M2	M3	M4
Sample 1	0.040	0.035	0.037	0.041	0.038	0.0028
Sample 2	0.041	0.041	0.042	0.040	0.041	0.0008
Sample 3	0.040	0.041	0.040	0.040	0.040	0.0005
Sample 4	0.041	0.040	0.041	0.038	0.040	0.0014
					0.040	0.0014

, with an overall average value of 0.040 ± 0.001 W/mK. Figure 8 shows the thermal conductivity measurements of the insufflated samples; these results indicate that the prototypes exhibit favorable thermal performance, suggesting that the material is suitable for building insulation and supporting the potential for developing new insulation materials based on agricultural waste. This result is comparable with other rice husk materials, with values of 0.37 to 0.042 W/mK [11]; and with other lignocellulosic materials, like wheat straw, hemp, eucalyptus bark fiber, algae, and mushroom insulation material, which are in the range of 0.32 to 0.40 W/mK [35,47,48,49]. The thermal conductivity values obtained for rice husk are comparable to those conventional insulation materials currently available on the market, like mineral wool (0.033–0.038 W/m·K) and expanded polystyrene (0.035–0.040 W/mK) [50,51], underscoring its potential as a competitive alternative for thermal insulation.

3.2.3. Density and Moisture Content Results

The density values averaged 130 kg/m³, with the lowest measurements of 121 kg/m³ corresponding to sample 1, with 121 kg/m³ shown in Figure 9. The moisture content of rice husk was found to have an average of 10.23 ± 0.1%, which is consistent with the behavior observed in other natural fiber materials. This moisture level is considered desirable, as it supports the preservation of the thermal properties of insulation materials. Excessive moisture can negatively impact thermal conductivity, increase condensation, and reduce biological resistance. Conversely, maintaining low-to-moderate moisture levels enhances the material’s biological resistance, making it well suited for use in building applications.

Materials with open porosity, such as rice husk, facilitate moisture movement driven by vapor pressure gradients, gravity, and capillary action. However, in well-designed systems, vapor diffusion is the dominant mechanism, preventing excessive moisture levels and contributing to the material’s effectiveness as a thermal insulator in energy-efficient buildings [52].

Furthermore, the results suggest that rice husk insulation would not promote the proliferation of pathogens if installed in homes in regions such as Chile. Previous studies, such as those conducted by Soto M. et al. (2023), have shown that similar natural fibers, like cellulose combined with wheat straw, do not support fungal growth or region-specific pathogens within comparable moisture ranges [28].

Figure 9 presents the measurement results, showing that the samples exhibit a thermal conductivity below 0.041, demonstrating their potential as a thermal insulation material. This value is comparable to conventional insulating materials, reinforcing their suitability for energy-efficient construction applications [53]. Unlike traditional insulation materials, where a higher density reduces thermal conductivity, lignocellulosic fibers show the opposite trend, with conductivity increasing as density rises [28]. This highlights the importance of optimizing density to maintain insulation efficiency properties.

3.2.4. Surface Morphology Analysis

Figure 10 presents three microscopic images of the rice husk, accentuating its suitability as a thermal insulation material. Image (A) shows the surface of the rice husk fiber, revealing a well-defined porous structure. This structure is further detailed in image (B), where the voids on the fiber surface are clearly visible, contributing to its ability to enhance thermal insulation performance [28,54]. Finally, the longitudinal view in image (C) illustrates the entire surface of the fiber, characterized by well-distributed pores, which play a key role in achieving optimal thermal insulation levels. These microscopic observations confirm that rice husk possesses optimal properties for use in insulation material applications.

3.3. Fire Behavior

3.3.1. Fire Reaction Test Results

Figure 11 shows the flame extinction time, for a total of 23 applications, reaching a total of 90.34 s., which nearly doubles the number of tests reported in previous research on wood dust [29]. The average of all the extinguishing times is 3.93, with the first ignition time being 3.93 s. A significant observation was made at the 9 s mark, when a carbonized protective layer (Figure 12) began to form on the surface of the sample. This layer acted as a thermal barrier, preventing further combustion and allowing the material to endure 23 flame applications without complete degradation. This behavior aligned with previous TGA findings, where the initial moisture release was followed by the decomposition of cellulose and hemicellulose, leading to the formation of a char layer that enhanced fire resistance.

Furthermore, a previous study [11] indicates that this self-protective carbonization effect in rice husk is primarily due to its high silica content. Silica contributes to the material’s resistance to combustion by forming a stable insulating layer that limits heat penetration and delays thermal degradation.

These results demonstrate the effectiveness of the material in delaying flame propagation compared to traditional passive fire protection strategies due to its self-protective charring mechanism. This natural flame-retardant behavior suggests its potential application in fire-resistant materials. The ability to withstand nearly twice as many flame applications as wood dust [29] indicates its suitability for use in thermal insulation and fire-resistant construction material.

3.3.2. Smoldering Combustion Performance

For this smoldering test, the reactor was filled with samples of (A) rice husk fiber, (B) recycled textile, and (C) cellulose, which were evaluated. During the experiments, the evolution of smoldering was visualized with an infrared camera. Figure 13 shows the results: The images are presented in a sequence of seven pictures every 10 min from 50 min to 110 min. The red color corresponds to temperatures around 240–261 °C. In samples (A) and (B), the evident smoldering begins at 65 min, while with cellulose, it starts at 55min.

Figure 13 shows that after conducting the smoldering test on three samples, it was observed that rice husk fiber resisted smoldering for 60 min, reaching a temperature of 343.2 °C at the first thermocouple at 0.5 cm before the process began. Similarly, the recycled textile sample also withstood smoldering for 60 min, but at a lower temperature of 270.3 °C. In contrast, the cellulose sample initiated smoldering earlier, at 55 min, with a recorded temperature of 308.9 °C. These results suggest that the variations in the thermal behavior of the samples can be attributed to their composition, density, and structural properties, and the performance of rice husk fiber highlights its potential as a more fire-resistant insulation material.

Figure 14 shows the temperatures recorded by the thermocouples on samples tested to a pre-set temperature of 360 °C; the dashed lines correspond to the temperatures measured at the hotplate surface. From these data, the smoldering velocity can be evaluated by determining the times at which each thermocouple position reaches a specific temperature, which here is chosen as 270 °C. (A) shows better behavior in the smoldering test than the (B) both in the temperature and velocity of smoldering, reaching maximum temperatures of 617 °C, while the recycled textile reaches the maximum at 652 °C, and the cellulose at 708 °C.

Figure 15 shows that the smoldering velocity is highest in the recycled textile at 6.30 mm/min, while the rice husk records a lower velocity of 3.40 mm/min, a result consistent with a previous study on rice panels produced using the pulping method [11]. The recycled textile exhibits a similar behavior to the corn pith [40], while cellulose demonstrates the slowest smoldering rate at 1.91 mm/min. This slower combustion is attributed to the presence of boron salts as a flame retardant, which are commonly added to commercially available insulation materials.

3.4. Thermal Performance of the Insulating Envelope and Energy Simulation

3.4.1. Thermal Inertia Results

Table 3. Results of thermal performance.

Property	Unit	Wall M Rice Husk	Wall M Recycled PUR
U-value	W/m2K	0.17	0.201
Time lag	h	14.50	8.50
Decrement factor	_	40.30	11
Heat storage capacity (whole component)	KJ/m2K	98	69
Thermal capacity of inner layers	KJ/m2K	49	34

shows the findings from the thermal inertia analysis, indicating that the rice husk fiber used in the test samples possesses an energy storage of 98 (KJ/m²K). This means that, when incorporated into a building’s component, this building will exhibit over 30% of the energy storage capacity compared to the use of recycled PUR, and similar behavior was observed in a previous study with sawdust material [29].

The thermal transmittance results indicate that rice husk fiber with a transmittance value of 0.17 W/m²K in the wall component outperforms recycled PUR, which has a thermal transmittance of 0.201 W/m²K. Both materials exhibit transmittance values well below the maximum limit of 0.45 W/m²K established by the atmospheric decontamination plan (PDA) for walls in Temuco [55]. The superior performance of rice husk fiber suggests a greater ability to reduce heat transfer, enhancing the thermal efficiency of the building envelope. This lower transmittance contributes to a more stable and comfortable environment, minimizing temperature fluctuations regardless of the external weather conditions.

Figure 16 shows the surface temperature behavior when the insulation material is changed from recycled PUR to rice husk fiber. Notably, replacing the insulation significantly increases the temperature wave delay from 8.5 h to 14.5 h, enhancing thermal storage capacity, reducing energy consumption, and improving indoor thermal comfort. This demonstrates the potential to enhance the energy efficiency of buildings [56].

Figure 17 shows the temperature behavior when recycled PUR insulation materials are replaced with rice husk fiber. A notable reduction in temperature fluctuation amplitude occurs, demonstrating its effectiveness in mitigating the impact of external temperature peaks. Such thermal performance improvements are typically not observed with conventional insulation materials like recycled polyurethane, which tend to closely follow external temperature fluctuations [29].

3.4.2. Energy Simulation Results

To evaluate the energy efficiency of different insulation materials, a simulation was conducted considering three different scenarios. The first scenario incorporated rice husk fiber insulation (A), the second scenario utilized recycled polyurethane insulation, and the third scenario represented the non-insulated case, serving as a baseline to assess the impact of insulation on energy consumption (Figure 18).

Temperature Homogeneity

The results presented in Figure 19 indicate that rice husk fiber maintains indoor temperatures within the comfort range throughout the year, demonstrating its effectiveness in stabilizing indoor thermal conditions and its provision of superior thermal regulation compared to the other scenarios. Recycled PUR insulation performs better than the non-insulated scenario by reducing temperature fluctuations; however, it is still below the lower comfort zone [55] during winter, indicating that while it offers some insulation, it is not as effective as a rice husk fiber. The non-insulated scenario exhibits the highest temperature variations, closely following outdoor temperatures, which indicates the importance of using insulation materials to enhance indoor thermal stability and energy efficiency. Additionally, rice husk fiber insulation shows the best performance in extreme temperature conditions, preventing excessive heat loss in winter and minimizing overheating in summer, making it a viable and sustainable option for energy-efficient construction climates like Temuco. The rice husk within the modular system achieves thermal comfort levels comparable to dwellings that comply with the technical buildings code standard [26]. Overall, these findings suggest that the rice husk fiber is a highly effective natural insulation material, confirming its potential application in sustainable building systems for enhanced thermal efficiency and occupant comfort.

Energy Consumption and CO₂ Emissions

The results presented in Figure 20 illustrate the energy consumption in kWh required for heating and cooling the modular unit under different insulation scenarios. The data clearly indicate that the rice husk fiber exhibits the lowest energy consumption, outperforming both recycled PUR and the non-insulated case. The non-insulated scenario shows significantly higher energy demand, particularly during the winter months, where heating consumption peaks at well above 643 kWh. This highlights the inefficiency of an uninsulated module at maintaining indoor thermal stability, requiring excessive energy to compensate for heat loss. The rice husk fiber demonstrates the highest energy efficiency. In

Table 4. Annual consumption results for heating, cooling, and associated CO₂ emissions.

Scenario	Heating Demand (kWh-year)	Cooling Demand (kWh-year)	Heating Demand (kWh/m2-year)	Cooling Demand (kWh/m2-year)	Total Demand (kWh/m2-year)	CO2 Emissions (kg eq-year)
Rice husk insulation	140.96	55.35	9.56	3.75	13.31	1956.81
Recycled PUR insulation	217.00	241.00	14.71	16.34	31.05	2116.04
Without Insulation	3909.14	350.06	265.03	23.73	288.76	4418.92

, the annual consumption and the rice husk scenario that maintains lower heating and cooling demand compared to the other two scenarios is shown, with 9.56 kWh/m²-year of heating demand, which is close to the 11.70 kWh/m²-year reported by [25] in a similar construction using wheat straw insulation. The heating energy demand observed using rice husk fibers is significantly lower compared to other thermally improved dwellings in Temuco, with a range between 40 and 80 kWh/m²-year [26]. This represents an approximate reduction of 81%, underscoring rice husk fiber’s substantial potential for enhancing energy efficiency in buildings.

This confirms its superior thermal performance, reducing the need for mechanical heating in winter and cooling in summer. The results suggest that rice husk fiber not only improves indoor comfort but also enhances energy efficiency, making it a more sustainable and effective alternative for insulation in modular construction systems.

Additionally, it achieves the lowest CO₂ emissions, associated with heating and cooling consumption, of 1956.81 kg per year, representing a decrease of about 60% compared to the scenario without insulation, as shown in Table 4, significantly reducing the building’s carbon footprint.

4. Conclusions

This study demonstrates that rice husk fiber is a viable and sustainable alternative to conventional thermal insulation materials, particularly when integrated into modular construction systems. Applied through the blowing technique, rice husk not only enhances the energy efficiency of buildings but also supports environmental sustainability by promoting the reuse of agricultural waste.

From a thermal performance perspective, the results demonstrate that rice husk fiber possesses favorable insulating properties. The measured thermal conductivity is 0.040 W/mK, a value comparable to conventional insulation material. Thermal simulations further indicate that its integration into modular construction, specifically through the blowing technique, can significantly improve building energy efficiency. In this context, the use of rice husk fiber achieves a U-Value of 0.17 W/m²K and reduces heating and cooling energy demand by more than 50% down to 9.56 kWh/m²-year compared to an uninsulated module. These findings highlight its potential for contributing to energy-efficient construction. Future studies should address the long-term durability and potential degradation of the material under variable and moisture conditions.

In terms of fire safety, the rice husk fiber insulation exhibits superior performance compared to the other evaluated materials. It demonstrates a smoldering velocity of 3.40 mm/min, lower than that of the other tested samples. This performance is attributed to its relatively high silica content, which promotes the formation of a protective char layer that enhances fire resistance. However, to broaden its applicability in more demanding conditions, further research on eco-friendly fire-retardant treatments is recommended.

From an environmental perspective, rice husk fiber aligns well with the principles of sustainable construction. Its use promotes the reuse of agricultural by-products and avoids the need for chemical additives. When applied through the blowing technique, it allows for efficient and uniform installation, supporting ease of implementation in sustainable building projects. In this study, the use of rice husk fiber results in a 55.7% reduction in CO₂ emissions compared to the uninsulated baseline module. Nonetheless, future studies should evaluate the potential environmental and occupational impacts associated with particulate emissions during the blowing process.

To maximize the potential of rice husk fiber insulation, it is necessary to optimize the blowing application method to ensure long-term stability. In addition, evaluating its performance under diverse climatic conditions and exploring potential hybrid formulations with other bio-based materials could further improve its thermal and fire-resistance properties.

In summary, rice husk fiber insulation stands out a promising bio-based solution for promoting energy efficiency and sustainability in the construction sector.