Addressing the Difficulties and Opportunities to Bridge the Integration Gaps of Bio-Based Insulation Materials in the European Construction Sector: A Systematic Literature Review

Abstract

Bio-based insulation materials (BbIMs) represent a potential alternative to conventional insulations, with their characteristics that favor a negative-carbon built environment. However, their use may face challenges that could prevent them from being used on a large scale in certain countries. The current study aims to provide focused insights into the practical difficulties and market opportunities for the application of BbIMs in Europe through a systematic literature review (SLR). The Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines were used as the basis for the conduct and reporting of this review. A keyword search was performed in Web of Science, Scopus, and ScienceDirect databases to select peer-reviewed English-language articles. HubMeta web tool was used to organize the selection process. The quantitative visualization of the literature was made by the Bibliometrix R package V4.1.4. Data were manually extracted and clustered in an Excel sheet. The review included 28 studies that have revealed interrelated insights. Difficulties range from regulatory and policy limitations and variability in performance, such as microbial growth and inconsistency in the behavior of materials under different conditions, to cost barriers. However, there are promising opportunities, including policy incentives and material performance benefits such as improved energy efficiency and indoor air quality. This research contributes to the literature by providing focused insights into the practical difficulties and market opportunities for the application of BbIMs in Europe. Research gaps and future perspectives point to the need for more field validation experiments, exploration of alternative production processes, and expanding life cycle assessment scopes to optimize their integration and performance. Stakeholders’ perceptions were made with a small sample in certain countries. Stakeholder perceptions were conducted with a small sample in some countries, so insights from stakeholders are needed to confirm or correct current findings.

1. Introduction

Construction activities contribute significantly to various environmental impacts, due to the effects of increased energy consumption and the embodied energy of materials. The operational energy share varies between 70% and 90%, whilst embodied energy accounts for 10 to 30% of a building’s whole life cycle [1]. The share of embodied energy may increase in energy-efficient buildings when operational energy is significantly reduced because of employing more effective technology and architectural solutions [2]. With the overall number of energy-dependent homes expected to increase by 115% and the global population by 41% by the year 2050 [3,4], it is imperative to optimize the performance of existing and new buildings while conserving natural resources [5].

Emissions from building materials are not only related to their impacts over the life cycle of a building but also affect human health and well-being [6]. The selection of construction materials is critical in preventing construction waste, enhancing overall sustainability [7], and assuring the energy efficiency of a building [8], alongside operational energy. Thermal insulation in particular plays an important role in balancing the goals of energy efficiency and environmental impact reduction in new and old buildings. In most cases, the choice of thermal insulation materials in the building envelope is based on the level of insulation, the surrounding climatic conditions [9], and the cost [10]. However, the selection criteria must consider all the technical, social, environmental, and economic aspects [10]. Yet, the environmental dimension is often neglected, even though insulation materials make an important contribution to global warming potential in general [11], and therefore require more attention. This is especially problematic for conventional insulation materials such as glass wool, stone wool, expanded polystyrene (EPS), extruded polystyrene (XPS), and rock wool, among others that are widely used. In this vein, it is essential to identify alternative low-carbon insulation materials to minimize their impact.

1.1. Background

In alignment with the recent adoption in March 2024 [12] of the negotiated text of the revised Energy Performance of Buildings Directive (EPBD) [13] to promote low-carbon materials and the adoption of renewable energies, the use of bio-based building materials has been recognized as a promising alternative [14]. As research and experiments progressed, the production of BbIMs began to increase gradually as insulation properties improved. Some bio-based insulation products are available on the market, including hemp, flax, wood fiber, sheep’s wool, and straw bales, as they are ideal for regeneration, reducing embodied carbon, promoting circularity [13,15,16,17], and improving human health and well-being. However, their use may face difficulties that prevent their widespread adoption in some countries, where they face strong competition from traditional insulation materials. According to the current and projected European thermal insulation market, traditional insulation materials continue to dominate according to the latest statistics for 2022 [18] (Figure 1). Glass wool dominates the European market with 33%, followed by stone wool with 23%. Meanwhile, renewable insulations including BbIMs currently have a small share of this market despite their promising hygrothermal performance and lower environmental impacts in comparison with traditional insulations [19] (Figure 2). It is, therefore, necessary to discuss the extent of the difficulties that BbIMs face in practice and to explore the drivers for scaling up their use.

The literature on various bio-based building materials or specific materials-focused studies contains many reviews on material performance and environmental impacts. In fact, some have addressed certain integration difficulties and opportunities of these materials. In this regard, Le et al. [20] reviewed circular bio-based building materials, in which valuable insights about sustainability assessment methods and research gaps were provided. Mainly, it indicates that there is a significant lack of studies regarding life cycle cost (LCC) and social life cycle assessment (S-LCA).

On the other hand, few reviews investigated BbIMs [19,21,22,23,24] with different parameters and scopes. For example, Cosentino et al. [19] reviewed physical and mechanical properties to consider, and the embodied environmental impacts of BbIMs extracted from existing insulation materials databases and literature. This review also discussed challenges such as thicker wall requirements, heterogeneity, and higher water absorption, alongside opportunities including reduced carbon emissions and the benefits of sourcing materials locally. Similarly, Raja et al. [21] discussed the importance of using BbIMs for improving energy efficiency, and global warming potential reduction in the construction industry. The study urged the need for policy support for the commercialization and widespread adoption of these insulation materials. Centura et al. [22] discussed the use of agro-industrial waste to produce various insulation materials for Euro-Mediterranean countries. In this vein, the authors presented classifications of relevant residues, statistics on global production, various chemical compositions, and physical and mechanical properties of BbIMs. In particular, the study identifies the adaptability difficulties of BbIMs to local building requirements and climatic conditions of regional mild climate conditions, in which buildings typically have little or no insulation. Additionally, since the use of local building materials reduces the effects of their production and transport, the presence or absence of BbIMs in an area depends primarily on the availability of the raw biomaterials locally. Schritt et al. [23], instead, compared the thermal properties of bio-composite and mycelium-based boards and mats based on various parameters such as density, thermal conductivity, specific heat capacity, water vapor resistance, water absorption, fire performance, and mechanical properties. Furthermore, Zhao et al. [24] focused on the environmental impacts of traditional insulators such as expanded polystyrene (EPS), extruded polystyrene (XPS), polyurethane (PU), and phenolic foam on plastic pollution, and hence proposed natural fiber insulators as alternatives. Technical and performance challenges such as mechanical properties, thickness requirements, flammability, high humidity, and water absorption were addressed. In addition, disadvantages such as low emissions, biodegradability, and reusability have also been covered.

1.2. Rational and Objectives of the Study

Although reviews of BbIMs exist, they cover a wide range of different BbIMs, including but not limited to their difficulties, opportunities, and scientific research gaps. The various findings are reported without sufficiently discussing the extent to which their challenges and opportunities are reflected. In this regard, there is a gap in the literature on how these materials meet European country-specific building regulations and standards. Building standards differ in each country, which can be a barrier or an advantage depending on the characteristics of the BbIMs to be used and the need to adapt them to the built environment [21].

A study that analyzes these findings along with a focus on the practical difficulties and market opportunities for the application of BbIMs in Europe is still needed. Thus, the current study aims to provide focused insights into these aspects through a SLR. Firstly, by explicitly identifying and emphasizing the difficulties limiting the widespread use of BbIMs, considering regulations, and technical performance, as well as exploring the factors driving the scale-up of their use. Second, covering research gaps is essential; therefore, the findings of this paper can be used as a basis for future research.

Thus, the research questions were defined as:

RQ1: 

What are the integration difficulties and opportunities of BbIMs in the European construction industry?

RQ2: 

What are the existing research limitations or knowledge gaps for further research in the academic field in the European context?

The structure of this paper after the introduction is as follows: Materials and methods are presented in Section 2. The results can be found in Section 3, which includes a description of the selected studies, integration difficulties, and integration opportunities of BbIMs in the European construction sector. Section 4 discusses the results and highlights existing research limitations, gaps, and future perceptions.

2. Materials and Methods

To assure replicability, transparency, and scientific rigor, this study presents a SLR following the PRISMA approach [25] (see Supplementary File S2). The HubMeta web tool, a free web-based tool for conducting SLRs and meta-analysis, was used to organize the selection process [26]. The quantitative visualization of the literature was made by the Bibliometrix R packages V4.1.4 [27], which is a comprehensive tool for data analysis and visualization.

2.1. Search Strategy

The first step was to conduct a keyword search related to BbIMs, in the Web of Science, Scopus, and ScienceDirect databases. The results were limited to only articles written in English.

In the Web of Science (WOS) database, within the “topic” field, the search query included was TS = (“bio-based insulation” OR “Plant fibers insulation” OR “crop by-products insulation” OR “crop-wastes insulation” OR “agro by-products insulation” OR “agro-wastes insulation” OR “biomass-based insulation” OR “animal fibers insulation” OR “food wastes insulation”), which resulted in 49 articles.

In the Scopus database, within the “Article title, Abstract, Keywords” field, the search query included the following: TITLE-ABS-KEY (“bio-based insulation” OR “Plant fibs insulation” OR “crop by-products insulation” OR “crop-wastes insulation” OR “agro by-products insulation” OR “agro-wastes insulation” OR “biomass-based insulation” OR “animal fibers insulation” OR “food wastes insulation”) AND (LIMIT-TO (DOCTYPE, “ar”)) AND (LIMIT-TO (LANGUAGE, “English”))), which resulted in 41 articles.

An additional search in the ScienceDirect database was performed to expand the sample. The search query included the following within the “Title, abstract or author-specified keywords” field: “bio-based insulation” OR “Plant fibers insulation” OR “crop by-products insulation” OR “crop-wastes insulation” OR “agro by-products insulation” OR “agro-wastes insulation” OR “biomass-based insulation” OR “animal fibers insulation” OR “food wastes insulation”, which resulted in 39 articles. Consequently, the total preliminary records identified were 128 papers.

2.2. Data Collection and Selection Process

The preliminary records identified from the databases (WOS, Scopus, and ScienceDirect) were exported to the HubMeta tool. After deduplication, 69 records were removed. The remaining records (n = 59) were processed for title-based screening and abstract reading, which excluded 20 based on the following two criteria: (1) publications that do not directly address BbIMs (n = 15), and (2) publications that studied BbIMs outside the European context (n = 05). Thereafter, after full paper screening of the remaining records sought for retrieval (n = 39), 19 articles were excluded. This exclusion was based on the following criteria: (1) publications that do not directly address BbIMs (n = 00), (2) publications that are not within the European context (n = 05), and (3) publications that do not discuss difficulties, opportunities, insights, solutions, or research limitations about BbIMs (n = 14). The screening in all the steps was conducted manually. The preliminary studies included based on this search resulted in 20 publications. Thus, the snowball backward [28] method seemed necessary to find other pertinent studies. After verifying the previously defined eligibility requirements, 08 publications were included. Finally, 28 papers were reviewed for systematic data extraction and analysis.

2.3. Data Items and Synthesis Methods

An overview of the selected studies includes a quantitative visualization of the literature, which was made using the Bibliometrix tool. It provides insights such as the yearly production of scientific papers, and a word cloud to understand interconnected concepts.

Key aspects were manually extracted and synthesized from the studies selected according to the outcome aspects, in response to the research questions (RQ1 and RQ2), and categorized in an Excel sheet (Version 2408) as follows:

(1)

Integration difficulties: refer to the difficulties related to policy and regulations at the European and national levels that may limit the use of such materials, as well as materials performance integration difficulties, which include the performance of BbIMs, their impacts on the environment, and cost barriers.

(2)

Integration opportunities: indicate opportunities for policies and regulations that motivate and emphasize the use of BbIMs, as well as the benefits of using certain insulations, and solutions to improve their production and performance.

(3)

Existing research limitations, gaps, and future perceptions: highlight areas of scientific studies uncertainties, knowledge gaps, and suggestions relating to BbIMs. The flowchart of conducting this SLR is presented in Figure 3.

3. Results

3.1. Description of the Selected Studies

The SLR identified 28 papers published in the WOS, Scopus, and ScienceDirect databases from 13 different journals. The review also shows the advancement and growing interest in BbIMs based on the increasing annual scientific production over the last period from 2015 to 2023 (see Figure 4). The information extracted from each study included in the final sample including all the assessed insulations and the measured variables are listed in Table S1 in the Supplementary Materials File S1.

Figure 5 shows the word cloud based on author keywords, which effectively captures the potential directions for further investigation and main topics. It presents the top 50 most frequently occurring author keywords. “Thermal conductivity”, “bio-based material”, “bio-based insulation”, and “thermal insulation”, shown in larger font size, were referred to extensively among the 50 authors’ keywords, which reflects the high relevance of the selected studies to the scope of this study. Other terms, such as “circular construction,” and “circular design “, are presented in a smaller font size, indicating their low occurrence in the selected studies. These less frequent terms point to potential future research directions, particularly in exploring the circularity aspects of BbIMs.

3.2. Integration Difficulties of BbIMs in the European Construction Sector

The findings refer to the difficulties related to policy and regulations at the European and national levels that may limit the use of such materials, as well as materials performance integration difficulties in the European construction sector, environmental impacts, and cost barriers. Figure 6 shows the Treemap (created using Excel version 2408) of the key integration difficulties identified. More details are presented in the Supplementary Materials File S1, Table S1.

3.2.1. Regulations and Policy Difficulties

Considering the building standards in some countries along with the lack of government regulations and incentives for BbIMs [29], the type of insulation material is not as important, i.e., the most thermally efficient material is prioritized, and its environmental impacts are generally not considered in construction projects [30]. Some BbIMs, if they are not made from waste biogenic materials, may have negative effects on eutrophication and land usage due to possible competition with food production [30].

Additionally, one of the most important criteria when using building materials is health and safety. In terms of fire retardants, the most environmentally friendly alternative is boric acid due to its effectiveness as a biocide and fire retardant, according to ref. [31]. However, its use is restricted by European regulations and the Globally Harmonized System of Classification and Labelling of Chemicals (GHS). As it is listed as a Category 2 reproductive toxicant, the regulations restrict its concentration to less than 5.5% (w/w) of the final product [31], which calls for looking for other alternatives. Nevertheless, it can also be temporary difficulties or related to some specific materials, since the possibility of improving these concerns is higher with continuous research.

3.2.2. Materials Performance Difficulties

The integration of BbIMs poses several challenges in terms of performance, assembly design [32], and potential microbial and mold growth [33,34]. Thus, comprehensive evaluation and understanding of these challenges are necessary for the successful integration of BbIMs in various applications. The heterogeneous nature of these materials [29] requires careful consideration of their efficiency and quality during the design process. Additionally, there is a risk of microbial and mold growth, especially in high humidity conditions, which has been a focus of research on several BbIMs commercial and newly developed ones [33,34]. The suitability of these materials for multi-story buildings is also a concern, as there is limited understanding of their moisture behavior and durability for this specific type of construction [35]. The generalization of theories about water absorption in different materials leads to varying experimental results [29]. Furthermore, inconsistencies in life cycle assessment methodologies, such as the exclusion of carbon sequestration and land transformation effects [36], can impact the perceived environmental performance of construction projects. In addition, maintaining clean indoor air is critical for near-zero energy buildings, but the use of BbIMs may make it difficult to meet non-assessable VOC criteria [37]. Although biobased materials have demonstrated functional performance and operating cost savings, there has not been a significant increase in their use in mainstream construction [36]. This poses a market penetration challenge compared to conventional materials.

Certain Materials Performance Difficulties

In comparative studies of BbIMs, several challenges related to performance under specific conditions have emerged. For instance, wood fiber and hemp-lime insulation face the challenge of an ‘energy performance gap’, where actual energy use surpasses expected performance due to energy consumption calculation errors, inadequate construction, or post-occupancy behavior [38]. The durability of BbIMs, particularly in terms of moisture absorption and mold growth, remains a critical concern. The hygrothermal performance of insulations like wood fiber and wood wool is less effective in ETIC systems compared to polystyrene, particularly when combined with Portland cement mortar, which may hinder moisture management [39]. Only wool and sawmill residues demonstrated effective latent heat and condensation control, emphasizing variability in BbIMs’ moisture performance [40]. Notably, the effects of the post-production drying period on insulation materials were highlighted in ref. [41]. Significant moisture retention was observed in hemp and lime-based insulations, which affects the thermal transmittance value and leads to mold growth and decreased thermal comfort, as observed in one sample at 80% RH for two months. Eventually, the extended drying time results in variability in the material’s thermal transmittance [41]. Similarly, mycelium composites exhibit vulnerability to mold [32].

3.2.3. Cost-Related Integration Difficulties

BbIMs are more expensive than conventional insulation according to many studies [30,42,43,44]. In practice, entrepreneurs focus only on profit, whilst its use for private homes depends on the owner’s desire and awareness of its properties [30]. Competing with widely used materials, synthetic materials made the integration of already available BbIMs into the EU market a challenge. Yet, if some materials have demonstrated their cost-effectiveness and environmental advantages over conventional insulation, such as EPS and stone wool, they still face the limitation of their commercial integration, as in the case of miscanthus [42]. Other studies [42,43,44] have suggested that one of the reasons for the high costs of bio-insulation is their thermal properties. For instance, the higher thermal conductivity of some BbIMs, such as wood fiber and hemp, means that they may require slightly thicker external walls to achieve the same insulation performance as conventional materials like EPS. Specifically, the thermal conductivity of EPS is approximately 0.033 W/m·K, whereas wood fiber and hemp insulation have thermal conductivities of around 0.044 W/m·K and 0.051 W/m·K, respectively [30]. This need for thicker walls can lead to increased construction costs [42,43,44].

Other factors that affected the price were the method of installation and the expense of the farming system, which resulted in miscanthus being the cheapest material and hemp fiber being the most expensive in comparison between BbIMs and synthetic insulations. Manual installation, modeled for hemp fiber, miscanthus, EPS, and stone wool, was found to be more expensive compared to mechanical blow-molding modeled for wood and flax fiber. This led to more time spent and hence higher labor expenses, which had a major effect on the overall life cycle costs according to the authors [42].

3.3. Integration Opportunities of BbIMs in the European Construction Sector

The findings in this section highlight opportunities relating to policies and regulations that incentivize and emphasize the use of BbIMs, as well as the benefits of using certain insulations, and solutions to improve their production and performance, which could help overcome some of the difficulties identified in the previous Section 3.2. Figure 7 shows the Treemap (created using Excel version 2408) of key integration opportunities of BbIMs in the European construction sector. Table S3 (in the Supplementary Materials File S1) presents more details of the integration opportunities identified.

3.3.1. Regulations and Policy Opportunities

Several studies have emphasized the environmental, economic, and health benefits of using BbIMs in the building sector, despite challenges to their widespread adoption [29,45]. The manufacture of these materials is based on the use of sustainable resources, making them an effective solution in decarbonizing buildings and improving energy efficiency [33,46].

At the European level, recently the bio-economy directive [47] suggested the use of BbIMs for building stock renovation as an alternative approach, aligning with sustainable building practices and circularity [48]. In addition, EU targets for near-zero energy buildings (nZEB) and CO₂ emission reduction highlight the compatibility of bio-based materials with these targets and incentivize their integration [49,50].

On the national level, the commercial availability of these materials is influenced by national policies and regulations, such as Switzerland’s commitment to reducing CO₂ emissions, creating a favorable environment for bio-based materials [30]. This is paralleled by France’s incentive programs and new environmental laws aimed at reducing the carbon impact of new buildings [34,51]. The French context illustrates the supportive environment for bio-based materials through policies like the Energy Transition Act for Green Growth and the RE 2020 environmental regulation. Policies on whole-life carbon favor bio-based materials for their lower embodied energy and diverse applications in countries such as the Netherlands, Denmark, and France, where CO₂ limits are imposed on new buildings [32]. Greenbelt planning and subsidies for crop growth could promote circular bio-based construction, suggesting a dynamic legislative approach to evaluate the impact of emerging bio-based materials in the construction market [36]. This implies that local supportive policies and regulations [52], compatibility with voluntary standards and green building rating [38] can facilitate the integration and promotion of bio-based materials to achieve adequate energy efficiency. Schemes such as AgBB (Committee for Health-related Evaluation of Building Products) in Germany present an example for developing new regulatory approaches, particularly concerning radon exhalation from conventional building materials [37]. BbIMs can therefore be considered a promising alternative.

3.3.2. Material Performance Opportunities

Several findings have presented promising opportunities in the use of BbIMs and highlighted their role in sustainable construction and energy efficiency [37,53]. Improved energy efficiency, sound insulation, and indoor air quality, along with the possibility of material recycling or reuse, are suggested as key performance opportunities of BbIMs. As the European residential building stock is old and needs to be renovated and decarbonized, many researchers have presented different approaches and tools. To achieve this goal, the use of BbIMs for external wall insulation could reduce environmental impacts in the residential sector by about 6–19% by 2050 compared to the global environmental impacts of the residential sector in 2010 [48]. Additionally, in terms of recyclability, many BbIMs, such as cellulose fiber, allow for easy disassembly without adhesives, proving their recyclability and biodegradability [30].

Certain Materials Performance Opportunities

Wood fiber has been extensively studied for its promising hygrothermal performance [38,54] and environmental positive impact [42]. It effectively controls moisture and thermal inertia [38], despite having higher thermal conductivity than polystyrene. Its performance can be further optimized with appropriate render types in ETIC systems [39].

Given the importance of considering the health impacts of building materials, BbIMs were proven to be able to maintain indoor air quality while providing effective thermal insulation. For example, wood wool insulation may not result in high levels of VOCs indoors [34] and is therefore suitable for building environments where air quality is taken into consideration. In addition, materials such as sheep’s wool and cellulose can withstand imperfections but do not exhibit any moisture content that would lead to mold growth [35]. This resilience emphasizes their suitability for wider use in construction, particularly in environments that prioritize the health impacts of construction materials.

The use of fast-growing plants like hemp and straw for energy retrofitting of historic public housing offers significant improvements in energy efficiency, cost savings, and interior comfort [55]. Particularly, straw can act as a carbon sink, contributing to environmental sustainability [35]. However, it is important to consider the fiber orientation of straw bale insulation to improve thermal and hygroscopic properties, thereby minimizing thickness and reducing cost [44]. It was proven that aligning the fibers perpendicular to the heat flow reduced thermal conductivity by 38% and improved moisture transport behavior, unlike the parallel orientation [44]. The findings suggest that straw bale insulation can be used more efficiently in construction with the development of bale machines suitable for large-scale production [44]. Furthermore, the straw-based insulation showed better performance in moisture regulation, thermal conductivity, and ease of installation compared to maize pith and recycled bedding (polyester duvet) materials, while also demonstrating higher thermal conductivity than mineral wool [43].

Developing eight BbIMs based on peat, recycled paper, wood shavings, and feathers, and evaluating their hygrothermal thermal characteristics, showed their ability to compete against conventional insulations. At 10 °C, the materials under investigation had a thermal conductivity ranging from 0.033 to 0.044 W/(m-K), and a considerable increase to 0.063 W/(m-K) at 50 °C. Notably, these materials have good thermal resistance, particularly when they possess low- to medium-density features [33].

3.3.3. Cost Opportunities

It was mentioned before that BbIMs are generally more expensive, yet there is a potential to have cost-effective bio-based insulations. This can be achieved by considering alternative production processes, such as cold sterilization or infrared illumination, to reduce the costs of BbIMs during the production stages [56]. It was found that cellulose fiber had the shortest impact payback time, whereas wood wool, despite having lower environmental impacts per kilogram compared to stone wool, required a greater volume of material due to its higher density (400 kg/m³ for wood wool boards compared to 50 kg/m³ for stone wool) and necessary thickness (12 cm), resulting in longer payback times [48].

4. Discussion

The current study on the integration of BbIMs in the European construction industry has revealed interrelated insights into the integration difficulties and opportunities of BbIMs. Hence, the following sections will discuss the findings of the studies to understand the critical drivers and barriers involved, and research trends. This discussion is structured to address RQ1 (“What are the integration difficulties and opportunities of BbIMs in the European construction industry?”) in Section 4.1, Section 4.2 and Section 4.3. Following this, we will address RQ2 (“What are the existing research limitations or knowledge gaps for further research in the academic field within the European context?”) in Section 4.4.

4.1. Regulations and Policy

Regulations and policies on the use of BbIMs play a critical role in either supporting or restricting the wider use of BbIMs. While some countries, such as France, have established supportive policies that favor bio-based materials, others continue to prioritize conventional materials due to existing standards that focus on thermal efficiency without fully considering environmental impacts. Despite the complexity and multiplicity of the criteria for using either conventional insulations or BbIMs, the challenge lies in balancing these priorities by the manufacturers and the regulatory bodies to create a low-carbon-built environment at the national level. For example, the restriction on the use of boric acid as a fire retardant, while driven by health and safety concerns, highlights the need for ongoing research into alternative, non-toxic fire retardants that can meet both health, safety, and environmental criteria. In fact, there are currently biobased fire retardants under development and commercially available that provide feasible alternatives [57].

On the other hand, opportunities appear in the progressive push towards decarbonized sustainable construction, as BbIMs are aligned with supportive regulatory frameworks and sustainability goals, such as the bio-economy directive [47,48] and CO₂ reduction targets [50]. These findings suggest that although difficulties may still exist, there is significant potential for BbIMs to play an important role in achieving decarbonization targets in the construction sector, as long as regulatory frameworks continue to support it more effectively.

4.2. Materials Performance

The performance and the features of BbIMs present many advantages and critical issues that need to be controlled. The heterogeneous nature of the constituent materials [29], the risk of microbial mold [33,34,35], fire resistance [31,58], and variability in performance under different environmental conditions cannot be neglected. This further emphasizes how crucial it is to understand the hygrothermal behavior of the materials in real-world settings. The development of novel insulations and optimization of manufacturing processes, such as the use of cold sterilization or infrared illumination, could mitigate these issues and enhance the usefulness of BbIMs. On the other hand, their ability to improve energy performance, recyclability, and indoor air quality, especially in older buildings [37,53], and the potential to reduce environmental impacts in the residential sector by 6–19% by 2050 [48], make them a better alternative to conventional insulations, to favor a negative-carbon built environment. These results have shown that there are many opportunities to improve the performance of BbIMs. Thus, it can be argued that current obstacles are not necessarily permanent barriers, but rather temporary ones that can be overcome or reduced by continuing research and development.

4.3. Cost Considerations

Cost remains one of the major obstacles to their wider use, as BbIMs are generally more expensive. The higher upstream and downstream costs of BbIMs, driven by the lack of demand as material type is not very important in building requirements [30,42], as well as the raw material cultivation system expenses, the requirement for thicker walls [42,43,44], and installer expenses [42], often discourage potential users and consumers. In the end, the homeowners’ willingness and awareness of these features determine whether they are used [30]. However, this study suggests that these costs can be offset by the long-term environmental and health benefits of BbIMs, such as lower life cycle impacts and shorter pay-back times, particularly for materials like cellulose fiber. To make BbIMs more competitive, efforts should focus on reducing production costs through alternative processes and increasing market awareness of the benefits of these materials. Additionally, policymakers could play a pivotal role by offering incentives or subsidies that reduce the financial burden on builders and homeowners, thereby accelerating the adoption of BbIMs in the construction sector. Further investigation into the whole life cycle costs of various BbIMs is recommended, since most studies focus on the environmental impacts of BbIMs. This would lead to an understanding of the overall economic value of BbIMs, and thus change the cost-effectiveness measures.

4.4. Existing Research Limitations, Gaps, and Future Perceptions

The chronological order of the literature from oldest to newest, presented some observations (for more details, see Table S4 in the Supplementary Materials File S1). Firstly, the measured variables changed gradually from materials hygrothermal performance, and related properties to the durability, environmental, and economic impacts. Secondly, despite the promising results of numerical and laboratory research, there is room for questioning how effectively it can be projected in the industry. This challenge extends beyond the manufacturing of materials to include logistics and distribution. Additionally, there is a lack of studies that explore socio-economic perspectives to get more insights about users/buyers’ acceptance of Bio-based construction materials. Research gaps and future perspectives indicate the need for more validation experiments, exploration of alternative production processes, and expansion of life cycle assessment scope to improve the use and performance of BbIMs. Fire reaction, circularity, indoor air quality, energy-environmental performance, and industry stakeholders’ insights on BbIMs are key thematic gaps with little relative progress, as seen in

Table 1. Key thematic gaps of BbIMs and their respective reference and insulations assessed.

Key Thematic Gap	Insulation	Ref.
Fire reaction	- Rice husk, corn pith, and barley straw, using corn starch and sodium alginate as binders	[58]
	- Rigid board based on vegetal pith and a natural gum (corn pith and sodium alginate)	[31]
Analysis techniques	- Earth-based blocks reinforced with barley straw, hemp shiv, and rice husk fibers	[53]
Circularity	- Wood fiberboard, foam glass, hemp fibers, and cellulose fibers	[30]
Hygrothermal performance	- Wood-hemp composite in timber frame wall panels with and without a vapor barrier	[46]
	- Hemp shives	[41]
	- Wood fiber and wood wool, and a panel made with corn pith and sodium alginate	[39]
	- Hemp-lime insulation	[49]
	- Wood fiber panel and assembly of wood fiber and hemp-lime	[38]
	- Wool, hemp, sawmill residue, wood, straw, cork, and polyethylene terephthalate	[40]
	- Pine needle insulation (needles + greenery of Pinaceae pine (Picea) + pine (Pinus) genus)	[52]
	- Peat, Sphagnum moss, mix of peat and Sphagnum moss, wood shavings, Recycled paper/paper wool, and feather	[33]
	- Hemp, cellulose wadding, bio-fibers, wood fibers, and recycled textiles.	[54]
	- Wheat straw	[44]
	- Chopped straw, sheep’s wool, and cellulose insulation	[35]
	- Maize pith, recycled bedding (polyester duvet) materials, and wheat straw insulating prototypes	[43]
Energy—environmental performance	- Wood boards, cork slab, and cellulose fiber	[48]
	- Wool, wood, cork, corn, hemp, and cotton	[59]
Environmental performance	- Wood fiber, hemp fiber, flax, and miscanthus	[42]
	- Straw and hemp	[55]
	- Mycelium-based composites	[56]
Indoor air quality	- Wood fiber, wood fiber (conifer)/cellulose core, and reed, calcium silicate	[37]
	- Wood wool	[34]
Industry stakeholders’ insights	- Includes hemp-lime, recycled paper, and mycelium	[36]

. Therefore, these gaps are considered opportunities for further exploration in future research.

5. Conclusions

The current study included 28 publications from indexed databases. It also showed the growing interest in BbIMs based on the increasing annual scientific production. The included studies revealed interrelated insights on the difficulties and opportunities related to the extent of BbIMs’ use. By reviewing the literature, the integration of BbIMs in the European construction sector seems to be a rather complex process. The first reason could be due to the lack of political and financial incentives. While policies have succeeded in supporting the use of bio-based materials, and insulations in particular, in some countries, especially France, they are facing difficulties in other countries. A major difficulty is the tendency to prioritize thermal operational efficiency over embodied environmental impacts in the evaluation and selection of materials for projects, even if some materials have demonstrated equivalent thermal performance. Other material performance, such as the risk of microbial and mold growth and changes in moisture behavior, further complicate their adoption. These challenges are further worsened by cost considerations. Further investigation on the whole life cycle costs of various BbIMs may lead to an understanding of the whole cost-effectiveness value of BbIMs.

Despite these challenges, there are significant opportunities for integrating BbIMs in the construction industry. There is a move towards sustainable materials selection practices with growing environmental, economic, and health awareness. This can be seen especially with the recent bio-economy directive [47] that recommended the use of BbIMs instead of conventional materials for building stock renovation.

The research gaps identified in the studies indicate the need for more real-world experiments to confirm the positive laboratory results. Future perspectives urge the optimization of production processes to reduce costs environmental impacts and enhance understanding of the hygrothermal behavior of BbIMs in different climatic conditions.

This research contributes to the literature by providing focused insights into the practical difficulties and market opportunities for the application of BbIMs in Europe. However, the current review may not reflect the current situation of using BbIMs in practice. Thus, it may be necessary to get insights from stakeholders from the industry, designers, and engineers to know the real situation and what can be improved to increase their use in practice. Hence, future research aims to extend the scope of LCA along with the cost impacts of different BbIMs with different production methods across European countries.