Building is the second largest sector in plastic consumption, remaining in range of 16% and 23%. Also, this sector consumes around 30% and 40% of energy consumption, responsible for more than 33% of energy related greenhouse gases emission in the world (Lopez Hurtado et al., 2016; Kumar et al., 2020; Pal et al., 2021; Zhao et al., 2022). Traditional insulation material used in building applications are mostly manufactured from petrochemical sources (Carlos Javier et al., 2023). Production and manufacturing processes of construction materials account a higher carbon footprint, polluting air, land, and water (Maraveas, 2020). Construction materials derived from agro-industrial waste are increasingly attractive in the construction sector, due to their sustainability and lower environmental impact. Hence, in recent years the amount of research and publications tending to the development of materials that take advantage of residues from agro-industrial activities has increased (Chabannes et al., 2014; Viegas et al., 2014; Lopez Hurtado et al., 2016; Liu et al., 2017; Antunes et al., 2019; Dixit and Yadav, 2019; Muthuraj et al., 2019; Maraveas, 2020; Moreno et al., 2021; Nandy et al., 2021; Zou et al., 2021; Ali et al., 2022; Amantino et al., 2022; Carlos Javier et al., 2023), to reduce the environmental impact generated by commercial materials such as fiberglass, rock wool, expanded polystyrene, polyurethane foam, polystyrene, phenolics, among other non-biodegradable materials. Thus, in the scientific literature studies have been reported with insulating materials based on cane fibers, coconut tow, cassava, rice, wood, coffee husks, cork, sheep’s wool, recycled newsprint, textile fibers, trays egg, and others.

One of the main characteristics of the use of natural fibers, in addition to their thermal resistance, is that they have low energy (gray energy or embodied energy) in their production process, transportation and to get the final product (Dixit et al., 2010; Pal et al., 2021). Normally the natural fibers used as insulation are the result of other processes, as in the case of rice husks and sugar cane, agricultural by-products are grown in large proportions.

Cellulose, which is obtained from recycled and shredded newspaper, is the most widely used thermal insulator, thanks to its thermoacoustic properties. Cellulose fiber is obtained mainly from the separation of newsprint by physical methods; material that is then processed and shredded (Lopez Hurtado et al., 2016; Garzón Bernal and Cuba Córdoba, 2020). Cellulose insulation can come as a pre-cast panel, in which cellulose fibers are molded with polyester or a similar binder, or as loose fibers, which are applied by hand or specialized equipment, in attics, ceilings, or walls. The first use of cellulose fiber as an insulating material date back to 1919 in Canada, but it was not until 1950 that commercial products were developed in the US. Pulp application gained popularity in the 1970s due to the US oil embargo (Lopez Hurtado et al., 2016).

The rice husk constitutes a residue, when the grain is separated from the husk when going through the piling process and represents around 20% of the gross weight of harvested rice. Rice hulls are mainly made up of carbon, cellulose, and silica (Dixit and Yadav, 2019). Silica represents 20% of rice husk, which makes it resistant to water penetration, thus protecting the rice grain when it is inside. Also, this material has been studied due to its thermal insulation properties when mixed with different types of binders and other materials, such as newsprint cellulose (Buratti et al., 2018). Rice husk has also been used as an aggregate material in concrete called “bio-aggregate concrete”, and in another study it replaces the use of aggregates such as sand in percentage, thus achieving a lighter material, with excellent performance as thermal insulator (Amantino et al., 2022). There are also studies on the potential as a thermal insulator of carbonized rice husks (Dixit and Yadav, 2019) and ash (Antunes et al., 2019; Muthukrishnan et al., 2019; Zou and Yang, 2019; Maraveas, 2020).

Globally, the construction sector represents between 30% and 40% of energy consumption (Lopez Hurtado et al., 2016; Das and Tiwari, 2018; Carlos Javier et al., 2023). During the design or conditioning of buildings, from the point of view of heat transfer, it is necessary to consider all the materials used during construction (specifically the thicknesses and thermal conductivity coefficient of each material), as well as outdoor and indoor temperatures. On the other hand, about 20% of the total energy consumption in buildings worldwide corresponds to the use of air conditioners (Hu et al., 2019; Bienvenido-Huertas et al., 2020). In this case, the role of thermal insultation materials in the building envelope is significant, especially in hot-humid region. The envelope is recognized as a key part for enhancing their thermal efficiency since it expends between 50% and 60% of the whole heat transfer (Kumar et al., 2020; Zhao et al., 2022).

According to the reviewed research, to establish sustainable houses and buildings or “green buildings and neighborhoods” (Zhang et al., 2019), as strategies to reduce energy consumption, it is necessary to verify the possibility of implementing new insulating materials, lighting changes and the implementation of systems that include renewable energy, prior to a more detailed feasibility study.

As a contribution in the generation of new materials from waste materials, this work presents the methodology that was carried out to develop an alternative insulation material based on newspaper (cellulose) and rice husk. Mechanical and thermal tests were performed. Moreover, with the results obtained, it is expected to contribute as a basis for future research in the field of construction materials derived from agro-industrial waste, as an alternative to be used in the field of environmentally friendly buildings.

Cellulose, which is obtained from recycled and shredded newspaper, is the most widely used thermal insulator. Cellulose fiber insulation has lower value of embodied energy and environmental impact compared to conventional insultation materials (Pal et al., 2021). Commercially it can be found in the form of a semi-rigid mat (boards) or blown insulation (loose-fill cellulose insulation). Cellulose boards as a thermal insulating material are usually used as fillers in false ceilings, partitions, doors, and furniture (Moreno et al., 2021). It can also be found as loose fibers, those that are applied manually or with specialized equipment, in attics, ceilings or walls. In recent years, there have been important efforts to improve the energy efficiency of buildings, through the application of thermal insulation in the building envelope, which includes the reduction of the thermal bridge effect, that involves the loss of energy at different points of the envelope (Lopez Hurtado et al., 2016).

Cellulose has been compared with thermal insulators made from mineral fibers, finding that its insulation capacity (R value) remains constant under sudden temperature changes. Likewise, the cellulose-based insulation offers low embodied energy, an important factor when selecting a material. Energy incorporated per kilogram of cellulose is between 0.94 and 3.3 MJ/kg, a reduced range of values compared to commercial thermal insulation that is between 10.8 and up to 45 MJ/kg (Lopez Hurtado et al., 2016; Pal et al., 2021).

In boards made from recycled paper, textile fibers and glue, values of the thermal conductivity coefficient (k) between 0.034 and 0.039 W/m∙K have been reported (Ricciardi et al., 2014; Pal et al., 2021) and 0.040 W/m∙K, when it is applied by blowing technics (Carlos Javier et al., 2023). Similarly, foamed cellulose has reported k of 0.039 W/m∙K (Yeon et al., 2014). In general, typical values of the thermal conductivity coefficients of cellulose fibers are around 0.040 W/m∙K (Lopez Hurtado et al., 2016). This can be explained by the microstructure of cellulose-based products involving voids, some of them connected and others not (Aksogan et al., 2018), depending on the arrangement of the components combined with the cellulose.

It should be noted that the values of k may vary depending on the manufacturing process, installation method, as well as the origin of the raw material. There is an inversely proportional relationship between conductive heat transfer and relative humidity (González-Serrud et al., 2022). The higher the relative humidity is present in the boards, the heat transfer with the environment decrease. In the case of cellulose fibers, due to their hygroscopic nature, moisture absorption can affect the final values of thermal conductivity (Lopez Hurtado et al., 2016). Hygroscopic cellulose insulation can be used to control indoors air humidity, increasing indoor thermal confort (Pal et al., 2021).

Fiber board from cellulose and without the use of additional bonding, using biodegradable, renewable and recyclable materials is also reported with a Young’s modulus of 17 GPa and a strength of 120 MPa (Arévalo and Peijs, 2016).

Likewise, when mixing cellulose with other materials, such as perlite and gypsum, k values between 0.083 and 0.12 W/m∙K are obtained (Aksogan et al., 2018), which, although not so low, correspond to studies that seek to improve other characteristics in cellulose, such as fire retardant and fungicide.

Rice husk insulation is made from the outer protective covering of rice grains, which is typically discarded as waste during the rice milling process. Rice husks are abundant, renewable, and readily available in many rice-producing regions, making them an attractive choice for sustainable insulation. Rice husk insulation is an emerging and innovative insulation material to create a sustainable and efficient thermal barrier. Rice husk insulation has been studied for its thermal properties, and it has been found that the addition of rice husk fibers to thermal insulation wallboards can decrease their thermal conductivity and increase their insulation performance. A k value of 0.047 W/m∙K was reported for a rice husk particle. Rice husk-earth based composite present k values between 0.102 and 0.197 W/m∙K, with different rice husk content (above 15%). Also, increase on rice husk content in defined proportion could produce decreases in thermal conductivity due to bulk density decrease (Antunes et al., 2019).

Studies that have been carried out on natural fibers materials have shown its good insulating characteristics when mixed with different types of binders. For example, in a study carried out on rice husks and cassava starch as a binder, a thermal conductivity of 0.0262 W/m∙K was obtained (Canto et al., 2018). In the work carried out in Universidad Tecnológica de Panamá, Azuero, thermal conductivity coefficient of 0.073 W/m°C and 0.049 W/m∙K, were obtained for rice husk and rice straw boards, respectively; these values are in the range of thermal insulators that goes from 0.020 to 0.080 W/m∙K (Gonçalves and Bergmann, 2007).

Material fibers that contain cellulose have a high resistance, as well as a low density, which is interesting when it comes to meeting thermal insulation needs in conjunction with lightweight construction systems. Regarding mechanical characterization results, specifically, compressive stress between 0.25 and 0.29 MPa have been reported in material based on newsprint, perlite, and plaster. The high compressive strengths of the samples added with zinc borax (between 0.35 and 0.45 MPa) could be due to the denser matrix formed by zinc borax compared to those formed by the other additives (Aksogan et al., 2018). In another material based on rice husk and geopolymer, a compressive strength between 0.26 and 7.24 MPa was obtained (Wang et al., 2020). A biocomposite made from rice husk, wheat husk, wood, and textile fibers, bonded with a biodegradable polymer, demonstrated compressive strength between 11 and 40 MPa (Muthuraj et al., 2019). Rice husk insultation material bonded with lime, presented compressive strength of 0.33 MPa and after 60 days of setting, and with a hemp insulation material the compressive strength was 0.48 MPa (Chabannes et al., 2014). In a carbonized wheat straw composite insultation cementing with a geopolymer, the compressive strength was between 0.49 and 0.82 MPa (Zou et al., 2021). Panels based on cardboard, cement, and sand reported values from 1.494 to 6.3 MPa in compressive strength, considering that strength increases with curing time. The tensile strength for a material based on cardboard, cement and sand is between 0.987 and 2.098 MPa. As in the case of compressive strength, it improves as the days of curing time (Ahmad et al., 2021).

Even though cellulose and rice husk based insultation material presents advantages to the environment, efforts must still be made in the optimization and development of these materials in terms of their properties, manufacturing, installation, and performance as a final product, also considering the origin of the raw material. Research has focused on the search for alternative raw materials that reduce or replace chemical additives with fungicidal and fire-retardant characteristics (Nagieb et al., 2011; Ricciardi et al., 2014; Yeon et al., 2014; Lopez Hurtado et al., 2016; Aksogan et al., 2018). In general, when it comes to cellulose and natural fibers, a percentage of boric acid and borax is added to give the material resistance to the appearance of fungi and give it fire retardant properties. The concentrations of borax and boric acid reported are between 15% and 20% of the total mass of cellulose-based fiber (Lopez Hurtado et al., 2016). Other authors indicate that the ideal concentration of fire-retardant materials to achieve good resistance to the flame is found to be 5% (Nagieb et al., 2011). Carbonized biomass partially and adopting fireproof inorganic cementing materials were proposed to enhance the flame retardancy in naturals materials as wheat straw insultation materials (Zou et al., 2021).

Mechanical, thermal, and acoustic study of polymer-based composite materials derived from expanded cork and rice husk by-products demonstrated that the incorporation of rice husk grains in composite formulations helps reduce thermal conductivity by 20% compared to composites without rice husk. Increasing rice husk content in composites can contribute to improved acoustic performance, with sound absorption up to 0.6 at low frequencies. Composite materials produced with expanded cork by-products and rice husk present a mechanical behavior suitable for use in construction systems such as walls and floors. An assessment of the suitability of these composite materials in building systems, such as walls and floors, was conducted, and the most suitable formulations were selected through a multi-criteria analysis. The findings suggest that construction solutions utilizing these composite materials can be applied in buildings, contributing to reduced energy consumption over the buildings’ operational lifetimes (Marques et al., 2020).

Bio composite made from recycled materials, specifically peanut shells, and rice husks, reinforced with polypropylene exhibited impressive tensile and flexural strength, as well as offering qualities such as thermal insulation, sound absorption, aqueous stability, and flame resistance. The thermal conductivity of these composites ranged from 0.156 to 0.270 W/m∙K. In summary, the article suggests that bio composites derived from agro-industrial waste can serve as a sustainable and environmentally friendly alternative to traditional building materials (Guna et al., 2020).

In other study, biocarbon was produced from hemp and switchgrass via pyrolysis at three different temperatures (450, 550, and 650°C). Composites were created with varying concentrations of biocarbon fillers and sizes. Biocarbon from both plants significantly reduces water absorptivity when compared to unfilled hemp fiber-reinforced composites. Tensile strength showed improvement with hemp biocarbon and switchgrass biocarbon, with the maximum tensile strength (10 wt% biocarbon of 50 microns, produced at 650°C). Other properties such as energy at tensile rupture, flexural strength, and impact strength varied based on particle size, filler loading, and pyrolysis temperature. Thermal conductivity was affected by filler-loading and particle size. The average thermal conductivity of the composite with hemp was 0.059 W/m∙K, while that of the switchgrass was 0.058 W/m∙K (Dahal, 2023).

Natural fibers are a renewable resource, making them a sustainable and environmentally friendly choice compared to synthetic fibers (Vidal et al., 2023). They also tend to have lower embodied energy and a smaller carbon footprint, further reducing the environmental impact of composite materials (Karimah et al., 2021). However, it is worth noting that natural fibers can exhibit more variability in properties compared to synthetic fibers, potentially affecting consistent performance in composite materials. Additionally, their susceptibility to moisture absorption can impact their mechanical properties and long-term durability (Dahal, 2023; Dahal et al., 2023; Martínez et al., 2023). Nonetheless, research is actively addressing these limitations through innovative processing techniques and the application of bio-based binders and coatings in order to improve the performance of natural fiber composites (Vidal et al., 2023).

Overall, both cellulose and rice husk insulation materials offer thermal insulation properties and can be used in construction. As insulation materials have their advantages and are considered environmentally friendly options. The choice between them may depend on factors such as availability, cost, and specific project requirements. Rice husk insulation, on the other hand, is derived from rice husks and offers the advantage of utilizing agricultural waste. It has been found to improve the insulation performance of wallboards.

New boards made of recycled newspaper and rice husk were developed and tested, as an alternative for insulation material. The studied material was fabricated and evaluated from three different compositions and tests, including tensile strength tests, compressive strength tests y thermal conductivity tests. The tensile, compressive, and thermal performance were evaluated to propose their application as insulting boards for the building sector, as an alternative for the reuse of materials considered as value-added waste, such as recycled paper and rice husks.

The maximum stresses in average tension are in a range of 1.31–1.76 MPa. Despite the fact that the results are consistent with what has been reported in the scientific literature, further research could include other configurations, such as the addition of long fibers in only one direction to reinforce the behavior of the material in tension, since a material insulation that is placed as part of the envelope of a building does not have a great resistance to tension as an essential condition, it could be desirable in structural cases.

Regarding the compressive strength, the values obtained are between 20.19 and 21.23 MPa. In Muthuraj et al., 2019, for a material based on rice husk, a compressive strength of 15 MPa was obtained. In this sense, through the tests carried out, it was possible to verify that the resistance of the developed material presented values like those reported in the literature.

The experimental results showed a good thermal behavior of all the investigated compositions: k-values vary in the 0.0409–0.04607 W/m∙K range. The developed material has competitive thermal conductivity compared to many natural and recycled insulation materials and may be a viable option for applications requiring efficient thermal insulation. However, there is still opportunity of improvement to meet or exceed the efficiency of the leading industrial insulators on the market.

Within the general analysis of the manufacturing process of the composite material, it was possible to determine very favorable results when verifying its behavior in tension and compression, even so, considering that the rice husk can be crushed to obtain particles smaller than 0.6 mm. As a result of this research, a material with good thermal insulation capacity and good compression performance.

As a result of the findings presented in this article, it can be concluded that the material developed could potentially have utility across various engineering domains, including the production of lightweight components, construction panels, sustainable packaging, energy dissipation, thermal insulation, among other. Nevertheless, further research and testing are imperative to assess its viability and performance in each specific application.

In general, the panels investigated could be a valid solution to replace the classic insulation systems, although the investigation remains open for carrying out additional tests on the developed material, such as bending, flammability, and degradability tests, among others. Also, the proposed material is presented as a sustainable alternative, which can be used in the field of environmentally friendly buildings, which contribute to reducing the carbon footprint. This research aims to promote the use of materials considered as agro-industrial waste in our country, demonstrating their applicability in different areas. Also is considered as a contribution in advances in the use of natural fibers and recycled paper as thermal insulation materials, which could accordingly help future development of sustainable green insulations. The obtained results indicates that recycling materials and agro-waste materials have the potential to replace conventional constructions materials and hence achieve environmental and economic sustainability.