Natural Insulation

The use of natural materials is essential and inseparable from the goal of reducing the carbon footprint of the building stock. The choice of materials is closely related to the embodied carbon in buildings, which currently represents 11% of total global emissions derived directly from the production of construction materials. [1]

Prioritizing locally sourced biomaterials not only allows us to reduce carbon impact but also contributes to improving the health of the indoor environment of our spaces by reducing the presence of harmful toxins and improving the breathability of the walls.

Furthermore, we must also consider the possibility of materials being recovered or transformed at the end of their useful life, as the environmental impacts of a building’s life cycle encompass everything from material production, transport, construction, use, and maintenance to demolition and waste management.

In the case of natural insulation, its very nature allows it to be reused for the same insulating purpose in another project, transformed for another use, or naturally decomposed and returned to the earth due to its organic origin.

Despite the long list of advantages of natural insulation, these materials are currently still in incipient use or were abandoned when replaced by petroleum-derived plastic insulation that promises durability and efficiency at a low cost.

But as we know, the reality is that all the low-cost products sold today hide hidden costs that we charge to the environment’s account in the form of global warming and pollution.

Insulation in Construction

In our latitude, traditional Iberian construction based on principles of bioclimatism did not typically use insulation as such; instead, it took advantage of the building’s thermal inertia with thick walls made of stone or earth, lime, and sand mortar.

In architecture, thermal inertia is the capacity of a construction element to store temperature. The walls of traditional houses had a great capacity to store temperature, whether heat in winter or coolness in summer. However, without insulation to buffer the transfer of temperature from one side of the wall to the other, thermal losses occurred rapidly, losing indoor comfort in a matter of hours.

Although the use of insulation to improve wall efficiency began during the 70s and 80s, the reality is that it was placed on the inner face, thinking that this would isolate us from the cold load-bearing brick or concrete wall.

In recent years, and as a result of research into passive construction systems, it has been shown that insulation should be located on the outer face of the wall. Unlike our traditional constructions that had few openings to capture sunlight, our current buildings have large openings that allow for solar heat gain, which is stored in inertia elements and not lost thanks to high external insulation and airtight joints.

Correct insulation is one of the main strategies of passive architecture. Unlike much of the architecture currently being carried out, where insulation deficiency is compensated for by heating and cooling systems running at full blast throughout the year, sustainable architecture reduces consumption to a minimum thanks to good insulation.

Thus, a passive house uses around 15 to 20 cm of insulation thickness, while the average insulation thickness in Spanish architecture is between 5 and 8 cm. Furthermore, different facades may have different insulation requirements, making it necessary to place more insulation on the north facade due to lower radiation. The principles of a passive house are based on generating similar comfort conditions in all rooms of the home, always ensuring the continuity of the insulation.

Insulation Used in the Last Century

The insulation materials most commonly used in Spain during the 20th century present a series of problems always related to their non-natural origin.

Mineral Wool Insulation

In the case of materials of non-renewable mineral origin such as glass or rock wool, chemical binders like formaldehyde are often used. This is a carcinogenic toxin that is constantly released into indoor environments, generating a bioaccumulation that is very harmful to the health of users.

For its part, glass wool is particularly harmful during its manufacture and handling due to the fibers it is composed of, which are small enough to be breathable and reach our lungs in the case of the smallest fractions.

Petroleum Derivatives

Petroleum derivatives, such as spray polyurethane or extruded polystyrene, have a high ecological footprint, are not renewable at the end of their useful life, and do not allow the wall to breathe properly, which can lead to humidity and mold growth in the inner layers of the wall.

Furthermore, their manufacture at high temperatures prevents the energy used from being replaced by energy from renewable sources, so the use of fossil fuels adds embodied carbon to the building. [2]

Advantages of Natural Insulation

When analyzing the advantages of natural insulation, we must take a holistic view that goes beyond mere technical performance such as heat loss and energy savings. We must value them from a holistic perspective that encompasses, beyond energy performance, the well-being and health of the indoor environment, breathability, useful life, and carbon emissions. [3]

While each type of insulation must be analyzed in detail, there is a list of qualities that are usually common to all of them:

– Thermal performance: beyond the thermal conductivity coefficient, natural insulation materials have higher thermal performance because they have a greater potential for moisture buffering.

– Acoustic performance: the lack of uniformity in the natural fibers that make up natural insulation usually helps to absorb sound.

– Durability: following the principles of the circular economy, natural insulation materials are usually non-processed products, making them chemically more stable and therefore more durable.

– Hygroscopicity: Hygroscopicity is the natural capacity to absorb and release moisture to the environment; a hygroscopic material has a lower risk of degradation and therefore less need for maintenance.

– Sustainability: natural insulation materials are recyclable and biodegradable, preventing them from becoming waste at the end of their useful life. Furthermore, they have a lower carbon footprint as they are minimally processed products that do not require large amounts of energy for their manufacture.

– Health: the absence of toxins, petroleum-derived components, or breathable fractions in their handling process means that natural insulation is not harmful to the health of the people inhabiting the spaces or those carrying out the manufacturing process. Furthermore, they have no impact on environmental health. [4]

– Comfort: natural insulation creates more comfortable spaces by ensuring health through aspects such as breathability, which prevents the appearance of humidity and mold growth that affects user health. [5] Its excellent insulating capacity provides greater climatic comfort, as it minimizes the need for high energy consumption in climate control.

– Fire resistance: many natural insulation materials are naturally fire-retardant, thus avoiding the need to apply toxic components harmful to health, such as flame retardants or additional fireproof finish layers that often incorporate toxic components.

– Ease of assembly: by reconciling various advantages stemming from the very nature of the materials, we reduce the need to combine layers of different specialized materials for each function, which facilitates assembly, anchoring, and recycling at the end of their useful life.

List of Natural Insulation Materials

We go into detail on the list of natural insulation materials.

Cork

Cork comes from the bark of cork oak trees. It is a material that is renewable every 10 years and is structured in polyhedral cells, which provide it with thermal, acoustic, and waterproof insulation capabilities.

Of the 340,000 tons of global cork production, 83% is produced in the Iberian Peninsula, making it very easy to find local producers near the site of the project to be developed. [6]

It is marketed in the form of agglomerated sheets, formed through pressure and heat without the need for glues; in the form of chips, granules, or crushed material for filling; or directly sprayed. [7]

Cork panels typically have a low thermal conductivity coefficient of 0.038 to 0.042 W/m*K, giving them a higher insulating capacity than granulated cork products, which range from 0.04 to 0.045 W/m*K. [2]

It is a weather-resistant insulation, so it can be left exposed, simply glued using a cork resin binder or anchored to the facade.

Furthermore, the product arrives on-site with a lifetime guarantee; it is recoverable, recyclable, reusable, and transformable, or it can simply be returned to the earth for natural degradation.

At a health level, it is a breathable material that contributes to generating a healthy indoor environment.

Finally, it is a fire-retardant insulation, resistant to abrasion and corrosion, and is not attacked by insects.

In the Iberian Peninsula, the Portuguese company Amorim, the world leader in cork production, distributes cork panels and granulated cork, as well as cork sandwich panels with an inner layer of coconut fiber that ensures acoustic insulation. Although it is distributed throughout Spain, all its manufacturing takes place in Portugal. [8]

In Catalonia, there is Barnacork, which distributes expanded cork boards for exposed exterior facade cladding resistant to fire under the trade name Aglocork MD, as well as granulated cork. Another very similar company located in Badajoz is Azucork.

The manufacturer Gotecork offers sprayed cork, an application method little known in the sector, although its application has a low cost and long duration. It is applied in the form of expanded ecological paint that functions as acoustic, thermal, anti-mold, waterproof, and fire-resistant insulation. With a thickness of only 3-4 mm, it is sprayed with an airbrush gun and an air compressor.

This solution can be applied to multiple surfaces such as roofs, terraces, floors, walls, and facades, and is even used as a coating on asbestos roofs, preventing the dispersion of microscopic fibers into the environment by creating a total encapsulation.

Wood Fiber

Wood fiber as a natural insulator is composed, as its name suggests, of small wood fibers obtained from wood waste, which can be both cutting remains from forests and the wood industry, as well as recycled wood of low commercial value.

Thus, this material follows the principles of the circular economy by definition, as it always utilizes waste material mixed with natural binders.

This is known as a process of upcycling, also known as creative reuse—the process of transforming by-products, waste materials, or unwanted products into new materials or products with a specific function, such as thermal insulation.

Its production process begins with the crushing of wood into small particles, which then undergo a thermo-mechanical treatment that allows their separation into individual fibers. Additives are added to these fibers to improve their fire resistance and protection against insects and fungi, such as boron salts, a substance that is not harmful to the health of the inhabitants. [9]

It is marketed in the form of dry-mounted boards, and its porous structure favors vapor diffusion and the absorption of sound waves. Two types of panels are distinguished, flexible and dense, which have a similar thermal conductivity coefficient: flexible panels have a lambda between 0.038 and 0.04 W/m*K and dense ones from 0.037 to 0.046 W/m*K. [2]

Furthermore, it can also be presented directly as fiber, so it must either be blown into a cavity between two wall leaves or blown directly into an open space such as an attic. [10]

In Spain, there are no manufacturers of wood fiber insulation, so the closest we can find is the company Buitex Recyclage in eastern France, which develops a complete range of hydrophobic rigid insulators for floors, roofs, and facades called Isonat. Its commercial delegation in Spain is called Davsa and is located in the province of Girona.

Among European manufacturers, the natural wood fiber insulation materials most widely distributed in Spain are those manufactured by Gutex, a German family business located in the southern Black Forest, which produces all types of boards and bulk wood fiber. Thus, the difference in density in the boards will depend on the position in which they are placed:

– In the case of exterior insulation that must be anchored directly and support a layer of plaster, in addition to being protected against moisture, a panel of high and homogeneous density is necessary.

– On the other hand, insulation in an air cavity between wall leaves can be a flexible board or directly granulated wood fiber, which is also ideal for irregular walls such as stone.

The general distributor of Gutex for Spain and Portugal is Biohaus, a supplier specialized in biomaterials for thermal and acoustic insulation such as cellulose, cork, and wood fiber, as well as natural paints.

Another supplier is Ecogreenhome, which distributes wood fiber products from the manufacturer Steico, a leader in the European market, with offices in Catalonia, Madrid, the Basque Country, and Aragon, which also offers innovative products such as beams with built-in lateral insulation.

Hemp

Hemp is the herb from the cannabis plant, which contains fibers, seeds, and oil and is used to manufacture products such as textiles, construction materials, paper, fabrics, soap, food, food supplements, and cosmetics.

Due to the 2022 regulation of cannabis for therapeutic purposes in Spain, our country has become the main producer and exporter of cannabis in Europe, a phenomenon that originated in Catalonia due to its previous legalization.

A surplus of this production is hemp fiber, which enjoys very rapid growth, does not need to be sprayed or use chemical fertilizers, and absorbs CO₂ during its growth.

The resulting insulation is resistant, hygroscopic, a good acoustic insulator, and highly durable, being rot-proof as it contains no proteins. At a health level, it is a toxin-free product with no VOC presence.

Natural hemp insulation is marketed in three different forms: in rolls, a type of fiber blanket, flexible panels, and in bulk, known as hemp shiv. Furthermore, the waste from this surplus—the woodier fiber from the main stalks—can be used in lightweight mortars in another example of ‘upcycling’. [11]

Between the roll and panel formats, the thermal conductivity coefficient falls within the same range of 0.038 to 0.042 W/m*K, while the bulk format has a higher lambda, meaning a lower insulating capacity, of 0.048 W/m*K. [2]

In Spain, the company Cannabric in Granada, known for manufacturing load-bearing hemp blocks, distributes hemp shiv, pellets, biocomposite, felt, panels, and hemp fiber boards manufactured by the Granada-based company Cannatektum.

The panels, which come in the form of rolls or strips, are composed of 75% hemp fiber and 25% recycled jute, providing high rigidity that allows for good adaptation in both timber frame constructions and stone building renovation works.

The boards are much more rigid and are manufactured with larger grain size particles to maintain their density. Additionally, there is a type of panel resulting from the combination of hemp biomass and clay—a collaborative, non-load-bearing material suitable for use in high thicknesses and non-exposed exterior insulation, thanks to its high resistance to water and fire.

In Catalonia, you can find the manufacturer RMT Insulation (Recuperación de Materiales Textiles), a family business specialized in the manufacture and trade of wool, which has expanded its product portfolio to include recycled cotton and hemp fiber blankets. This is a company that is currently using synthetic binders in its composition.

In Navarra, we find the company BioKlima Nature, which distributes flexible thermo-acoustic insulating rolls from the Navarrese manufacturer Aislanat. In this case, it is composed of 85% hemp fiber with 15% thermofusion fiber acting as a binder.

Straw

Straw is a rapidly renewable material, an agro-fiber left over from wheat harvesting, used as a construction material, much like hemp.

It is a material that, due to its thermal properties, is very suitable for use as thermal insulation, which is why it has been a widely used material in bioconstruction in the USA and Canada. Furthermore, it also acts as a good acoustic insulator. [12]

What is known as the Nebraska style is based on load-bearing walls built with straw bales, resolving structural function, thermal insulation, and acoustic insulation. However, it is essential that the straw bales are pressed to achieve high density, that a design is made considering it will only support compression forces, and that reinforcement elements are placed for bracing and stabilization, as well as the foundation and crowning of the wall.

Lacking a tradition of building with straw in Spain and specific regulation, its recent implementation is closely linked to its industrialization, thanks to various Spanish manufacturers who have developed prefabricated compressed straw panels within a wooden structure. In other countries, such as our French neighbor, there is already regulation for building with straw bales directly as structural support. [13]

In Spain, companies like Okambuva, a manufacturer located in Valencia, have been able to study and develop a technical data sheet that quantifies environmental criteria such as the product’s low ecological footprint, its high energy efficiency, and justification of its mechanical performance, making it suitable as a load-bearing and enclosure element. Thanks to these tests, we know that its thermal conductivity coefficient is 0.067 W/mºC, one of the least insulating, although it must be considered that the minimum width of the panels is 50 cm, which ultimately results in high insulation.

Construction with these panels consists of a construction method different from Nebraska, known as Alfawall, which is based on a lightweight pine wood frame structure filled with pressed straw bales. Its manufacture involves placing the straw bale using a press into the wooden box at high pressure, increasing the system’s resistance and, in turn, its insulating capacity. These prefabricated modules are stacked like large bricks in small-scale constructions, facilitating a very agile and simple assembly. For larger buildings, panels with straw as natural insulation act only as an enclosure.

There are two other manufacturers in Madrid: the company EcoCocon has also developed a similar prefabricated panel, 40 cm thick, which combines with an outer layer of wood fiber to ensure adequate thermal insulation. The other is the company Bala Box, which manufactures and markets much smaller blocks, the size of a straw bale, which must be laid together. The company Ecopaja in Vitoria has developed, in addition to 45 cm thick modules and 24 cm frames, 24 cm thick roof insulation panels up to 12 m long.

Cellulose

Cellulose is a highly effective natural insulation that presents numerous similarities with wood fiber, as both are wood derivatives. Cellulose is the main molecule present in plant cell walls, so it is found in large quantities in materials such as paper and cardboard. For this reason, its manufacture is based on the reuse of recycled paper, specifically newspaper. The material is crushed and mixed with water, forming a mass from which the ink is extracted, and to which additives are added to improve its fireproof, insecticidal, and fungicidal properties, such as boron salts. [14]

This mixture is dried and pressed in the same way as wood fibers, using low-impact processes. It is marketed in the form of blankets or in bulk, known as cellulose wadding, which can be blown or sprayed, as in the case of wood fiber. Its thermal conductivity coefficient ranges between 0.038 and 0.044 W/m*K.

In Madrid, you can find the Aislanat factory, a family business that in 2021 received the Sustainable Company award for its promotion of sustainability and the circular economy through the manufacture and installation of cellulose wadding.

In the province of Zamora, there is the company Aisedecel, which manufactures and markets a range of cellulose insulators, in blanket or bulk form, improved by a treatment that makes it hydrophobic, thus reducing its water absorption from 16-35 liters for untreated cellulose to 2.5-4.5 L.

This hydrophobic treatment ensures that, in the event of potential humidity, it does not lose its original state or settle, leaving thermal bridges, and complies with current regulations on water absorption and vapor (UNE-EN 1906:2013 Standard).

In Navarra, the distributor Biohaus, which provides natural insulation from cork, wood fiber, and cellulose, has created its own brand of bulk cellulose called Biocell, manufactured at its plant in Alsasua with 90% recycled cellulose and holding the European Technical Certificate (ETA 04/0080).

The reality is that, although surplus newspaper exists in almost every country equally, it is very difficult to find Spanish manufacturers; however, there are many companies that market products from Isocell, which has factories in Austria, France, and Belgium.

Sheep’s Wool

Sheep’s wool is a fiber of animal origin and is made from the protein known as keratin. It is a renewable material obtained through the shearing of sheep, a process necessary for their care regardless of its subsequent use. Its high insulating capacity places it as a competitive natural insulation with a historical tradition. [15]

It has a high hygroscopic capacity as it can retain water between 40% or 45% of its weight, making it a material that buffers changes in environmental humidity. In fact, its insulating capacity increases when it becomes damp. Furthermore, it regulates temperature, is breathable, and prevents potential condensation. However, a treatment against xylophages and moths with borax salt is necessary. [16]

It is marketed in the form of blankets or panels, with a thermal conductivity coefficient of 0.035 to 0.042 W/m*K. [2]

In Spain, there are several manufacturers, such as RMT Insulation in Catalonia, which markets 3 mm thick wool felts that can be applied both on finishing surfaces and inside wall linings and false ceilings, in addition to functioning as acoustic insulation, both as an impact absorber under flooring and as a coating that prevents reverberation. [17]

In the Basque Country, the company BioklimaNature markets sheep’s wool blankets manufactured by the brand AislaNat, which are easy to cut manually or with scissors and to install by stapling them to the load-bearing structure.

The Valencian company Wool4Build manufactures panels of different densities and thicknesses of 4 or 5 cm in this case, thanks to a project co-financed by the European Union within the framework of the ‘Cip Eco-Innovation’ environmental program.

Cotton

Cotton is currently the most common natural fabric, derived from seeds. Widespread cotton planting has caused controversy in recent years due to the environmental impact it entails, as the current exploitation model consumes large amounts of fertilizers, insecticides, and water. However, initiatives have recently appeared for the development of controlled crops, without pesticides or chemical additives, respectful of the environment, the people working in their cultivation and transformation, and the end users, thus obtaining organic cotton. [18]

The marketing of cotton as natural insulation is found in the form of blankets, semi-rigid panels, or in bulk. It is usually recycled material from discarded clothing or scraps from the garment industry. It is a hygroscopic, breathable, organic, renewable, and recyclable material, with a thermal conductivity coefficient for boards between 0.034 and 0.036 W/m*K and in bulk from 0.04 to 0.042 W/m*K.

The company RMT in Barcelona manufactures 5 and 10 cm thick cotton boards and bulk material, all made from 85% recycled cotton from scraps and other textile fibers, treated against fungi and with fire retardant.

In La Rioja, Logroño, we find the company Geopanel, specialized in the regeneration of textile fibers to produce construction products, geotextiles, upholstery, footwear, and carpets, among others.

Due to the high thermal but also acoustic insulating capacity of cotton, blankets and panels are developed for both purposes. There are also acoustic insulators designed as colored decorative elements.

Flax

Linen is one of the oldest textile fibers, a plant-based fiber characterized by good air permeability, hygroscopicity, with the ability to absorb and retain water in a proportion between 50% and 60% of its weight, and low susceptibility to electromagnetic charges.

From an ecological and health perspective, linen is a fast-growing plant that does not require pesticides or fertilizers, and consumes very little water for its cultivation. This natural insulation is marketed in the form of rolls and panels, with thermal conductivity coefficients ranging between 0.037 and 0.047 W/m*K for panels and 0.037 W/m*K for rolls. [2]

In La Rioja, the Logrotex company from Logroño has developed a brand called Isolgreen, which manufactures and distributes natural linen insulation in the form of rolls and panels.

However, it is a natural insulation that is not widely used in Spain and the rest of Europe. Today, Russia is the main global exporter of linen. [19]

Coconut

Coconut fiber consists of natural fiber extracted from the coconut husk, which is uncommon in the European construction sector, despite being commonly used in traditional construction in many countries of the Global South as a clay binder for earth walls, as it allows them to work under tension. Furthermore, the agricultural industry itself, which produces this byproduct, often reintroduces it into the production cycle mixed with vegetable substrate, as this improves water retention. [20]

This same property makes it a good natural insulation, for which the longest fibers are used, marketed in the form of rolls and boards, initially designed as acoustic insulation against impact noise, although its thermal conductivity coefficient is comparable to other natural insulations, ranging between 0.043 and 0.045 W/m*K.

In Spain, the company Barnacork also markets products from the Portuguese company Amorim, such as coconut fiber boards and rolls. The difference between both formats, apart from their thickness and rigidity, is their composition, since the boards require the use of natural resins as a binder, while the rolls do not, although neither contains any toxic substances or poses a health risk to users.

Posidonia

Posidonia is an aquatic plant very common in the Mediterranean coastal area that is vital for maintaining water quality. However, once dead, the sea carries it to beaches where it accumulates and is eventually discarded. IBAVI, the Balearic Housing Institute, has included posidonia in the catalog of sustainable materials for the Balearic Islands. [21]

In this catalog, which serves as a true guide to biomaterials, they propose that local councils manage the annual removal of posidonia from beaches for use in construction. This extraction is done by hand and does not use any energy-consuming machinery, except for its transport, making it a material with a zero carbon footprint. Furthermore, it does not require any treatment because salt acts as a biocide, and it is practically fireproof. Therefore, it is a material with unlimited durability, evidenced by samples from the 14th century found in perfect condition.

The calculation performed in this study quantifies its low thermal conductivity at 0.043 W/m*K, which is why its use in dry form is recommended as natural roof insulation without any industrialization.

Through the Life Reusing Posidonia project, the Balearic government developed a social housing promotion built entirely with local and reused biomaterials, among which posidonia was the protagonist.

Insulation from Recycled Products

There are several nascent projects that seek to apply the circular economy to everyday products, giving them a second life through awareness campaigns and collections for subsequent recycling. [22]

An example is the Bio Circ project, developed in collaboration between France and England, funded by the European Union, which recycles bed fibers such as sheets and pillows, generating a final product that is a mixture of materials like cotton, wool, linen, and silk. They produce 5 cm thick circular bio-insulation panels that guarantee a lambda of 0.044 W/m*K.

On the other hand, the Catalan company Isoltex collects jeans for shredding and forming into recycled cotton panels for insulation with a thermal conductivity coefficient of 0.032 W/m*K. These panels can be used in interior partitions, dividing walls, separation elements with party walls, ventilated facades, and ceilings.

Finally, the company Socyr manufactures, in addition to cork and granulated ETICS, insulation based on recycled cork stoppers. The company, founded in Valencia, created Biosocyr in 2018, an ecological association that promotes ecological and bio-construction products and uses the opportunity to disseminate information through the Recycled Cork project, which aims to close the complete circuit of manufacturing products with recycled corks by financing the planting of native trees. It also has the help of collaborating associations such as schools, municipalities, waste collection companies, and wineries that carry out cork collection.