Bio-based insulation

What is bio-based insulation?

Bio-based insulation is a family of thermal insulation products whose main raw material comes from renewable biological sources: wood residues, recycled paper, animal fibre, tree bark, hemp or cereal straw. It is an umbrella term, not a single product. Each member has its own manufacturing route, behaviour inside a wall and limits, so the specification always names the exact product.

The family sits inside the wider group of bio-based materials, which also covers structural timber. In Slovak practice the umbrella term is prírodná (biogénna) izolácia, and clients usually meet the individual products by their own names.

Which materials belong to the bio-based insulation family?

Five groups dominate the market in Slovakia and Central Europe. The table shows the family at a glance.

Material	Raw material	Typical form	Typical use
Wood fibre (drevovláknitá izolácia)	Wood residues, usually softwood	Flexible batts and rigid boards	Roof sarking, timber-frame walls, render substrate
Cellulose (celulózová izolácia)	Recycled paper	Loose fill, blown into cavities	Attics, retrofit of closed wall cavities
Sheep wool (ovčia vlna)	Washed fleece, often with binder fibres	Rolls and batts	Timber-frame walls, roofs, partitions
Cork (korková izolácia)	Bark of the cork oak	Expanded boards and granules	Facades, floors, ground-level slabs
Hemp and straw	Hemp shiv, cereal straw	Hempcrete infill, straw bales	Specialist walls with lime or clay finishes

How does bio-based insulation keep a house cool in summer?

Its main advantage in summer is heat capacity. Dense, fibrous products such as wood fibre and cork store heat and release it slowly, so the peak of outdoor heat reaches the inside of a roof or wall later and in a weaker form. Lighter products store much less heat for the same thickness.

This is why dense wood fibre boards are often chosen for a roof or a west-facing facade that takes the afternoon sun. The effect is a time lag and damping, not a barrier. The building still needs shading and night ventilation. Because the result depends on density, thickness and the rest of the assembly, check summer behaviour with a simulation, not a rule of thumb.

How do bio-based materials handle moisture?

Most bio-based insulation is vapour-open and hygroscopic. Hygroscopic means the fibres take up water vapour when the air is humid and release it when the air dries, which smooths out indoor humidity peaks. Engineers call this moisture buffering. Sheep wool and cellulose are especially good at it, and wood fibre is also strong.

The same openness has a limit. Vapour can pass through the layer, but liquid water and long wetting do not dry out easily. A wet fibre loses insulating value and can rot or grow mould. The design rule is therefore to keep bulk water out with a wind barrier, a water-shedding underlay or a rain screen, and to keep the build-up open to diffusion so that any moisture can leave. Adding a vapour-tight layer to a build-up designed to be open can trap moisture inside the wall.

How is bio-based insulation different from mineral wool and EPS?

The fair comparison is with the two products most often used as defaults: mineral wool, which is non-combustible and vapour-open, and EPS, a petroleum-based foam. Bio-based insulation wins on moisture buffering and, for dense products, on summer behaviour. It usually loses on price and on fire behaviour unless it is treated. The ratings are qualitative, because performance varies between products.

Axis	Bio-based insulation	Mineral wool	EPS
Summer heat protection	Good in dense products, moderate in light ones	Moderate, low heat capacity	Modest, low heat capacity
Moisture buffering	High for wool, cellulose and wood fibre	Low, but vapour-open	Low
Fire behaviour	Combustible at fibre level, treated products do better	Non-combustible	Combustible, flame retardants are usual
Pests	Cellulose and wool usually need treatment, cork is naturally resistant	Not a food source for insects	Not a food source for insects
Price	Usually higher per unit of performance	Moderate	Usually among the lowest
Embodied carbon	Can be lower with local sourcing, depends on processing and binders	Energy-intensive production	Fossil raw material, energy-intensive production

Are bio-based insulation materials safe from fire and pests?

Not automatically. Organic fibres burn, and the question is how a given product behaves and whether it carries a treatment. Cellulose is commonly treated with borate compounds for fire and insect resistance. Sheep wool is often treated against moths and beetles, and untreated wool is less protected. Wood fibre products vary: some are treated for fire or insects, and some are not, so the material type alone does not tell you the fire performance.

Borate treatments carry their own limit. They are water soluble, so they can wash out if the insulation gets wet, which makes keeping the material dry even more important. Ask the supplier for the declared fire and pest performance of the exact product, and ask whether that performance holds after wetting.

What does bio-based insulation cost, and is it low-carbon?

Bio-based products usually cost more per unit of thermal performance than mineral wool or EPS. The reasons are smaller production volumes, more handling, and more labour at installation, especially for loose fill and natural batts. Prices vary, so ask for current quotes for the same assembly and performance.

The low-carbon claim has a real basis. The raw material is renewable and carbon stays locked in the fibres. But low-carbon is not automatic. Processing energy, binders, treatments and transport all add to the footprint, and the credit for stored carbon depends on accounting rules. Read the environmental product declaration of each product and compare like with like.

What are the honest limits of bio-based insulation?

• Cost per unit of performance is often higher, sometimes much higher, than the default products.
• Most products need a treatment against fire, insects or both, and the treatment is specific to the product.
• Loose fibres such as cellulose settle over time if they are not installed at the right density, and settling reduces insulating value.
• Water is the main enemy. Wet insulation loses performance, so storage on site and dry detailing matter from the start.
• Less stiff products need careful fitting around services and penetrations, because gaps cost more than with rigid boards.
• Availability in Slovakia is narrower than for mineral wool and EPS, so lead times can be longer.

Where does bio-based insulation fit in a Slovak house?

Bio-based insulation fits best where its strengths matter and the detailing can be controlled. Wood fibre boards suit a roof sarking layer or a ventilated timber-frame wall. Cellulose suits attics and retrofits of existing cavities, where a specialist installer checks density. Sheep wool suits timber-frame walls and roof voids with an open build-up. Cork suits floors and some facade and slab details. Straw bale construction (slamostavba) is a specialist route that needs lime or clay finishes.

Choose the build-up in design, not on site. The moisture and summer checks should include the bio-based layer, and the product should be named in the specification with its declared fire and pest treatment. Ask a Slovak supplier for current availability before the design relies on a particular product.