What’s Inside Your Walls Affects the Air You Breathe
Insulation shapes indoor air quality in two ways: through the compounds it releases (or doesn’t) into your home, and through how it handles moisture within the wall structure. Wood fibre and hemp insulation contain no formaldehyde binders and no halogenated flame retardants, while managing moisture in ways that conventional alternatives cannot. Here’s why the material you never see deserves your attention.
Quick Takeaways
1
Insulation hidden within walls can off-gas into living spaces; choosing formaldehyde-free options removes a source of indoor pollution you’d otherwise never suspect
2
Wood fibre and hemp insulation buffer moisture, reducing condensation risk and the mould that follows
3
Conventional options (mineral wool, PIR foam, EPS) insulate effectively but don’t interact with moisture and may contain chemicals of concern
The Invisible Neighbour
You’ll never touch your insulation once it’s installed. It sits behind plasterboard, under floors, above ceilings. Out of sight, out of mind. But it occupies more volume inside your walls than any other material, and what it’s made of influences what reaches your indoor air.
Off-gassing from insulation materials can migrate through gaps, service penetrations, and imperfect air barriers into living spaces. This migration is slow but continuous. In a bedroom where you spend eight hours breathing, a ceiling void filled with material that releases formaldehyde at low levels adds a quiet, persistent source to your exposure.
Most people choosing insulation focus on thermal performance (the lambda value, or how well it resists heat flow) and cost. Both matter. But the material’s chemical composition and its behaviour around moisture matter too, and these rarely feature in the conversation.
What’s in Conventional Insulation
Mineral wool (glass wool or rock wool) is the most common insulation in European homes. Effective, fire-resistant, affordable. Older formulations used formaldehyde-based binders; many current products have moved to formaldehyde-free alternatives, but not all. Check the product data sheet. Mineral wool is vapour-permeable but not hygroscopic: water vapour passes through it, but the material doesn’t absorb or buffer moisture.
PIR and PUR foam boards deliver high thermal performance in thin profiles. These foams may contain flame retardants and are vapour-impermeable, acting as barriers to moisture movement. In a breathable wall system, they don’t belong. In a sealed construction with mechanical ventilation, they function as intended.
Expanded polystyrene (EPS) is inexpensive and widely used. It contains no formaldehyde, but it’s petroleum-based, doesn’t buffer moisture, and can release styrene at low levels.
How Wood Fibre and Hemp Work Differently
During a loft insulation project last autumn, we handled three materials side by side: standard mineral wool batts, wood fibre flexible batts from Steico, and hemp fibre batts from Biofib. The mineral wool required gloves and a mask; fine glass fibres irritated skin and airways within minutes. The wood fibre batts felt like handling dense, dry felt. No itch, no dust, no respiratory irritation. The hemp was similar, with a rougher, more fibrous texture.
That handling experience reflects a deeper difference in composition.
Wood fibre insulation is made from softwood processing residues, bound with natural resins or polyolefin fibres. No formaldehyde binders. No halogenated flame retardants (the dense wood fibre itself meets fire regulations through its charring behaviour). Lambda values range from 0.036 to 0.043 W/mK, comparable to mineral wool at similar thicknesses.
Hemp fibre insulation uses bast fibres from the hemp stem, bound with polyester or starch-based binders. Again, no formaldehyde. No synthetic flame retardants. Thermal performance sits around 0.038–0.042 W/mK.
Both materials are hygroscopic: they absorb moisture from the surrounding air when humidity is high and release it when conditions dry. Wood fibre can buffer up to 15% of its weight in moisture without losing insulation performance. This turns the insulation layer into part of the building’s humidity regulation, working with breathable finishes on the interior to smooth out the peaks and troughs described in “Humidity: The Goldilocks Zone.”
Both are also vapour-permeable, allowing moisture to move through the wall assembly. In older buildings with solid walls, where sealing moisture in causes more problems than it solves, this compatibility is essential. “Your Walls Can Breathe” covers the broader principle; here, insulation is the hidden layer that makes it work.
Cost and the Honest Trade-Off
Wood fibre costs roughly 50–100% more than standard mineral wool. Hemp sits in a similar range. For a typical loft insulation project (50 m² at 200 mm depth), the premium might be £500–1,000 over mineral wool.
That premium buys moisture buffering, healthier composition, and superior summer heat protection (wood fibre’s higher density slows heat transfer, keeping rooms under the roof cooler). If you’re spending £8,000–15,000 on a whole-house insulation retrofit, the difference between mineral wool and wood fibre is a relatively small proportion of the total. The insulation stays in your walls for decades.
Putting It All Together
Insulation works best as part of a system. Wood fibre or hemp behind the plasterboard, a breathable membrane managing vapour flow, clay or lime plaster on the interior surface. Each layer contributes to moisture management while avoiding adding pollutants to your air.
If you’re working with an installer, ask about the whole wall build-up. A wood fibre batt behind vapour-sealed vinyl paint loses much of its moisture-buffering benefit. The layers need to work in the same direction.
Products to Explore
Steico and Gutex produce a range of wood fibre insulation products, from flexible batts for DIY loft work to rigid boards for external wall systems. Biofib and Technichanvre offer hemp fibre batts in standard dimensions. Look for products carrying natureplus certification, which confirms both health and environmental standards. For wall assemblies, breathable membranes from Intello (pro clima) or similar brands complete the system.
Common Questions
Can I install wood fibre insulation myself?
Flexible batts handle much like mineral wool and suit DIY installation in straightforward applications: between ceiling joists, between wall studs, in accessible loft spaces. More complex work (external insulation, breathing wall assemblies in older buildings) benefits from an experienced installer who understands moisture management.
Is wood fibre insulation a fire risk?
Wood fibre is combustible, but it achieves good fire ratings through density, charring slowly and predictably. No chemical flame retardants are needed. Dense wood fibre boards meet Euroclass ratings required for most residential applications. Check the specific product’s fire rating before specifying.
How does the cost compare over the long term?
Wood fibre’s durability and stability mean it doesn’t sag or settle over time the way some lighter insulation products can. Its moisture tolerance reduces the risk of damp-related problems that might require remedial work later. The cost per year of service is often closer to mineral wool than the upfront price suggests.
Will natural insulation attract pests?
Modern wood fibre and hemp products are treated with boron-based compounds (low toxicity to humans, effective against insects and fungi) to resist biological degradation. In a properly detailed wall assembly, pest problems are no more likely than with conventional insulation.
What about sheep’s wool insulation?
Sheep’s wool is another natural option with excellent moisture-buffering properties and no harmful emissions. It’s softer to handle, carries a higher price point, and works well in smaller applications like between-joist insulation. Thermafleece and Isolena are established brands. It complements wood fibre and hemp as part of the natural insulation range.
