Wood fibre insulation
Thermal insulation made from wood fibres, supplied as flexible batts or rigid boards. Its strengths are summer stability in a loft and low embodied carbon.
What forms does wood fibre insulation come in?
Wood fibre insulation is made from wood fibres bound into batts or boards, and it comes in two forms that do different jobs. Flexible batts are soft and conform to the opening; they go between rafters and timber-frame studs, and their conductivity is roughly 0.038 to 0.040 W/(m·K). Rigid boards are pressed to a higher density and serve as a sarking layer over the rafters, as a render substrate on a facade, or as an internal lining layer; their conductivity is 0.040 to 0.048 W/(m·K).
Rigid boards frequently do two jobs at once, which is where their real economy lies. Over the rafters they form a diffusion-open, water-shedding layer with an equivalent air-layer thickness of 0.3 m or less that also adds thermal resistance and stops wind washing the insulation beneath. On a timber frame facade they take on the role of substrate for a render system, which removes the need for a separate support plane and one whole trade from the sequence.
Why does it look worse on paper and perform well in summer?
Compare conductivity values and wood fibre loses to white polystyrene at 0.036 to 0.040 W/(m·K) and clearly to graphite grades at 0.030 to 0.032 W/(m·K). For the same U-value the build-up is therefore thicker, typically by a few centimetres in a pitched roof. That is the honest price of what the material delivers elsewhere.
What matters elsewhere is specific heat capacity together with density. A wood fibre board stores substantially more heat than foam of the same thickness, which slows the heat wave passing into the interior, lowers its peak and pushes it into later hours. In a loft, the critical space in a Slovak summer, that is the difference between a usable and an unusable bedroom. Thermal storage does not remove the gain, it only spreads it over time, so glazing ratio, orientation, external shading and night ventilation remain the decisive levers, and the criterion for the highest daily air temperature in a critical room sits in STN 73 0540 Part 2.
| Criterion | Wood fibre | Mineral wool | EPS and XPS |
|---|---|---|---|
| Conductivity | 0.038 to 0.048 W/(m·K) | 0.032 to 0.045 W/(m·K) | 0.029 to 0.040 W/(m·K) |
| Heat storage per unit volume | High | Low | Low |
| Reaction to fire | Class E, some products D | Class A1 | Class E as a board |
| Humidity buffering | Strong | Moderate | None |
| Embodied carbon | Lowest of the three | Middle | Highest per unit mass |
| Price at equal U-value | Highest | Middle | Lowest |
Where does wood fibre belong in a build-up?
The material is hygroscopic: it takes moisture into the fibre structure, stores it temporarily and releases it when conditions change. That buffers short humidity peaks and reduces the risk of condensate appearing faster than the build-up can dry it out. It only makes sense, though, in a vapour-open build-up that has somewhere to dry to.
- Inside face: a vapour retarder of roughly 0.2 to 10 m, not a full barrier, and often a variable membrane.
- Structural plane: flexible batts between rafters or studs, laid without gaps and without compression.
- Outside face: a rigid wood fibre sarking board or a diffusion-open membrane of 0.3 m or less.
- Layer order: vapour resistance falls from inside to outside, roughly by a factor of five or six across the build-up.
If a layer with a higher resistance ends up on the outside than on the inside, the build-up stops working and the timber inside it is at risk. Wood fibre is therefore never combined with a vapour-tight external layer such as fully supported sheet metal on solid boarding, and never sealed between two tight membranes, which is the fastest way to waste both the material and its price.
What does wood fibre insulation cost you?
| Cost item | What it means in practice |
|---|---|
| Price of the material | Usually the most expensive of the common options at equal U-value |
| Build-up thickness | A few centimetres more, which also raises the cost of rafters, reveals and flashings |
| Fire class | Combustible, so it needs a fire assessment on anything larger than a house |
| Weight | Heavier to handle than mineral wool, which slows installation on a roof |
| Protection on site | Must stay dry, so the sequence has to close the envelope before it goes in |
Only a priced schedule for a specific project gives a real figure, because the thickness penalty ripples into the structure. What the material returns is a genuinely better summer, a build-up that forgives a wetting, and a materially better carbon profile.
Why does wood fibre score so well on embodied carbon?
The fibres come from timber that has bound carbon during growth, and the manufacturing process is less energy-intensive than foaming polystyrene or melting rock. In embodied carbon assessments wood fibre therefore comes out substantially better than either of its main competitors, and in a whole-life assessment of a timber house it can shift the total materially.
Two cautions belong with that claim. The benefit is real only if the material stays in the building for its full service life, so a build-up that has to be opened and replaced early loses most of it. And the carbon stored in the fibre is not permanently removed from the atmosphere; it is deferred for as long as the board remains in place. Both points argue for the same thing: design the build-up so that it never has to be disturbed.
Frequently asked questions
- Is wood fibre a worse insulator than polystyrene?
- On conductivity alone, yes. Flexible wood fibre batts sit at roughly 0.038 to 0.040 W/(m·K) and rigid boards at 0.040 to 0.048, against 0.036 to 0.040 for white polystyrene and 0.030 to 0.032 for graphite grades. For the same U-value the build-up is therefore a few centimetres thicker. What the material buys back is summer behaviour and moisture buffering.
- Why does wood fibre perform better in a loft in summer?
- Because of its specific heat capacity combined with its density. A wood fibre board stores substantially more heat than foam of the same thickness, which slows the heat wave passing into the room, lowers its peak and pushes it into later hours. In an attic bedroom that difference decides whether the room is usable in August.
- Can wood fibre be combined with a vapour barrier?
- It should not be. The material belongs in a build-up that can dry, so the inner layer is a vapour retarder of roughly 0.2 to 10 m, often a variable membrane, and the outer layers are diffusion-open. Sealing it between two tight layers removes the drying reserve that is the whole reason for choosing it.
- Is wood fibre insulation combustible?
- Yes. Wood fibre boards are reaction to fire class E to EN 13501-1, with some products reaching class D, so unlike mineral wool at class A1 they contribute to a fire. On a single-family house this is usually not an obstacle; on larger buildings the fire assessment to STN 92 0201 and the fire authority sign-off decide the matter.
- Does wood fibre rot if it gets damp?
- It buffers humidity safely but it does not tolerate bulk water. A soaked board loses thickness and its fibres degrade, and repeated wetting invites mould. The material is therefore protected during construction and placed only in build-ups that have a demonstrated drying route, verified by a moisture calculation rather than by the material’s reputation.