What is rigid wood fibre board?
Wood fibre board is made from softwood broken down into fibres and pressed into boards, with a small amount of binder and wax for water resistance. Most external and roofing boards have tongue-and-groove edges, so they lock together into a continuous layer over the structure.
The boards are vapour open, so moisture can pass through and dry out of the construction. Wood fibre also has a high specific heat capacity (around 2,100 J/kgK), which means it stores heat and slows its passage through a wall or roof.
Four main uses
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External wall insulation. Board fixed outside the wall and rendered directly, or used behind ventilated cladding.
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Internal wall insulation. Board bonded and fixed to the inside face of the wall, then lime plastered.
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Above-rafter insulation. Board laid over the rafters as a continuous sarking layer, with counter battens, battens and roof covering above.
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Below-rafter insulation. Board fixed to the underside of the rafters, then lime plastered as for internal wall insulation.
Schematic sections, not to scale. Shaded layers show rigid wood fibre board; the batts between studs and rafters are flexible insulation. Roof membranes (breather, airtightness and vapour control) are not shown; include them as specified for the roof pitch and build-up.
External wall insulation. Boards are fixed to the outside of masonry or timber frame walls. They are either rendered directly, as part of a wood fibre render system, or used behind ventilated cladding. Insulating outside the wall keeps the structure warm and reduces thermal bridging at floors and junctions.
Internal wall insulation. Boards are bonded and fixed to the inside face of a solid wall, then lime plastered. This suits buildings where the external appearance must not change, such as listed or terraced buildings.
Above-rafter insulation. Boards are laid over the rafters as a continuous insulating sarking layer, beneath counter battens, battens and the roof covering. They are usually combined with flexible insulation between the rafters, and they provide temporary weather protection during construction.
Below-rafter insulation. Boards are fixed to the underside of the rafters and lime plastered, in the same way as internal wall insulation. This adds insulation and closes the thermal bridge through the rafters, and it can be used with or without board above the rafters. It suits roofs where the coverings are staying in place, and rooms in the roof where a breathable plaster finish is wanted.
Choosing a board
We stock three rigid wood fibre boards. Two are external boards for walls and roofs; the third is made for insulating the inside of masonry walls.
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STEICO Special Dry |
Pavatex Isolair Multi |
STEICO Internal |
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|---|---|---|---|
|
Density |
140 kg/m³ |
145–200 kg/m³, by thickness |
160 kg/m³ |
|
Thermal conductivity λD |
0.040 W/mK |
0.041–0.044 W/mK, by thickness |
0.038 W/mK |
|
Vapour resistance μ |
3 |
3–4 |
5 |
|
Edges |
Tongue and groove |
Tongue and groove |
Square edge |
|
Thicknesses made |
60–200mm |
30–200mm |
40–100mm |
|
Weather protection |
Up to 4 weeks on roofs of 14° and above |
Up to 3 months |
Internal use only |
|
Main uses |
External walls, above and below rafters |
Above and below rafters, internal and external walls |
Internal wall insulation on masonry |
|
Reaction to fire |
E |
E |
E |
STEICO Special Dry is the lightest of the three and insulates best per millimetre among the external boards. It is used rendered or behind cladding on walls, and above or below rafters in roofs.
Pavatex Isolair Multi is denser in thinner boards and lighter in thicker ones. It is water-repellent and windproof, and gives the longest temporary weather protection, which makes it well suited to sarking on roofs that may stay uncovered for a while. It has the widest range of uses of the three: above and below rafters and on both internal and external walls.
STEICO Internal is made for the inside face of masonry walls. It has the lowest thermal conductivity of the three and takes lime and clay plasters directly. Its moisture content should be 13% or below before plastering.
In Bio Space's IWI build-up, Special Dry and Isolair Multi are also options on the inside of the wall, alongside STEICO Internal. Cork board is an alternative in the same position. Check each product page for the thicknesses we stock.
Fixing and finishing
On masonry, boards are bonded with a lime mortar such as Secil Isovit Lime, applied to the back of the board with a notched trowel, then fixed with insulated masonry fixings.
On timber frame, boards are pinned to the frame. For cladding, counter battens are screwed through the boards into the frame, then battens and cladding follow.
Below rafters, boards are screwed to the underside of the rafters with insulation fixings, then plastered in the same way as internal wall insulation.
Rendered and plastered finishes go on as a mesh-reinforced base coat followed by a thin lime finish coat. The mesh spreads stresses across board joints and is essential to prevent cracking.
Moisture and fire
Wood fibre manages moisture well, but it does not cure a damp wall. Before insulating solid masonry, deal with rain penetration, defective pointing, leaking gutters and high ground levels. On walls exposed to driving rain, or where the construction is unusual, a moisture risk assessment is worthwhile before choosing internal insulation.
Wood fibre boards are combustible; STEICO Special Dry, for example, is reaction to fire class E. The use of combustible materials in external walls is restricted on some buildings, depending on height and use. Check Approved Document B, or its equivalent in Wales, Scotland or Northern Ireland, before specifying.
