Wood Fibre Sarking & Insulation Board – Breathable Timber-Frame Roof Board, 3 Thicknesses

Foam insulation boards do one job well: they stop heat escaping. What they don't do is let a roof breathe — trap the wrong kind of moisture behind a foil-faced PIR board on a timber-frame or period building and it has nowhere to go. Wood fibre board is the answer roofers reach for on breathable build-ups: a rigid board pressed from compressed softwood fibre, vapour-open enough to let a roof structure dry out in both directions while still doing genuine insulating and wind-bracing work.

We stock it in two distinct roles. The 22mm board is a racking and sarking board — fixed straight across rafters as a breathable alternative to OSB or plywood sarking, giving the roof structure rigidity and wind resistance before the counter-battens and covering go on. The 80mm and 100mm boards are the insulation-grade version of the same material, used where a genuinely vapour-permeable insulation layer matters more than squeezing the last millimetre of thickness out of a warm roof build-up — timber-frame extensions, listed and period reroofs, and anywhere the design calls for a breathing roof rather than a sealed one.

Key benefits

Vapour-permeable — lets moisture pass through and disperse rather than trapping it against timber. Hygroscopic — the fibre structure buffers and releases moisture vapour, which helps manage condensation risk in breathable constructions. Tongue-and-groove long edges on every board for a tight, draught-resistant joint without extra tape. Genuine wind-bracing value on the 22mm sarking board when fixed correctly across rafters. Natural, largely bio-based material for anyone specifying toward a lower-impact build-up.

Suitable applications

Pitched roof sarking/racking under tiles, slates or shingles on timber-frame construction. Warm roof and rafter-level insulation on breathable build-ups where PIR isn't the right fit. Period and listed-building reroofs where the original structure needs to keep breathing. Companion insulation layer alongside our breathable roofing membrane, which is the vapour-permeable underlay this board is designed to work with.

Specification

Installation & ordering guidance

Fix across rafters with the tongue-and-groove edges engaged, following your roofing contractor's or the manufacturer's datasheet for fixing centres and counter-batten detail — this varies with rafter spacing and roof pitch, so we don't state a single fixing pattern here. Keep boards dry and off wet ground until they're covered; like any wood-based board, prolonged exposure to standing water before installation isn't good for it. Pair the 22mm sarking board with our breathable roofing membrane over the top, and see our PIR insulation board if your build-up calls for a sealed, foil-faced warm-roof board instead of a breathable one — they solve different problems and it's worth checking which one your design actually needs before ordering.

FAQs

What's actually different between the 22mm board and the 80/100mm boards? The 22mm is a thin racking/sarking board — its job is structural bracing and breathability under the roof covering, not meaningful insulation value. The 80mm and 100mm boards are the insulation-grade version of the same wood fibre material, thick enough to contribute real thermal performance in a breathable build-up.

Should I use this instead of your PIR insulation board? Not automatically. PIR is the simpler, higher-performance-per-millimetre choice for a standard sealed warm roof. Wood fibre earns its place specifically where the design needs to breathe — timber-frame, period or listed work, or anywhere a vapour-open build-up is specified. If you're not sure which your roof needs, that's worth checking with whoever specified the build-up before you order.

Does it need to be protected once fixed? Yes — like any wood-fibre or timber board, it should be covered by the roofing membrane and covering within a reasonable window and not left exposed to prolonged wet weather.

What fixings should I use? Standard roofing screws or nails sized for the board thickness and your batten/counter-batten detail; we don't sell a proprietary fixing kit for this board, so use fixings rated for the load and spacing your roof design specifies.

How quickly does this dispatch? The 22mm and 80mm boards are quick-turnaround stock items. The 100mm board is a lower-volume line with a longer lead time — please allow around 4–5 weeks for that thickness specifically, and order early if it's on your critical path.

Custom, unbranded product — see our breathable roofing membrane and PIR insulation board for the companion products most often ordered alongside this one.