Prepare your attic conversion site: substrate, centres and real-world constraints
Identify the roof structure type and the condition of the underlay before starting
Before installing wood fiber insulation on roof slopes, identify the roof structure (traditional framing or prefabricated trusses) and check the straightness of the rafters. Inspect the roof underlay (HPV or not), counter-battens, and air inlets/outlets at the eaves and ridge. Check for moisture, signs of leaks, and blackened timber. Without a sound underlay, there is no insulation.
Measure the rafter centres and set out your layout for a gap-free installation
Measure the centre-to-centre spacing across 3 to 5 bays, because the actual spacing varies. Lay out your semi-rigid boards with an oversize allowance of 5 to 10 mm so they stay in place without excessive compression. Correct twisted rafters before closing up.
Anticipate singular details: purlins, valleys, hatches, downlights and service runs
Treat purlins, valleys and trimmers as areas requiring cuts and adjustments. Prepare openings for hatches, IC-rated downlight housings and service runs. Maintain continuity of the vapour barrier and airtightness with tapes and collars, then plan the service cavity framing. To secure this point, also see the typical air leaks around hatches and service penetrations.
Choose the right thickness and build-up for high-performance wood fiber roof-slope insulation
Determine the wood fiber thickness in roof slopes according to the available space (between rafters, under rafters, or a combination)
On site, check the usable height of the rafters, flatness, services and openings. Size the thickness from the declared lambda (ACERMI) and a target thermal resistance, then choose the build-up. Between rafters only if the depth is sufficient. Under rafters if you need to top up without reworking the structure. A combined build-up if you are aiming for thermal continuity and a clean services route. The installation of wood fiber insulation on roof slopes quickly becomes more reliable in a combined build-up, because you keep a continuous layer on the inside face.
| Target thermal resistance R (m².K/W) | Conductivity λ (W/m.K) | Theoretical thickness e (mm) |
|---|---|---|
| 6 | 0,040 | 240 |
| 7 | 0,040 | 280 |
Choose the right format: semi-rigid vs rigid boards, density and long-term stability
For roof slopes between rafters, favour semi-rigid boards with a slight oversize for stable fit without sagging. Rigid boards are mainly justified in a continuous support build-up (sarking, counter-battening) or when the external layer requires better dimensional stability. In your quote, note the product reference, thickness, certified λ and the density range stated by the manufacturer.
Manage thermal bridges: crossed double layer and rafter treatment without unnecessary extra thickness
A crossed double layer limits the thermal bridges at the rafters and simplifies layout. Aim for a continuous layer under the rafters to “break” the structural timber, without stacking unnecessarily. Keep a continuous air-tight membrane and a variable-perm vapour control layer where the build-up requires it. Hangers and supports must be compatible with the total thickness to avoid settling and air leaks. For more detail on order-of-magnitude values and the method, use this thermal resistance calculation. Argile’s free thermal resistance calculator stacks the layers and gives the wall R and U-values.
Installing wood fiber insulation on roof slopes correctly: step-by-step method (cutting, holding, continuity)
Cut accurately and fit tightly: tolerances, compression to avoid and joint continuity
For wood fiber insulation installation on roof slopes, start from a layout. Measure the actual centres, then cut each board to the correct width so it holds by friction without forcing it in. Wood fiber that is compressed too much loses effective thickness and creates depressions on the underside. Install with staggered joints between layers, trim ends at purlin lines and avoid any continuous gap, especially at the perimeter.
Keep the insulation in place on roof slopes: hangers, battens, lashing and supports without crushing the insulation
Plan the retention before closing up. Compatible hangers, well-aligned battens and, if needed, temporary lashing or strapping to hold the insulation until the lining is installed. The key point is no settling. Check flatness, and keep compression even only where the fixing system requires it.
Pay attention to critical junctions: eaves, ridge, verges and wall connections
At singular details, aim for continuity. At the eaves, close the joint against the masonry element and fill any gaps. At the ridge and verges, trim neatly and block voids. Finish with air and vapour sealing. Membrane, tapes and collars installed on dust-free substrates. This is where singular details are decided.
Vapour barrier in attics: airtightness, vapour management and continuity of connections
Understand the role of the vapour barrier on the warm side: preventing condensation in the wood fiber
On the heated side, the vapour barrier does two things. It limits parasitic air movement, which carries far more moisture than simple diffusion. It also regulates vapour flow to keep the assembly dry. In wood fiber roof-slope installation, this is what prevents interstitial condensation at the contact with cold zones (rafters, purlins, counter-battens). To quantify the risk, rely on the dew point calculation and adapt the membrane type to the intended build-up and its vapour openness.
Achieve a continuous installation: overlaps, tapes, collars and perimeter connections
The site rule is continuous connections. Install taut sheets, regular overlaps, systematic taping. Treat singular points with dedicated tapes, adhesive sealant and collars. Around the perimeter, connect the membrane to walls, wall plates, openings and roof-slope bases on a clean, stable substrate. Counter-batten to protect the sealed joints before lining.
Watchpoints in 2026: recessed lights, electrical boxes and penetrations (zero leaks)
Goal: zero leaks. Recessed lights should be handled with protective covers and maintained safety clearances, without puncturing the membrane. For electrical work, prefer a service void or airtight boxes installed on the inside. Any penetration (MVHR, antenna, ducts) must be made with a collar, perimeter sealing and a visual check, with photos for the file.
Air gap and condensation risk: avoid mistakes that ruin the job
Know when a ventilated air gap is needed — and when it is a false good idea
On roof slopes, the need for an air gap is determined first by the roof underlay. With an HPV membrane, the insulation can be placed in contact. Adding an air gap on the cold side often becomes counterproductive. You lose thermal resistance and create parasitic air movement. Without an underlay or with a non-HPV underlay, provide a continuous ventilated air gap between the roof covering and the insulation, connected to the air inlets and outlets required by the DTUs.
Common moisture-causing mistakes: poorly sealed vapour barrier, unsuitable underlay, compressed insulation
In a wood fiber roof-slope installation, the insulation can handle humidity peaks, but not trapped water. Typical causes are a vapour barrier that is interrupted or poorly sealed, unsuitable tapes, penetrations that are not sleeved, an external membrane that is not compatible with the system, and insulation compressed at the supports.
End-of-installation checks: visual inspection, continuity, cold spots and correction of defects before closing up
- Check the continuity of the membrane.
- Verify corners, ends and connections to the purlins.
- Treat every penetration and every duct.
- Identify cold spots before lining.
- Correct the defects, then close up.
