Installing Wood Fibre Insulation in Timber Frame Walls
Brettstapel wall panel showing structural timber, sheathing board, and external wood fibre insulation layer. Source: Wikimedia Commons / Jamesh251 (CC BY-SA 3.0)
Typical timber frame wall assembly
A timber stud wall intended to meet current Polish thermal requirements (WT 2021) typically consists of the following layers from interior to exterior:
- Interior finish (plasterboard or timber board)
- Service cavity battens (for electrical and plumbing routes)
- Vapour control layer (VCL) — airtightness layer
- Structural stud frame with insulation between studs (mineral wool or wood fibre batt)
- Structural sheathing board (OSB/3 or wood fibre sheathing)
- Wind-tight breather membrane or rigid wood fibre board
- Ventilated rainscreen cavity or render on rigid board
- External cladding or render
The exact configuration depends on the structural system, climate exposure, and the U-value target. Some designs use a hybrid approach: insulation between studs plus an external layer of rigid wood fibre board, which improves thermal bridging performance at the stud lines.
Vapour control layer placement
The VCL is placed on the warm side of the insulation — in a standard Polish climate, this is the interior face of the stud frame. Its function is to limit moisture migration from the interior into the insulation layer, where condensation could occur during winter.
Common materials for the VCL include polyethylene membrane (sd value typically 50–150 m) and variable-resistance membranes (sd value 0.3 m in summer, up to 10 m in winter). Variable membranes are increasingly specified in wood fibre assemblies because they allow the wall to dry inward during summer, reducing accumulated moisture risk.
Airtightness of the VCL is as important as its vapour resistance. All laps must be taped with a compatible adhesive tape; penetrations for electrical boxes, pipework, and wall ties must be sealed individually. An untaped VCL joint performs well below its rated sd value.
Between-stud insulation — flexible batts
Wood fibre flexible batts are cut to a width slightly larger than the stud spacing (typically 600 or 625 mm centres) so they compress slightly and hold friction-fit without adhesive. Standard batt thicknesses range from 60 mm to 200 mm; multiple layers can be used for deeper stud sections.
Batts should fill the stud cavity completely without voids at edges or corners. Cutting around noggins and blocking requires care: pieces must be cut and fitted individually rather than compressed around an obstruction, which creates uninsulated zones.
External rigid board — fixing methods
Rigid wood fibre boards on the external face of a timber frame are fixed mechanically through the sheathing board into the stud frame. Two fixing types are in common use:
- Large-head plastic fixing screws: designed to grip the insulation board without crushing. Head diameter typically 60 mm minimum for boards up to 160 mm thick. Penetration depth into structural timber minimum 40 mm.
- Batten-and-screw systems: vertical timber battens are fixed through the insulation into the studs, then external cladding is fixed to the battens. This creates a ventilated cavity and allows longer insulation thicknesses without overstressing individual fixings.
The number of fixings per board (typically 8–12 for a 1200 × 600 mm board) and the fixing pattern (perimeter and field) are specified by the system supplier and should be followed to ensure wind load resistance.
Breather membrane or rigid board as wind barrier
The external face of the insulation layer must be wind-tight. Two approaches are used:
- Breather membrane over flexible insulation: the membrane acts as a wind barrier and resists rain ingress to the cavity. The membrane must be rated for the roof or wall wind load class.
- Rigid wood fibre board as sheathing and wind barrier: dense facade boards (typically 200–350 kg/m³) can function as the structural sheathing, wind barrier, and insulation in a single element. Joints between boards must be taped or overlapped to prevent air infiltration paths.
Thermal bridging at stud lines
Timber studs have a thermal conductivity of approximately 0.13 W/(m·K) — significantly higher than the 0.036–0.042 W/(m·K) of the insulation. In a 145 mm stud frame with studs at 600 mm centres, the effective U-value of the wall as built is substantially higher than the calculation for the insulation alone would suggest.
The correction for thermal bridging is quantified using the isothermal planes method described in EN ISO 6946. Adding an external layer of continuous insulation — wood fibre board without interruptions — substantially reduces the bridging correction factor (ΔU). A 60 mm external layer of rigid board reduces the correction from a typical 0.04–0.06 W/(m²·K) to around 0.01–0.02 W/(m²·K).
Common installation errors
Gaps at stud flanges
Flexible batts installed in a hurry often leave a narrow uninsulated gap along the face of the stud. The batt should be pressed firmly to the stud face and held in position by the VCL or sheathing before fixing.
Unsealed VCL penetrations
Electrical back-boxes recessed into the insulated stud cavity without a pre-formed airtight box create a permanent leak point. Pre-formed airtight back-boxes or surface-mounted conduits within the service cavity are the standard correction.
Compressed insulation at junctions
At window reveals, eaves, and sill junctions, insulation is sometimes compressed rather than cut to fit. Compressed insulation increases density locally but may increase λ and creates discontinuities in the vapour control layer.
Missing cavity closer at party walls
The cavity between external cladding and insulation board must be closed at the top and at any internal junction to prevent fire spread through the cavity. This is a specific requirement under WT.