Pitched roof — termPIR® over rafters

When to use this system

The over-rafter system is used when the attic space is to be utilised to the maximum while still meeting WT 2021 requirements (U ≤ 0.15 W/m²·K) without sacrificing headroom. termPIR® boards are laid above the rafters, forming a continuous insulation layer uninterrupted by structural elements — this eliminates the thermal bridges that, in a conventional between-rafter layout, can reduce insulation efficiency by 15–25%.

The solution is particularly beneficial for:

• New energy-efficient and passive buildings — allows U values below 0.10 W/m²·K at 220 mm.
• Renovations and conversions — rafters remain exposed on the inside (an architecturally attractive solution).
• Rooms with limited ceiling height — low structural thickness in relation to the thermal layer.

Installation requirements

Key technical aspects derived from the system technology (source: Technical Catalogue of termPIR® Boards — Residential Buildings, pp. 11–20):

• Counter-battens fixed with self-drilling screws through the termPIR® board directly to the rafters
  • Spacing approx. every 40 cm in the internal (green) roof zone
  • In the edge zones (red) +1 screw per linear metre of counter-batten
  • Every second screw installed at a 67° angle (for better transfer of wind loads)
• Self-drilling screws — length selected according to termPIR® thickness + counter-batten + rafter (e.g. for 150 mm termPIR® → 300 mm screw).
• Board joints sealed with aluminium tape (consumption 1.25–1.5 lm/m²) or low-expansion PIR foam.
• Wind barrier laid directly on the termPIR® boards on the external side.
• In rooms with elevated humidity (bathroom, sauna) — an additional PE vapour barrier foil is required under the boards on the internal side.

Advantages of the over-rafter system

Aspect	termPIR® over rafters	Conventional between-rafter mineral wool
Thermal bridges at rafters	none	present, reduce efficiency by 15–25%
Visibility of structure from inside	rafters exposed (aesthetics)	rafters hidden in insulation
Insulation thickness required for U ≤ 0.15	150 mm	~280 mm of mineral wool
Existing roof retrofit	requires removing the covering	installed from inside, no need to strip the covering
Workmanship	quick, dry, single contractor	longer, multi-stage

Recommended termPIR® variants

• termPIR® AL — universal 50 µm aluminium foil on both sides, gas-tight. Default choice for over-rafter roofs.
• termPIR® AL R-eco — variant with recycled content, for orders with environmental requirements (LEED, BREEAM, CAM criteria for public procurement).
• termPIR® MAX 19 AL — λ reduced to 0.019 W/(m·K). Achieves U ≤ 0.15 at just 130 mm where structural height is limited.

Installation components from the BOKKA range

Apart from the termPIR® boards, the system requires standard roofing elements (covering, battens, counter-battens, wind barrier). From our range, directly for this system:

• Timber-frame screws — for fixing counter-battens to rafters through the termPIR® layer, length selected according to board thickness (e.g. 300 mm screw for 150 mm termPIR®)
• Aluminium tape — sealing of all board joints and cuts (consumption 1.25–1.5 lm/m²)

Flashings OB-XX are used in sandwich panels (insPIRe®, GS MW). In a pitched termPIR® roof, the external finish is the roof covering (tile/sheet metal) on battens and counter-battens.

Technical documentation

The system is described in the manufacturer’s catalogue “Technical Catalogue of termPIR® Insulation Boards — Residential Buildings” (Gór-Stal, 2022-04-26, pp. 11–20). Detailed connection drawings are available in the Construction Junctions section (counter-batten fixing, ridge junction, gable wall junction, roof window detail, eaves detail).

Layer composition

U-value by insulation thickness

Related catalogue items