Under-rafter roof — ceiling junction

Junction function

Junction of pitched roof with flat ceiling below the collar beam (15) — a typical detail of habitable attics, where the horizontal part of the ceiling is formed by insulation suspended below the collar beams (15), and the sloped part by under-rafter insulation (07) + between-rafter insulation (06).

A two-layer system (W I) — complete elimination of the rafter thermal bridge:

• Between-rafter layer (06) — between the rafters, insulating the inter-beam space.
• Under-rafter layer (07) — beneath the rafters, eliminating the thermal bridge through the timber of the rafters (timber lambda 0.15 vs PIR lambda 0.022 → ~7× worse).

At the junction with the flat ceiling, both layers must transition continuously onto the ceiling plane — without this, a corner thermal bridge forms at the collar beam.

Critical installation aspects

• Between-rafter thermal insulation (06) — cut to ±2 mm to the between-rafter width. Joints sealed with aluminium tape (08).
• Under-rafter thermal insulation (07) — a second course beneath the rafters fixed to the sub-structure (10). Thickness typically 40–60 mm.
• Horizontal ceiling beneath the collar beam — continuation of the under-rafter termPIR AL on the sub-structure of the ceiling framework (10) — without any break.
• Joint gap (12) slope → horizontal MUST be filled with low-expansion PIR foam. The slope of the panels and their cutting leaves small gaps that form a thermal bridge.
• Vapour barrier (11) continuous from the eaves edge to the collar beam — any break in the vapour barrier → condensation within the insulation.
• Ceiling hanger (09) with a thermal-insulating washer at the penetration point — eliminates a point thermal bridge.

Typical U-value

For termPIR AL 200 mm between rafters + 50 mm under rafters:

• Roof U ≈ 0.10 W/(m²·K) (passive class)
• Rafter thermal bridge Ψ ≈ 0.01 W/(m·K) — with under-rafter insulation
• Without under-rafter layer: Ψ ≈ 0.12 W/(m·K) → 12× greater losses

Used in the under-rafter pitched roof termPIR system.

Documentation

termPIR catalogue — Single-Family Houses, p. 23 — pitched roof W I, junction with ceiling.

Components in this junction