Ceiling detail — under-rafter roof W1 (termPIR® AL × 2 layers)

Junction function

Attic ceiling in the under-rafter W1 system = logic analogous to the roof, but applied to the horizontal attic ceiling. Two layers of termPIR® AL:

1. Between collar beams (item 03) — boards pushed between the collar beams, cut to size.
2. Below collar beams (item 04) — boards across the entire surface beneath the collar beams, eliminating the thermal bridge at every collar beam (timber λ ≈ 0.13).

This arrangement only makes sense when the attic is non-habitable = a traditional loft. For a habitable ceiling (walked on) a single thicker layer between the beams plus an OSB board as flooring is preferable.

Critical installation aspects

• Inter-beam boards (item 03) — cut to ±5 mm relative to the clear span between collar beams; fill any gaps with PUR mounting foam (note: this time HIGH-EXPANSION, not low-expansion — the gap is between heavy timber elements).
• Under-beam boards (item 04) — fixed to the collar beams with self-drilling screws with a 60 mm washer, spacing 30 cm between fixings; the thermal bridge at each collar beam is eliminated.
• Aluminium tape (item 05) — a key element: at ALL joints (inter-beam to collar beam, under-beam to under-beam, under-beam to inter-beam). Failure to comply = water vapour loops + condensation on the OSB.
• OSB (item 01) above — with a 3 mm expansion gap between boards, to allow for timber movement; fixed to the collar beams (NOT to the termPIR).
• Hanger (item 06) fixed ONLY to the collar beam — an anchor in termPIR® will pull out at the first gust.
• PE vapour barrier (item 08) — between the plasterboard substructure and the under-beam board, required for bathrooms / kitchens / drying rooms.

Occurs in the system pitched roof — termPIR® under-rafter.

Documentation

Technical Catalogue termPIR® — Residential Buildings (Gór-Stal 2022), page 22 — Pitched roof W I, ceiling detail. Scale 1:10.

Components in this junction