Roof window — under-rafter W2 roof (termPIR® AL + AL GK)

Junction function

Roof window in the W2 system — analogous to W1 but with an under-rafter termPIR® AL GK layer (item 07) instead of separate insulation + separate plasterboard. Dimensions from the catalogue:

• 150 mm termPIR® AL between rafters (item 06) — full depth of a standard 5×16 cm rafter + small allowance.
• 50 mm + 12.5 mm AL GK under rafters (item 07) — composite: 50 mm PIR + 12.5 mm plasterboard.

This package delivers U ≈ 0.11 W/m²K with zero insulation over the rafters (the continuous under-rafter layer eliminates the thermal bridge). Meets WT 2021 (U ≤ 0.15) with a large margin towards “passive” requirements.

Window edge insulation (item 11) is essential in both W1 and W2 (thermal bridge at the window frame).

Critical installation aspects

• Window geometry — the recess between the inter-rafter boards must be a perfect fit for the frame supplied by the window manufacturer. Velux GZL = 78×140 cm → 76×138 cm clear opening between rafters + 2 cm allowance for foam.
• Window edge insulation with termPIR® AL (item 11) — typically 40 mm around the frame, pressed against the frame + the inter-rafter board; bonded with PUR installation foam.
• AL GK board around the window — cut to the frame outline with a 10 mm allowance for adjustment; joints filled with gypsum compound, corner tape applied.
• Manufacturer’s window flashing kit — according to the window’s instructions (EDW/EDJ for Velux, XDP for Fakro). Never improvise the flashing — water will get in under the counter-battens.
• Screws (item 09) — through the plasterboard substructure → into the rafters; length = thickness of the package + 60 mm penetration into the timber.
• Internal finish — reveal corners finished with an aluminium profile protected against impact (lofts often have children, cats and general clumsiness).

Occurs in the system pitched roof — termPIR® under rafters.

Documentation

Technical Catalogue termPIR® — Residential Buildings (Gór-Stal 2022), page 29 — Pitched roof W II, roof window detail. Scale 1:10.

Components in this junction