Roof window — under-rafter system W1

Junction function

A roof window in a pitched roof is the detail most prone to water leaks and thermal bridging. The structure is complex — the window’s timber frame intrudes into the insulation layer, and 5 functions converge around the window:

1. Thermal insulation — termPIR® between rafters + under rafters (W1 variant).
2. Windproofing — vapour-permeable membrane on the exterior side.
3. Vapour barrier — PE film on the interior side.
4. Waterproofing — window collar + roof flashings draining water away.
5. Interior finish — plasterboard on framing.

Without careful sealing of all layers around the window perimeter — water ingress, condensation within the insulation, and heat losses increased by 5–8% for the entire building.

Critical installation aspects

• Sealing collar (item 07) — MANDATORY from the window manufacturer (Velux EDW for tile, EBW for sheet metal, FAKRO EZW). Without the collar — a tight installation is impossible.
• Vapour-tight sealing tape BG1 (item 11) around the frame perimeter on the interior side — connects the vapour barrier to the window frame. Class A1 (vapour-tightness).
• Vapour-permeable sealing tape BG3 (item 12) around the frame perimeter on the exterior side — connects the windproofing to the frame. Sd ≤ 0.5 m (vapour permeability).
• Under-rafter insulation min. 40 mm (item 03) around the window frame — in the W1 variant, continuity of insulation requires a termPIR® AL strip on the window frame along all 4 sides. Eliminates the thermal bridge at rafters adjacent to the window.
• Corner zone — at the window corners (4 corners), increased sealing with BG1/BG3 tapes (min. 50 mm overlap) + rigid PVC corner pieces are required.
• Water drainage — the EDW/EZW collar directs water sideways and downwards towards the roof covering, never directly onto the insulation.

Three sealing zones

Zone	Element	Material
1. Exterior (water)	roof flashing + collar	metal + EPDM
2. Middle (wind)	BG3 tape around perimeter	vapour-permeable
3. Interior (vapour)	BG1 tape around perimeter	vapour-tight

The “3 sealing zones” system — separate tapes with different vapour permeabilities MUST NOT be swapped.

Used in the under-rafter pitched roof system.

Documentation

termPIR® Catalogue — Residential Buildings (Gór-Stal, 2022), p. 25 — roof window in the W1 variant.

Components in this junction