Basement window — two-layer basement wall (termPIR® WS)

Junction function

Basement window = the most demanding window installation in the house — below grade, in a damp environment, directly exposed to splashback and capillary moisture rise. Requires:

1. WS thermal insulation around the frame (item 07) — continuation of basement wall insulation along the frame perimeter.
2. Bitumen behind the window frame (item 06) — bonded with the wall waterproofing; lack of continuity = capillary water will penetrate into the basement interior.
3. External sill with a 10% slope (vs. typical 5% for windows above grade) and a 30 mm drip edge.
4. Low-expansion foam in the frame joint.

In practice: basement windows in a single-family house are tiny (typically 60×40 cm) and often enclosed “from the outside” using plastic basement light wells Velux UWB / Aco SF to drain rainwater.

Critical installation aspects

• Bituminous compound behind the window frame (item 06) — continued BEHIND THE FRAME (between the frame and the lintel concrete); without this, water will enter through capillary action.
• External sill with a slope of at least 10% (vs. 5% standard) — drains water faster from the recessed geometry.
• External light well (Velux UWB, Aco SF) — recommended for all basement windows with opening level below grade; eliminates splashback + soil collapse into the opening.
• Drainage grate in the bottom of the light well — channels rainwater to the perimeter drainage of the basement wall.
• Low-expansion foam (item 12) — low-expansion only; in basements humidity fluctuations are greater than in living rooms = greater movement of the structure = greater sensitivity to foam expansion.
• Ring beam / lintel (item 02) — reinforced concrete, insulated with termPIR® WS in the layer around the window (continuation of the basement wall).

Documentation

Technical Catalogue termPIR® — Residential Buildings (Gór-Stal 2022), page 45 — Two-layer basement wall, basement window detail. Scale 1:10.

Components in this junction