Basement window — 3-layer wall — termPIR® AL

Junction function

A basement window is a detail with elevated risk of moisture exposure — the window is partially below ground level, meaning continuous exposure to rainwater and groundwater. Three critical functions:

1. Waterproofing around the frame perimeter — torch-on bitumen membrane + bituminous compound in a 30 cm band around the window.
2. Drainage well — discharge of rainwater from the external sill into the perimeter drainage.
3. termPIR® insulation continuity around the frame in the 3-layer cavity — elimination of thermal bridging.

Critical installation aspects

• Window frame (item 04) — PVC with reinforcing core or aluminium with thermal break. Wooden windows are unsuitable for basements (moisture + fungal growth).
• Drainage well (item 05) — minimum depth 20 cm below the window sill. Connected to perimeter drainage (item 08) via a short pipe section.
• Liquid waterproofing (item 06) continued around the frame perimeter — with 15 cm overlap onto the frame. Critical — without it groundwater seeps between the frame and the wall.
• PIR assembly foam (item 10) fills the gap between the frame and the wall from the inside — ordinary PU foam is insufficient (absorbs moisture).
• Vapour-tight tape (item 09) around the frame perimeter on the basement side — eliminates condensation of basement vapour within the frame.
• External sill with drip edge (item 11) — the bent edge directs water beyond the outline of the drainage well.
• Stainless steel A2 anchors (item 13) — connecting load-bearing wall + cladding wall + window frame. Black steel is not permitted (corrosion in damp environments).

Drainage well — types

Well type	Application	Price
Prefabricated PVC (e.g. Aco)	typical single-family house	base
Cast-in-place concrete	public utility buildings	+50%
Steel with grating	industrial facilities	+30%

Featured in the three-layer foundation system.

Documentation

termPIR® Catalogue — Residential Buildings (Gór-Stal, 2022), p. 49 — basement window in 3-layer wall.

Components in this junction