Knee wall — over-rafter roof (termPIR® AL + ETX)

Function of the junction

The knee wall is that short wall section (typically 60–120 cm) between the slab of the top storey and the wall plate — the most challenging element of a pitched roof to insulate properly. In poorly designed projects this is where a thermal bridge and water vapour condensation in the insulation layer appear.

The Gór-Stal catalogue solution:

1. Wall plate insulated with a termPIR® AL board (item 11) — essential, it eliminates the thermal bridge where the wall plate is seated on the masonry.
2. Knee wall externally insulated in the termPIR® ETX ETICS system (item 09) — continuation of the façade insulation up to the roof.
3. Over-rafter termPIR® AL boards (item 07) run down onto the wall plate and meet the wall plate insulation and, internally, the ETX insulation.
4. Aluminium tape (item 06) seals all board joints — maintaining vapour tightness.

Critical installation aspects

• Wall plate insulation with termPIR® AL (item 11) — an element often omitted; without it a linear thermal bridge ψ ≈ 0.15 W/m·K develops between the wall plate (timber, λ ≈ 0.13) and the steel anchor. With insulation ψ < 0.03 W/m·K.
• ETX (wall) to termPIR® AL (roof) joint — requires aluminium tape along the full length of the seam plus silicone on the outside (beneath the wind barrier).
• Wind barrier (item 05) carried from the roof onto the wall with an overlap ≥ 200 mm, bonded to the ETX with butyl tape.
• Boards trimmed at the joint with the wall plate — seal the gaps with PUR foam, cut off the excess once cured and cover with Al tape.
• Damp rooms under the roof — additional PE vapour barrier on the inside, beneath the termPIR® layer, with an overlap onto the knee wall.

Used in the pitched roof — over-rafter termPIR® system.

Documentation

Technical Catalogue termPIR® — Residential Buildings (Gór-Stal 2022), page 16 — Pitched roof, over-rafter system, detail of the knee wall to roof junction. Scale 1:10.

Components in this junction