Top-storey floor + knee wall

Junction function

The top-storey floor / knee wall junction is the trickiest detail when insulating an attic from the room side. It requires continuity across 3 insulation layers:

1. termPIR® AL on the floor (item 12) — from the attic side.
2. termPIR® AL on the wall plate (items 10, 11) — in the strip above the knee wall.
3. termPIR® ETX on the knee wall (item 06) — from the outside.

All 3 layers meet at the critical point in the wall-floor-wall plate corner. Without proper insulation of this corner, a linear thermal bridge ψ ≈ 0.3 W/m·K forms = condensation on the ceiling of the top-storey room in winter.

Critical installation aspects

• Wall plate insulation (items 10, 11) — 2 layers (as shown in the drawing): the first under the wall plate, the second on top of the wall plate; 60–80 mm of termPIR® AL in total.
• ETX on the knee wall (item 06) — continued all the way up to the wall plate; the insulation strip fully wraps the floor ring beam (item 08).
• termPIR® AL on the floor (item 12) — runs from the roof down onto the wall plate (a narrower strip), meeting the wall plate insulation (item 11); gaps are to be filled with installation foam.
• Aluminium tape (item 13) — on all board joints; particularly critical at the wall plate-floor corner.
• PUR adhesive (item 14) — Ceresit CT 84 or PCI Pecimörtel; full-surface bonding of boards to reinforced concrete / masonry.
• Additional insulation material (item 16) — the catalogue lists optional mineral wool as a filler for geometric gaps; in a modern house it is better to use PIR throughout to maintain uniform vapour tightness.
• Reinforced concrete ring beam (item 08) — must be fully wrapped in termPIR® (underneath, on the side, on top); this is the most commonly neglected detail in older houses.

Documentation

Technical Catalogue termPIR® — Residential Buildings (Gór-Stal 2022), page 62 — Top-storey floor, knee wall insulation. Scale 1:10.

Components in this junction