Between-rafter roof — knee wall junction

Function of the junction

The most challenging detail in a single-family house with a habitable attic — a triple-junction point:

1. Pitched roof with between-rafter termPIR AL insulation (item 06).
2. Knee wall insulated with ETX (item 11) on the façade side.
3. Wall plate (item 13) — timber penetrating the thermal envelope.

Unlike the over-rafter system (where insulation runs above the wall plate), here the insulation is between the rafters — a geometric gap remains around the wall plate, which must be filled with low-expansion PIR foam (item 12); otherwise a thermal bridge running through the wall plate and rafters is created.

This is the most commonly observed thermographic defect in houses with habitable attics — visible as cold streaks along the rafters at the knee wall.

Critical installation aspects

• termPIR AL boards between the rafters (item 06) must be cut to ±2 mm accuracy of the rafter spacing. A fit that is too tight deforms the rafter; too loose → gap + thermal bridge.
• Geometric gap at the wall plate and the knee wall (item 12) — fill with low-expansion PIR foam (expansion ≤ 30%). Standard (PU) foam expands 200%+ → will push the boards out.
• Under-rafter insulation (a second layer beneath the rafters) eliminates the bridge running through the timber rafter itself — without it, the rafter is a thermal bridge along its entire length.
• The wall plate (item 13) must be wrapped in an insulating strip — either from above (before the wind barrier) or from below (before the vapour barrier); ideally on both sides.
• The PE vapour barrier (item 09) must run continuously from the ceiling onto the knee wall strip. Lack of continuity → vapour condensation in the insulation.

Between-rafter vs over-rafter

Aspect	Between-rafter (this)	Over-rafter (see)
Insulation location	between rafters	on rafters (from above)
Rafter thermal bridge	present (timber lambda 0.15)	eliminated
Wall plate thermal bridge	requires separate insulation	eliminated
Roof height	unchanged	+18–25 cm
Cost	base	+30–40%
Typical use	renovations, refurbishments	new low-energy builds

Used in the pitched roof termPIR between-rafter system.

Documentation

termPIR catalogue — Single-Family Houses, p. 22 — pitched roof W I, junction with the knee wall.

Components in this junction