A knee wall is a short wall in a loft that closes off the space between the sloping roof and the floor. It stands at the point where the sloping roof meets the floor — the “knee” of the roof structure. The height of the knee wall determines how much usable space there is in the loft.
Where is the knee wall?
In a gable roof, the sloping roof surfaces run down on both sides. At the point where the roof slope approaches the floor, the knee wall is placed. The knee wall runs from the floor up to the sloping roof surface.
Behind the knee wall there is an unusable triangular space that is often used for storage (accessed through a small door).
Height of the knee wall
The knee wall height has a direct impact:
| Knee wall height | Usable floor area | Application |
|---|---|---|
| 0 cm (no knee wall) | Minimal — only the centre is usable | Uninhabited loft |
| 60-80 cm | Moderate — no standing height along the edges | Bedroom with sloping ceiling |
| 90-120 cm | Good — more usable space | Standard in new-build homes |
| 150 cm+ | Large — almost like a straight wall | Spacious loft room |
> Building regulations: For a habitable room (bedroom, study) at least 50% of the floor area must have a clear height of 2.10 m or more.
Insulating a knee wall
The knee wall is one of the most important places to insulate, because there is an unheated space directly behind it. Without insulation, you lose a lot of heat.
Applying insulation
1. Install battens — Timber battens (45×70 mm) at 60 cm centres against the knee wall
2. Fit insulation — Mineral wool or PIR boards between the battens (minimum R-value 3.5)
3. Vapour barrier — Apply a vapour barrier foil on the warm (room) side
4. Finish — Plasterboard or timber cladding over the top
The space behind the knee wall
The space behind the knee wall must remain ventilated (to prevent moisture problems). Ensure there is ventilation via the eaves or the ridge.
Knee wall in self-build projects
When building a garden room, shed or garage with a gable roof and a loft:
- Garden room with sleeping loft — A knee wall of 80-100 cm makes the loft habitable
- Shed with storage loft — A knee wall is not needed if the loft is only for storage
- Garage with loft room — Insulate the knee wall well if you want to heat the space
Related terms
- Roof structure
- Truss
- Gable roof
- Insulation value (R-value)
- Mineral wool
- Breathable membrane