What is a knee wall?
A knee wall is a short wall in an attic that closes off the space between the sloping roof and the floor. It stands at the point where the sloping roof approaches the floor — the “knee” of the roof structure. The height of the knee wall determines how much usable space there is in the attic.
Where is the knee wall located?
In a gable roof, the sloping roof surfaces descend 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 to the sloping roof surface.
Behind the knee wall is an unusable triangular space that is often used as storage (accessed through a small door).
Height of the knee wall
The knee wall height has direct consequences:
| Knee wall height | Usable floor area | Application |
|---|---|---|
| 0 cm (no knee wall) | Minimal — only the centre is usable | Uninhabited attic |
| 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 attic floor |
> 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 the knee wall
The knee wall is one of the most important places to insulate, as there is an unheated space directly behind it. Without insulation, you lose a lot of heat.
Applying insulation
- Install battens — Wooden battens (45×70 mm) at 60 cm centre-to-centre against the knee wall
- Place insulation — Mineral wool or PIR boards between the battens (minimum Rd 3.5)
- Vapour barrier — Apply a vapour barrier foil on the warm (room) side
- Finish — Plasterboard or wood 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 gutter or the ridge.
Knee wall in self-build projects
When building a garden house, shed, or garage with a gable roof and an attic floor:
- Garden house with sleeping loft — A knee wall of 80-100 cm makes the attic habitable
- Shed with storage loft — A knee wall is not necessary if the attic is only for storage
- Garage with attic room — Insulate the knee wall well if you want to heat the space
Related terms
- Roof frame
- Truss
- Gable roof
- Insulation value (R-value)
- Mineral wool
- Vapour-permeable membrane