There are two main methods to insulate attic knee walls.
attic knee wall insulation: Method 1
One involves the insulation of the rafter space along the sloping ceiling.
Before insulating, seal all possible leaking areas. Any cracks or holes between the insulation material and the knee wall should be carefully caulked. The top plate of the knee wall should also be properly sealed. Rafters should be sealed with an airtight membrane or other strong air barrier.
To insulate, use rigid foam insulation along the after space of the sloping ceiling (option 1, image below). The foam is usually notched in order to fit over the floor joists.
The Oak Ridge National Laboratory suggests a second method (option 2, image below).
- Sealing the perimeter joints and any cracks or gaps with a caulk or spray foam (see Option 2 in the image above).
- Insulating the knee wall, itself: cut and install batt insulation or rigid foam in the attic-side of the knee wall.
- Insulating the open joists below the knee wall.