attic knee wall insulation

The knee walls of finished attics should be properly insulated, sealed and sheathed. Attic knee wall insulation is vital for effective attic comfort and home energy-efficiency, and to avoid overheating problems in summer or heat losses in winter…

Attic knee walls are the short walls – say 5 feet high (1,5m) – that enclose the triangular portions of attics (where the top plate of the wall meets the rafters). They are built on the floor of the attic, and tipped into place against the rafters in order to hide the extra triangular portions. They make the attic a truly living space, defining and protecting it from air infiltration. However, by themselves, they will not provide enough insulation and sealing.

attic knee wall insulation: Method 1

There are more two main methods of attic knee wall insulation. We will describe them in this page.

A first method involves the insulation of the rafter space along the sloping ceiling of the knee wall.

To achieve it, use rigid foam insulation along that after space of the sloping ceiling (see option 1, in the image below).

The rigid foam insulation material is usually notched in order to fit over the floor joists.

The recommended insulation value varies with climate zones. See: Insulation and climate zones

In this method the knee wall and the open joists below the knee wall aren't insulated. Just conveniently sealed in their possible leaking points.

Any cracks or holes in the space between the insulation material and the knee walls should be careffuly caulked, and the attic knee door properly insulated. The top plate of the knee wall should also be conveniently sealed.

Any ducts between the new conditioned space do not need insulation.

Rafters should be sealed with drywall or other good and strong air barrier.

attic knee wall insulation: Method 2

An alternative method is the one described in the option 2 of the image below (from Oak Ridge National Laboratory).

Attic Knee Wall Insulation

This second method of attic knee wall insulation involves the following steps:

- Insulating the knee wall. To achieve it you may cut and install batt insulation in the attic-side of the knee wall, or use other insulation methods or materials.

- Insulating the open joists below the knee wall with rigid insulation, insulation fibers, batt insulation or other materials...

- Sealing perimeter joints and any cracks or gaps with a caulk or spray foam (see Option 2 in the image above).

Obviously, this methods also demand the insulation of the knee wall door.