Reviewed and Revised on 10/10/2013
Knee walls are found in houses with finished attics and multiple ceiling heights. The knee wall is the short wall that reaches from the sloped ceiling to the floor, or connects the upper ceiling to the lower ceiling inside the attic. Not only are air leaks common in these areas, but they are frequently not insulated properly.
Insulation itself does not stop air flow. For wall insulation to do its job, it must have an air barrier on all sides, so that it is enclosed in the wall cavity. An air barrier is any material that restricts air flow. In most walls, the exterior (outdoor) air barrier is typically a combination of sheathing and either building paper, house wrap, or board insulation. The indoor air barrier is often sheetrock. For the air barrier itself to be effective, it must create a continuous layer around the insulation. This means the insulation must be air sealed on all 6 sides in order to be effective. All holes and cracks should be fully sealed, and the air barriers must be in full contact with the insulation.
Knee walls generally have exposed studs on the attic side, and can be insulated with batts or blankets. Your climate will dictate where you will place the vapor barriers.If the studs are covered on the attic side, you can insulate the knee wall by drilling a hole in the covering near the top of the wall, and filling the cavity with blown insulation. Once the insulation is added to the cavity, it is very important to complete the air seal on the attic side. Add tape or caulk to the seams and perimeter of the covering.
The sloped area above the ceiling is more difficult to insulate. Block the rafter cavity at the bottom, and fill it with a loose-fill insulation poured from the attic over the ceiling. It is important not to restrict the pathway of attic ventilation between the soffit (lower attic) and the ridge (upper attic) of the sloped ceiling area. You can insulate the floor behind the knee wall and the attic above it the same way you would a regular attic.
If you are unsure about the best way to air seal and insulate these areas, consult a trained professional.