Answer: Knee walls are not found in all attics, and they are not absolutely necessary. But they are awfully good to have.
Go in the attic or simply imagine yourself in the attic. If your attic is unfinished, it has a triangular shape. At the bottom points of the triangles, the rafters meet the top plates of the exterior walls.
Three Problems With That Space
- It is a completely useless space because of its odd shape and small size.
- It is a point that may allow a significant amount of air infiltration.
- If you are trying to build some usable space in there, it is very unsightly.
So, a knee wall is a short vertical wall roughly two or three feet high that “blocks in” that useless triangular space. The exact height of the knee wall in your choice. The higher the knee wall, the greater the amount of useful wall space you create in your finished attic. But you do so at the expense of floor space. So, it is a delicate balancing act.
The knee wall, by itself, can provide some degree of insulation against air infiltration from the outside. However, the knee wall is usually not enough to provide all the insulation you need.
To accomplish this, the U.S. Department of Energy recommends that you complete several steps:
- Insulate the rafters.
- Cover rafters with an air barrier.
- Caulk that barrier.
- Caulk all other holes or cracks or stuff with fiberglass batt insulation or spray foam.