Home Improvement Interior Remodel Walls & Ceilings

When to Install a Knee Wall in Your Attic

Knee wall in luxury attic

Caiaimage/Martin Barraud / Getty Images

Knee walls are not found in all attics, and they are not absolutely necessary. But they are awfully good to have. If your attic space is framed with rafters and is unfinished, it has a triangular shape. At the bottom points of the triangles, the rafters meet the top plates of the exterior walls. The exact height of the knee wall is your choice.

What Is a Knee Wall?

A knee wall is a short vertical wall, roughly two or three feet high, thats used to support rafters in the roof. It also blocks off the space that's too small (due to the sloping roof) to use for anything in an attic.

Energy-Saving Measure

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 reduce air infiltration, the U.S. Department of Energy recommends that you complete several steps:

  1. Insulate the rafters.
  2. Cover rafters with an air barrier.
  3. Caulk that barrier.
  4. Caulk all other holes or cracks or stuff with fiberglass batt insulation or spray foam.

The energy saving measures listed above are only necessary if the space is to be occupied in some form. If not, adding more ceiling insulation would be the preferred method. When insulating between rafters, a space for airflow is needed to help keep insulation dry from condensation.