What is a knee wall?

A knee wall is probably the LEAST COMFORTABLE and most ENERGY INEFFICIENT part of your home. 

A knee wall is an area you've seen hundreds of times and may have no idea what it is till now. 

A knee wall is also an area that do-it-yourselfers normally 'fix' wrongly. 

Physically, a knee wall is an area of a home that would normally be a full attic space. Instead of a full attic, a room is built in that space. The room is typically conditioned (heated and/or cooled) and is drywalled off. 

Now what does a knee wall look like? A few key indicators that you have a knee wall:

  • Low sloping ceiling with attic or roof space above that leads down to a wall. The blue wall in the picture below is the knee wall. 
  • An attic access that is perpendicular to the floor

From the inside of the attic, a knee wall looks like the image to the left. The wall on the left with yellow insulation is the same wall as the blue wall in the image above. As you can see in the image below, the joists run perpendicular to the wall, which means the joists run below the floor of the living space. As insulation does not stop air flow, cold air in the winter can pass under your floor as well as hot air in the summer. As a side note to this image, the insulation on the knee wall floor was installed upside down. About half of the Do-It-Yourselfers install insulation incorrectly.

From the outside, what does a knee look like? Below is a picture of the same house. The orange represents the walls that form the conditioned space in the bedroom. This is the livable space shown in the first image of this blog post. 

Then you can see the ventilation where air enters into the knee wall space. Unwanted air will travel under the floor, causing discomfort and a severe lack of energy efficiency. 

Now, what are the solutions?

We want to air seal any penetrations or top plates, as we do in all attic spaces. But then for the knee wall, it's important to air seal the wall with Tyvek or foamboard. This will help reduce the wind damage on the insulation to keep it in good shape as well as increase its effectiveness. Then, we bay block the cavities to create a continuous thermal boundary, and block air from passing under the floor. 

Knee walls are tight, complicated areas. Our recommendation is to get an energy audit to first determine your home's air leakage. The recommended solutions all stop air from entering or leaving the house. Not knowing your Building Airflow Standard nor your blower door number can be severely dangerous as you air seal. Air seal too much, and you can create moisture and health problems - bad for both the inhabitants and the home. 

Our recommendation is to get an Energy Audit and have those knee walls air sealed and insulated properly.