knee walls

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.

Do you need wall insulation?

Do you need wall insulation? The short answer, yes.

Now for the long answer: 

Wall insulation is always good to have in the home. The chances are if your home was built in the 1970's or later, you have wall insulation. We hear a lot of homeowner complaints about cold rooms and drafts because "my walls aren't insulated!" Most times this is due to air penetrations from attic top plates or from the basement rim joist.

To the right, we see a thermal image of a room. As this test was performed in the winter, the blue spots are cold air infiltration caused by attic penetrations from above. You can see thin blue lines which are the joists. The reason it is warm between the joists is because there is insulation in the wall cavities. This particular home was built in 1969.

The home in the images below was built in 1947, and does not have insulation in the wall cavities. 

Not having insulation in the wall cavities is obviously a concern, but we also noticed the home was not properly air sealed in the attic, knee walls, nor the basement. Without touching the walls, we reduced the homes airflow by 55% - a personal record for us - by bay blocking the knee walls, air sealing the attic, and spray foaming the rim joist. Upon completion of the work, the homeowners felt an immediate impact in comfort.

In fact, the investment to insulate the walls would be nearly as much as air sealing and insulating the attic, knee walls, and basement. Not only would it cost nearly as much, but it was not be as effective as the approach we took since the air would still escape out the attic at far too high of a rate. More importantly though, it was impossible to put insulation in these wall cavities as the cavities were not deep enough.

So what does putting insulation in the wall cavities involve? It involves drilling a hole in each cavity, then pumping insulation into the cavity, followed by plugging up the hole. 

As you can see, insulating wall cavities is a very invasive process. It can provide good benefits, but we recommend addressing the attic and the basement of the home before insulating the wall cavities. 

So the long answer made short: yes, you should insulate your wall cavities if you have no insulation in the walls, if the wall cavities are deep enough to be insulated, and only if you addressed your attic and basement first.