Knee Wall Foam Board Insulation

The most common use for foam board insulation at Star Companies is to properly insulate and air seal knee walls. Most people’s next question is: “What is a knee wall?”. A knee wall is an interior wall that has living space on one side of the wall and attic space on the other side. Historically, they were short walls around knee level but in modern construction knee walls can be as tall as the interior space, sometimes even 12-15 feet tall in larger homes. This presents a problem for conditioning the living space because not only is there attic space above the ceiling, now there is attic space behind the walls. Some clues that you have knee walls in your home are: if you have a walk-in attic, a split-level home, cape cod style home, or a small door in the wall to access your attic.

How Should a Knee Wall Be Insulated?

At minimum, the knee wall should have batting insulation in between each stud cavity. However, only installing fiberglass insulation in the knee wall does not stop the unconditioned air in the attic from heating or cooling the knee wall which in turn permeates into the living space. The solution to fiberglass being air permeable is installing rigid foam board to the face of the stud, then using spray foam to air seal the knee wall, effectively creating a closed system. In sum, the fiberglass batting insulation provides good R-Value, while the foam board insulation and spray foam bolster the R-Value as well as provide the air sealing properties that fiberglass on its own could not.

What is the Cost of Knee Wall Insulation?

There are a few factors that impact the cost of insulating knee walls. First, is the total square footage of the project, higher square footage requires more materials thus a more expensive project. Next, is whether there is existing insulation within the knee walls and whether that insulation is intact and sufficient. If there is no insulation or if the existing insulation has been damaged or is lacking R-value changes the cost due to installation or replacement of the batting insulation.

Please give us a call at (816) 353-2160 or contact us, and we will provide you a free estimate.