Attic knee walls are useful in finished attics but not strictly necessary for energy efficiency.
Are you concerned that your attic has hidden weak spots that are allowing air infiltration and inflating your energy bills?
It’s true that attics often harbor air leaks or inadequate insulation that can allow indoor air to escape from your home and/or let outdoor air in. Either way, your energy bills are going to suffer as your HVAC system works overtime to keep up with heat loss in the winter or heat infiltration in the summer.
In their search for any and all possible ways to reduce air loss, somehow people came up with the idea that adding a knee wall was a great idea. In reality, adding a knee wall to an unfinished attic provides very limited benefits from an HVAC perspective.
…Knee Walls Are Not a Substitute For:
- A properly sealed building envelope: It is true that the place where the roof meets the attic floor can be prone to air leaks. However, the solution is not to just enclose this area behind a wall. If you do this the leaks will still exist. The leaks need to be properly sealed.
- Adequate insulation: If you do not have appropriate insulation blown in between the rafters in the roof and the joist in the floor, simply putting up a knee wall is not going to make up for it. Heat will still be able to travel freely through the uninsulated areas.
What Attic Knee Walls Are Really For
The primary purpose of knee walls is to finish out an attic space in an attractive manner.
Imagine your typical triangular attic space. The bottom points of the triangle, where the roof meets the floor, create an awkward, virtually useless space. By enclosing this space with a low, roughly knee-high wall, you can make your attic much more attractive. If you make the knee wall taller than knee height, you can give yourself more wall area in the attic and make your attic rooms look more normal, though you will be losing floor space.
Assuming your attic was already properly insulated (as a finished attic definitely should be), adding the knee wall isn’t really going to do anything in terms of reducing air loss or air infiltration. What it may do is help reduce energy consumption, as the total volume of the heated or cooled attic living space will be smaller and therefore slightly easier to keep at a comfortable temperature.