Kneewalls

Another topic that seems to make most homeowner’s eyes glaze over is kneewalls. Many may not be exactly sure what a knee wall is, or even if their house has any, so first let’s clear that up. Simply put a knee wall is any wall in your home that has living space on one side and attic on the other. It is considered an exterior wall, but really it is even more susceptible to energy loss than other walls. The reason that these walls are so problematic is that when it is 95 degrees outside the home, it is more like 140 degrees in the attic. So how would you like to try to cool your home in 140 degree weather? If your kneewalls are not sealed...you do!

The new energy codes have addressed how to handle kneewalls in new construction, but it is really not being enforced in most areas. It does add a few dollars of cost to a home, but the energy savings/payback is rather fast.  The following is how the new code handles knee walls:

 All attic kneewalls must be insulated and air-sealed. Attic kneewalls may be insulated using R-13+R-5 insulated sheathing, R-15+R-3 insulated sheathing, or R-19 compressed into a 2×6 cavity. The attic-side of the kneewall must have a rigid air barrier (with seams sealed)

The only real difference in how they were handled in the past (wall cavities stuffed with batts) is that they must now have an air barrier (insulation board) covering the batt insulation on the attic side. Think of it this way; what if your home had no siding and only exposed batt insulation on the exterior? Sure there is no wind and rain in the attic, but how much hot or cold air would leak in your home if that was all that was separating you from the outside. The answer is a lot and when you raise that outside temperature to 140 degrees it gets even worse.

The fix is easy and it is do it yourself friendly. The work can be ugly because no one will ever see it but you unless you invite friends to your attic. Simply nail the insulation board to the wall studs with roofing or plastic-cap nails a seal the joints with spray foam. Be sure you get all the way to the bottom because you need to cap off the floor system below.