Air Seal Attic Knee Wall

Air seal and insulate the kneewall if you plan to insulate a kneewall and the attic floor behind the kneewall protect the insulation with an adjacent air barrier.

Air seal attic knee wall.

What happens then is that the batts don t make contact with the air barrier the drywall air moves through and around them and they fall out of the attic kneewall. Now an attic kneewall has to perform better than an exterior wall in summer because there s a bigger temperature difference across it than there is across an exterior wall. The air barrier should have no leaks especially in the areas where the floor meets the wall and the wall meets the roof. Air sealing and insulating the knee wall can make the attic space warmer more efficient and more comfortable.

Adding a rigid foam air barrier to the cold side of the batt is a great way to take care of this issue on a knee wall between finished and unfinished attic space. We want to air seal any penetrations or top plates as we do in all attic spaces. It is important to choose an air barrier and insulation material that will reduce the air leakage and heat loss as much as possible. If it s 95 f outside it could be up to 130 f in the attic.

The problem with many kneewalls is that they have fiberglass batt insulation with nothing covering them on the attic side as shown in the photo below. This will help reduce the wind damage on the insulation to keep it in good shape as well as increase its effectiveness. But then for the knee wall it s important to air seal the wall with tyvek or foamboard. Properly sealing and insulating attic knee walls can significantly reduce air leakage and heat loss between the conditioned space and the unfinished attic.

Fiberglass batts work best if they re enclosed and air sealed on all 6 sides.