Vapor Barrier In Attic Knee Wall

Despite all the bad publicity reflective insulation is getting lately i like to use it quite a bit on knee walls.

Vapor barrier in attic knee wall. Water vapor can pass through building materials in several ways including direct transmission and by heat transfer but studies suggest that fully 98 percent of the moisture transfer through walls occurs through air gaps including cracks around electrical fixtures and outlets and gaps along baseboards. However if you are in a cold climate then adding radiant barrier to the attic side of the knee wall will help the room retain heat in the winter. If the knee walls are sharing space with an attic that already has radiant barrier on the rafters then the room on the other side of the knee wall is already benefiting from not gaining heat. We use a perforated variety to avoid the double vapor barrier.

The few holes in this barrier required for exterior lights switches can be easily sealed with a can of foam or silicone mushroomed on both sides of the vapor barrier. Adding a second vapor barrier could cause condensation to become trapped in the insulation between the two vapor barriers. Attic vapor barrier detail for cold climates. Over time this trapped moisture can rot the wood framing in your attic or cause mold and mildew to appear.

Install a continuous air barrier on the exterior side of the attic knee wall framing with a rigid air barrier or other supporting material to prevent the knee wall cavity insulation from sagging and to create a continuous thermal barrier. The science of moisture movement. They are 2 4 construction kraft faced r 11 batts in the wall with the paper facing the drywall and a plastic vapor barrier in the knee wall attic. We like it because the 4 rolls that fit most older knee walls are easy to drag in and install.

We treated the knee wall as an outside wall stapling up house wrap tyvek visible in the picture above on the unconditioned side. It s even made by your favorite company guardian. This provides a backing to prevent the batts of insulation from falling into the attic and also acts as a barrier to help stop the cold air from penetrating the insulation. Because it s nearly impossible to seal penetrations in the ceiling the reality is water vapor will get into the attic.

Rigid air barrier material could include rigid foam insulation drywall plywood or osb among others. If i place vertical runs of r 30 unfaced against the current plastic barrier i d use a perforated barrier to hold them in place. To thwart the comments of the expense of the double wall headed my way i would like to add that the second wall that i add to the interior is totally non structural 24 o c.