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.
Vapor barrier in attic knee wall.
Over time this trapped moisture can rot the wood framing in your attic or cause mold and mildew to appear.
The science of moisture movement.
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.
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.
It s even made by your favorite company guardian.
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.
Thus installing vapor barriers on wall surfaces must.
Despite all the bad publicity reflective insulation is getting lately i like to use it quite a bit on knee walls.
Even though we apply vapor barriers to exterior insulated walls in cold climates it s no longer recommended for ceilings floor of the attic.
We treated the knee wall as an outside wall stapling up house wrap tyvek visible in the picture above on the unconditioned side.
Rigid air barrier material could include rigid foam insulation drywall plywood or osb among others.
We like it because the 4 rolls that fit most older knee walls are easy to drag in and install.
Attic vapor barrier detail for cold climates.
We use a perforated variety to avoid the double vapor barrier.
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.
Because it s nearly impossible to seal penetrations in the ceiling the reality is water vapor will get into the attic.
I ve got some kneewalls that i d like to beef up.
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.
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.