It's important to consider the permeability of batt facings when specifying types -- some, like kraft paper, allow drying and inhibit wetting. Foil facings are impermeable and unfaced batts are often coupled with a plastic vapor barrier.
The kraft-paper facing that accompanies some fiberglass batts is a vapor retarder with a permeance of about 0.30 perm (although this value varies from manufacturer to manufacturer, and even from batt to batt). Like the permeance of asphalt felt. the permeance of kraft-paper facing varies with its moisture content. As the paper becomes damper, its permeance increases.
As typically installed, kraft-paper facing is not an air barrier.
Because kraft facing allows some drying to a home's interior to occur during the summer, it doesn't have the disadvantages of interior plastic. Builders who prefer to install unfaced fiberglass batts may want to use a vapor-retarder paint or a product