In addition to being insulated, living areas should also be sealed with a moisture-proof layer or vapor barrier that’s applied to the inside of the wall studs to prevent the movement of moisture from the living areas into the insulation. Insulation will lose some of its thermal qualities if it becomes damp or wet, and if the moisture within the living spaces is retained within the house, the occupants will still feel comfortable with less heat, due to the inside humidity. Consult local building codes for guidance on specific vapor barrier requirements. Depending on your climate, whether it’s hot, humid, or dry, vapor barrier needs vary.

There are several methods for applying a vapor barrier to exterior walls: install insulating batts or blankets faced with vapor barrier backings such as treated Kraft paper or aluminum foil. The vapor barrier should always be placed closest to the living area that’s heated. If unfaced batts or blankets are used, a vapor barrier or polyethylene film not less than 3 mils thick (or an equivalent) should be applied.

If you elect loose blown insulation, establish a vapor barrier by stapling or nailing polyethylene sheet material to the interior of the wall studs. You can also install aluminum foil–backed drywall before the insulation is blown between the studding.