Do I need to use housewrap and a vapor barrier?
Vapor barriers are placed on the warm-in-winter side of a wall—generally the last layer before sheetrock—in most U.S. climates. The need for a vapor barrier depends on climate—the colder the climate, the more a vapor barrier is needed. Check with codes in your area to see the vapor barrier requirement. Kraft-faced fiberglass batt insulation may be sufficient.
Housewrap is one type of weather resistant barrier (WRB) that is placed over the structural sheathing. It keeps any water that has penetrated the siding from causing moisture accumulation and possible decay of the structural sheathing. Asphalt paper is another type of WRB used to protect the sheathing. Housewrap and asphalt paper are not vapor barriers—they permit water vapor transmission but prevent bulk water from penetrating. And this is a good thing because if the wall cavity gets wet, WRBs need to permit drying of the wall to the exterior. Housewrap should be installed prior to windows and doors (to provide protection in case windows leak). Review the manufacturer’s installation instructions for more detailed information about installation. The installer’s guide, Moisture Protection of Wood Sheathing, discusses protection of structural sheathing in more detail.