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How to install vapor barriers for your house

Moistureproofing your home

In winter, warm, moist household air moves toward colder, drier air outside. Where insulation has been installed without a proper vapor barrier, water vapor migrating into the walls condenses against the cold inner surface of wall or roof sheathing and inside the insulation. The result is wet, inefficient insulation, and the possibility of mold growth, exterior paint peeling, and structural rot.

To prevent moisture damage, ventilate your house well and use insulation with a vapor barrier-usually a layer of nonpermeable aluminum foil, kraft paper, or polyethylene-facing the warm-in-winter side of the wall, floor, or ceiling being insulated. When insulating with loose fill or with unfaced batts or blankets. install a separate vapor barrier of Emil-thick polyethylene sheets. More convenient but less efficient as a vapor retarder are insulation batts and blankets with an attached vapor barrier; consider adding polyethylene sheeting to faced insulation.

When installing faced insulation between studs, staple the vapor barrier’s flanges to the studs so that the flanges overlap. When covering insulation with polyethylene sheeting, always join sheets at a stud, overlapping and stapling their edges as shown. Be careful not to puncture or tear a vapor barrier; patch any holes with duct tape for polyethylene.

Use polyethylene as a vapor barrier over an unpaved crawl-space or basement floor. To add a vapor barrier to a finished wall, paint ‘the wall with vapor-barrier paint; vinyl wallpaper or two coats of oil-based paint also provide vapor resistance.