Exterior Wall Insulation Vapor Barrier

A vapor retarder is a material used to prevent water vapor from diffusing into the wall ceiling or floor during the cold winter.

Exterior wall insulation vapor barrier.

The cool surface is the sheathing assuming no exterior insulation. Very hot and humid climates may benefit from an exterior vapor barrier that keeps outside humidity from penetrating into walls. This barrier is meant to keep moisture from getting to the insulation in the walls and ceilings and it is required by building codes when insulating most houses. The whole vapor barrier retarder business can be quite confusing.

In the worst case moisture can actually freeze within the walls accumulating until a thaw melts it and causes visible damage to your home such as wall or ceiling staining. Usually composed of a thin film such as polyethylene vapor barriers are used to retard or prevent water vapor diffusion into a wall ceiling or floor during the cold winter. Do not use faced insulation in that wall as the foil on the hot room side and kraft paper on the changing room side will create a vapor barrier sandwich which will trap any moisture that gets in that wall. A vapor barrier is designed to keep moisture in your home from getting inside your walls.

Most inspectors don t even understand it. So you are correct 6 mil poly is indeed a better vapor retarder than kraft faced insulation with regards to stopping vapor diffusion. Using a tyvek weather barrier in combination with a vapor barrier depends on how the rest of the wall system is constructed and the climate the building is in. Plastic specifically 6 mil polyethylene plastic is.

Vapor barriers are sheets of plastic or other material placed on one side of insulation sheets. Without a vapor barrier in place condensation inside the walls could ruin the insulation and promote the growth of harmful mold and bacteria. Not every wall does. In any case the vapor barrier must point to the warm side.

Of course you can have moisure problems here even without the exterior vapor barrier because of what bill rose calls the rule of material wetting. Changing room has faced insulation on the exterior walls but unfaced insulation on the interior shared wall with the hot room. If water vapor diffuses or infiltrates into the wall cavity and finds the cool surface moisture problems can occur. After the insulation is in place you will want to add a vapor retarder sometimes called a vapor barrier if you need one.

As mentioned earlier batt and roll insulation come with a vapor barrier attached. If the majority of the year is spent heating the home where the inside temperature is greater than the outside it is generally good to use a vapor retarder behind the interior drywall. But you might not want the best vapor barrier in your wall assembly if you have lousy walls.