Foam Board Insulation Over Vapor Barrier

All foam boards absorb water vapor.

Foam board insulation over vapor barrier.

It will degrade its r value. However an installation between the studs can pose a challenge. Seal the insulation or the ceiling surface below against air leakage. It s best to use only a class iii vapor retarder such as painted drywall.

If the wall cavity get wet for any reason then the wall will need to dry primarily to the interior. You don t want moist air leaking into the roof cavity. When installing the foam use caulk or tape for maximum insulation between the material and the board or floor. The goal is to keep the inside face of the foam sheathing above the dew point of the interior air.

Unfortunately the language in the joe lstiburek article that you linked to is imprecise. Use closed cell spray foam closed cell foam board extruded polystyrene such as dow styrofoam or foil faced iso board. Keep foam board insulation away from direct sunlight prior to installation. 2 don t use a vapor barrier such as poly on the interior.

Many types of rigid foam including foil faced polyiso and faced eps are vapor barriers. However an installation between the studs can pose a challenge. Over the years i ve come to rely on two approaches for basement insulation projects and remodelling including closed cell spray foam and closed cell xps foam board. Not every wall does.

To complete the vapor barrier you would want to foam seal or tape all the sides where the rigid meets timber because these gaps will facilitate vapor flow. Cover the insulation with a vapor barrier before you install it. R matte plus 3 thermasheath 3 is rigid foam plastic thermal insulation r matte plus 3 thermasheath 3 is rigid foam plastic thermal insulation board. After the insulation is in place you will want to add a vapor retarder sometimes called a vapor barrier if you need one.

The previous owners went over the plaster lathe with 1. A vapor retarder is a material used to prevent water vapor from diffusing into the wall ceiling or floor during the cold winter. Insulation radiant and vapor barriers rigid foam board over fiberglass i have an 80 year old house in upstate ny with a very drafty kitchen. Although lstiburek talks about the risks of vapor barriers in the article he s really talking about the risks of interior polyethylene not vapor barriers.

Rigid foam insulation is usually a qualified vapor barrier.