Foam Board Insulation Over Vapor Barrier

If the wall cavity get wet for any reason then the wall will need to dry primarily to the interior.

Foam board insulation over vapor barrier.

It will degrade its r value. 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. When installed in the correct thicknesses these to methods result in a proper vapor barrier and a superior insulation. All foam boards absorb water vapor.

It s best to use only a class iii vapor retarder such as painted drywall. Use closed cell spray foam closed cell foam board extruded polystyrene such as dow styrofoam or foil faced iso board. Cover the insulation with a vapor barrier before you install it. When installing the foam use caulk or tape for maximum insulation between the material and the board or floor.

Keep foam board insulation away from direct sunlight prior to installation. Unfortunately the language in the joe lstiburek article that you linked to is imprecise. Not every wall does. However an installation between the studs can pose a challenge.

A vapor retarder is a material used to prevent water vapor from diffusing into the wall ceiling or floor during the cold winter. You don t want moist air leaking into the roof cavity. Many types of rigid foam including foil faced polyiso and faced eps are vapor barriers. The goal is to keep the inside face of the foam sheathing above the dew point of the interior air.

Rigid foam insulation is usually a qualified vapor barrier. It s made of environmentally sound closed cell polyisocyanurate foam bonded to a durable white matte non glare aluminum facer and a reflective reinforced aluminum facer. 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.

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. 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. 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. The previous owners went over the plaster lathe with 1.

Do no use a vapor barrier under the insulation.