Foam Board Insulation Over Vapor Barrier

The goal is to keep the inside face of the foam sheathing above the dew point of the interior air.

Foam board insulation over vapor barrier.

Keep foam board insulation away from direct sunlight prior to installation. You don t want moist air leaking into the roof cavity. However an installation between the studs can pose a challenge. Cover the insulation with a vapor barrier before you install it.

If the wall cavity get wet for any reason then the wall will need to dry primarily to the interior. 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. A vapor retarder is a material used to prevent water vapor from diffusing into the wall ceiling or floor during the cold winter. 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.

Do no use a vapor barrier under the insulation. Rigid foam insulation is usually a qualified vapor barrier. Not every wall does. 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.

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. The previous owners went over the plaster lathe with 1. It s best to use only a class iii vapor retarder such as painted drywall. After the insulation is in place you will want to add a vapor retarder sometimes called a vapor barrier if you need one.

When installed in the correct thicknesses these to methods result in a proper vapor barrier and a superior insulation. However an installation between the studs can pose a challenge. Use closed cell spray foam closed cell foam board extruded polystyrene such as dow styrofoam or foil faced iso board. Unfortunately the language in the joe lstiburek article that you linked to is imprecise.

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. All foam boards absorb water vapor. It will degrade its r value.

Seal the insulation or the ceiling surface below against air leakage.