Vapor Barrier Over Concrete Roof Deck

Some debate still occurs over how necessary vapor barriers are but a consensus is growing closer.

Vapor barrier over concrete roof deck.

Resolution of the vapor barrier debate. From a practical standpoint a vapor retarder of less than 0 01 perm is effectively a vapor barrier almost no moisture passes through to the roof system above. Exceptions to this idea include roofs over indoor swimming pools or other high humidity producing activities and over new concrete roof decks to prevent the moisture in the concrete from drying into the roof system. Therefore one will often see it installed directly on a roof deck such as a concrete or wood deck or on a continuous substrate such as gypsum board or wood panels that is installed directly over a metal deck.

In a low slope roof assembly the vapor retarder is normally installed under the primary roof insulation. Another exception is a class i vapor retarder should be installed over a new concrete deck to prevent the moisture in the concrete from drying into the roof system. This method is shown in the image above. Rolling out a course of the new vapor barrier.

The midwest roofing contractors association mrca is advising a vapor retarder of less than 0 01 perm is necessary over new concrete roof decks. When we use a vapor retarder in a roof system it will also act as an air barrier as long as it is sealed at all perimeters and penetrations and is tied to the wall air barrier. Roof system designs using mechanical fasteners penetrating a vapor retarder should be avoided. Here s a key takeaway all vapor retarders block air but not all air barriers block vapor diffusion.

Likewise a self adhering vapor barrier will not stick to concrete over the long term. The old vapor barrier was attached with hot asphalt and it s really fused to the concrete deck. Typically a vapor retarder has to have a perm rating of 1 0 or less to be successful. The moisture in the concrete will release into the glue causing the self adhering vapor barrier to fail.

Everyone knows that water in roof systems is never a positive. Most authorities now agree that vapor barriers are important under certain conditions but not necessarily as a whole house solution for every home. Despite this concern for concrete decks a self adhering vapor barrier protects the insulation and provides a temporary roof. A vapor retarder is a material or system that is designed as part of the roof system to substantially reduce the movement of water vapor into the roof system where it can condense.

In reroofing situations where the existing roof deck is known to be lightweight structural concrete or there is evidence of concrete deck related moisture problems nrca maintains its earlier recommendation for using a high bond strength vapor retarder adhered directly to the roof deck.