Vapor Barrier Over Concrete Roof Deck

Resolution of the vapor barrier debate.

Vapor barrier over concrete roof deck.

Despite this concern for concrete decks a self adhering vapor barrier protects the insulation and provides a temporary roof. The moisture in the concrete will release into the glue causing the self adhering vapor barrier to fail. This method is shown in the image above. Rolling out a course of the new vapor barrier.

In a low slope roof assembly the vapor retarder is normally installed under the primary roof insulation. Typically a vapor retarder has to have a perm rating of 1 0 or less to be successful. In circumstances where conditions inside a home or office are much different than outdoor conditions water. Roof system designs using mechanical fasteners penetrating a vapor retarder should be avoided.

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. A vapor retarder is commonly installed on the warm interior side of a roof or wall. 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. If a roof system is installed over a concrete deck without a vapor barrier the moisture in the concrete will likely dry into the roof insulation.

Here s a key takeaway all vapor retarders block air but not all air barriers block vapor diffusion. 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. The midwest roofing contractors association mrca is advising a vapor retarder of less than 0 01 perm is necessary over new concrete roof decks. Some debate still occurs over how necessary vapor barriers are but a consensus is growing closer.

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. The old vapor barrier was attached with hot asphalt and it s really fused to the concrete deck. 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. Likewise a self adhering vapor barrier will not stick to concrete over the long term.

Most authorities now agree that vapor barriers are important under certain conditions but not necessarily as a whole house solution for every home.