Vapor Barrier On Concrete Roof Deck

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.

Vapor barrier on concrete roof deck.

Roofs over concrete decks should always have a vapor barrier installed. Concrete contains a significant amount of moisture and there is no test to accurately tell if the concrete is dry enough to install the roof on top of it. Vapor barriers installed on the interior of assemblies prevent assemblies from drying inward. Incorrect use of vapor barriers is leading to an increase in moisture related problems.

This method is shown in the image above. Vapor barriers were originally intended to prevent assemblies from getting wet. 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. Rolling out a course of the new vapor barrier.

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. Here s a key takeaway all vapor retarders block air but not all air barriers block vapor diffusion. Everyone knows that water in roof systems is never a positive. This can be accomplished either because of the local climate or as a result of design principally through the use of rigid insulation installed above the roof deck or air impermeable spray foam insulation installed.

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. In a low slope roof assembly the vapor retarder is normally installed under the primary roof insulation. 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.

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. The key is to keep the roof deck the principle condensing surface in roof assemblies sufficiently warm throughout the year. However they often prevent assemblies from drying. A vapor retarder is commonly installed on the warm interior side of a roof or wall.

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. 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.