Vapor Barrier On Concrete Roof Deck

A vapor retarder is commonly installed on the warm interior side of a roof or wall.

Vapor barrier on concrete roof deck.

This method is shown in the image above. The old vapor barrier was attached with hot asphalt and it s really fused to the concrete deck. 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.

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. 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. The key is to keep the roof deck the principle condensing surface in roof assemblies sufficiently warm throughout the year. Vapor barriers were originally intended to prevent assemblies from getting wet.

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. Vapor barriers installed on the interior of assemblies prevent assemblies from drying inward. Roof system designs using mechanical fasteners penetrating a vapor retarder should be avoided. 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.

Here s a key takeaway all vapor retarders block air but not all air barriers block vapor diffusion. Incorrect use of vapor barriers is leading to an increase in moisture related problems. 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. Everyone knows that water in roof systems is never a positive.

The midwest roofing contractors association mrca is advising a vapor retarder of less than 0 01 perm is necessary over new concrete roof decks. 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. However they often prevent assemblies from drying. Roofs over concrete decks should always have a vapor barrier installed.

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. 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.