Vapor Barrier On Roof Deck

Everyone knows that water in roof systems is never a positive.

Vapor barrier on roof deck.

A vapor retarder is commonly installed on the warm interior side of a roof or wall. This can be a problem in any below grade space. 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. That means that when we use a vapor retarder in a roof system it s also acting as an air barrier.

Roofs over concrete decks should always have a vapor barrier installed. It is therefore suitable for most roofs. This can be a problem in any air conditioned enclosure. The roof membrane on the top is clearly a vapor barrier.

Another option for the roofing system is to recognize a hardboard or sheathing layer can be a vapor open air barrier. In fact it is almost a perfect vapor barrier. Concrete contains a significant amount of moisture and there is no test to. V gard provides an excellent vapor barrier with a vapor resistance value expressed by an sd equivalent air layer thickness of 1 500 m.

Traditionally we ve insulated the roof panel to try and keep the temperature on the panel from reaching the dew point. 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. 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.

Resistant to foot traffic the engineered composition of v gard provides high resistance to static loading and dynamic impact. The bottom membrane s primary function is that of an air barrier but it s material. We ll add a vapor barrier so the moist air does not flow up through the insulation and condense on the roof. The solid substrate for fluted metal decks is typically a gypsum underlayment and the concrete is the solid substrate for concrete decks.

The caveat is that the vapor retarder needs to be sealed at all perimeters and penetrations and tied to the wall air barrier so air does not bypass the vapor retarder layer. In a low slope roof assembly the vapor retarder is normally installed under the primary roof insulation. This can be a problem when there is also a vapor barrier on the exterior. This method is shown in the image above.

The midwest roofing contractors association mrca is advising a vapor retarder of less than 0 01 perm is necessary over new concrete roof decks. A gypsum fiber board has a perm rating of approximately 24 to 30 perms depending on thickness and if this board is fastened to a steel deck and the joints and transitions are taped it could be an effective air barrier that allows some drying. This can be a problem where brick is installed over building paper and vapor permeable sheathing. And the membrane on the bottom is also a vapor barrier.

Most fully adhered vapor barriers are also air barriers and can serve as a temporary roof during construction.