Vapor Barrier On Roof Deck

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

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. It is therefore suitable for most roofs. Roofs over concrete decks should always have a vapor barrier installed.

Concrete contains a significant amount of moisture and there is no test to. The bottom membrane s primary function is that of an air barrier but it s material. Like fully adhered insulation fully adhered vapor barriers need a solid substrate below. We ll add a vapor barrier so the moist air does not flow up through the insulation and condense on the roof.

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. That means that when we use a vapor retarder in a roof system it s also acting as an air barrier. The roof membrane on the top is clearly a vapor barrier. Resistant to foot traffic the engineered composition of v gard provides high resistance to static loading and dynamic impact.

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. Classic compact flat roof the most famous double vapor barrier of them all. But a preinstalled membrane may be even more effective at removing condensation. And the membrane on the bottom is also a vapor barrier.

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. V gard provides an excellent vapor barrier with a vapor resistance value expressed by an sd equivalent air layer thickness of 1 500 m. In a low slope roof assembly the vapor retarder is normally installed under the primary roof insulation. The solid substrate for fluted metal decks is typically a gypsum underlayment and the concrete is the solid substrate for concrete decks.

Another option for the roofing system is to recognize a hardboard or sheathing layer can be a vapor open air barrier. This can be a problem when there is also a vapor barrier on the exterior. 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. This can be a problem in any air conditioned enclosure.

The midwest roofing contractors association mrca is advising a vapor retarder of less than 0 01 perm is necessary over new concrete roof decks. This can be a problem where brick is installed over building paper and vapor permeable sheathing. This method is shown in the image above. Traditionally we ve insulated the roof panel to try and keep the temperature on the panel from reaching the dew point.

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