Vapor Barrier On Roof Deck

Like fully adhered insulation fully adhered vapor barriers need a solid substrate below.

Vapor barrier on roof deck.

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. But a preinstalled membrane may be even more effective at removing condensation. 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.

It is therefore suitable for most roofs. In fact it is almost a perfect vapor barrier. V gard provides an excellent vapor barrier with a vapor resistance value expressed by an sd equivalent air layer thickness of 1 500 m. Typically a vapor retarder has to have a perm rating of 1 0 or less to be successful.

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

That means that when we use a vapor retarder in a roof system it s also acting as an air barrier. 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 in any air conditioned enclosure. A vapor retarder is commonly installed on the warm interior side of a roof or wall.

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. Everyone knows that water in roof systems is never a positive. Roofs over concrete decks should always have a vapor barrier installed.

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. 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. And the membrane on the bottom is also a vapor barrier. Traditionally we ve insulated the roof panel to try and keep the temperature on the panel from reaching the dew point.

Classic compact flat roof the most famous double vapor barrier of them all. In a low slope roof assembly the vapor retarder is normally installed under the primary roof insulation. This can be a problem where brick is installed over building paper and vapor permeable sheathing. Resistant to foot traffic the engineered composition of v gard provides high resistance to static loading and dynamic impact.

This can be a problem in any below grade space.