Vapor Barrier On Roof Deck

In fact it is almost a perfect vapor barrier.

Vapor barrier on roof deck.

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. A vapor retarder is commonly installed on the warm interior side of a roof or wall. Everyone knows that water in roof systems is never a positive.

This can be a problem where brick is installed over building paper and vapor permeable sheathing. This can be a problem in any below grade space. Classic compact flat roof the most famous double vapor barrier of them all. 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.

Most fully adhered vapor barriers are also air barriers and can serve as a temporary roof during construction. Resistant to foot traffic the engineered composition of v gard provides high resistance to static loading and dynamic impact. This can be a problem when there is also a vapor barrier on the exterior. 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. 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. The solid substrate for fluted metal decks is typically a gypsum underlayment and the concrete is the solid substrate for concrete decks. Traditionally we ve insulated the roof panel to try and keep the temperature on the panel from reaching the dew point.

This method is shown in the image above. Typically a vapor retarder has to have a perm rating of 1 0 or less to be successful. Another option for the roofing system is to recognize a hardboard or sheathing layer can be a vapor open air barrier. Like fully adhered insulation fully adhered vapor barriers need a solid substrate below.

This can be a problem in any air conditioned enclosure. The roof membrane on the top is clearly a vapor barrier. But a preinstalled membrane may be even more effective at removing condensation. That means that when we use a vapor retarder in a roof system it s also acting as an air barrier.

It is therefore suitable for most roofs. 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. Concrete contains a significant amount of moisture and there is no test to.

Roofs over concrete decks should always have a vapor barrier installed.