Moisture vapor is created inside buildings. A vapor retarder helps keep vapor from getting into the insulation. It is usually installed between the deck and insulation (see picture).

Materials used for vapor retarders include felt plies, elastic sheets, aluminum foil and kraft paper sheets. Vapor retarders are not used with every roof system. It depends on the moisture in the building and climate of the area.

Remember that not all roof systems have vapor retarders. It depends on the building and climate.

A common vapor retarder is bituminous. Bituminous and modified bitumen are words you will hear a lot in roofing. They refer to materials called bitumen. Bitumen is asphalt or coal tar. Vapor retarders aremade with asphalt bitumen.

One type of vapor retarder is asphalt with reinforcing material. The reinforcing materials are either glass fiber or organic felts. The materials are laid in asphalt to make a vapor-retarding membrane. Another type of bituminous vapor retarder is called self-adhering modified bitumen.

Other types are made of polyvinyl chloride or polyethylene plastic sheets. These are thin plastic sheet vapor retarders. They are often loose-laid (which will be explained later).

Sometimes contractors do a sandwich-type of construction on steel decks. First, one thin layer of insulation is attached directly to a deck. It is usually attached with mechani-cal fasteners, such as nails or screws. Then, a vapor retarder is laid on top of the insula-tion. Layers of insulation are then installed on top of the vapor retarder.