Loose floor tile, musty odors, peeling paint, moist floors, rusting metal and other problems of dampness are not limited to leaky buildings. They occur in some of the otherwise best designed buildings and the cause is likely to be water vapor moving through the concrete floor and basement wall. All concrete slabs on or below grade are exposed to ground moisture and will transmit water vapor unless they are protected by a vapor barrier. The slab may appear to be dry and the surface may look dry but below the surface the concrete may hold a considerable amount of moisture. This moisture contributes to all the bad effects already mentioned as well as mildew growth, deterioration of stored documents, efflorescence and the swelling and warping of upper floors made of wood. There are four types of vapor barriers for use below concrete floor slabs: roof felts, plastic films, rubber sheet membranes, and prefabricated asphalt panels. The selection of a vapor barrier should be made on the basis of the moisture conditions of the ground and the intended use of the building, rather than on the basis of initial cost. Roofing felts probably were the first moisture barrier used below concrete slabs. They are available in three foot rolls which are lapped four to six inches and sealed with hot asphalt. polyethylene film is very low in cost and easily installed but is very easily punctured and not a complete barrier at the joints. Polyethylene-coated kraft paper, which has a comparable perm rating, is more durable. Butyl rubber sheeting is a good moisture barrier that is rugged and long-lasting; but it is expensive and must be prefabricated for the job. Prefabricated asphalt and asphalt plastic panels have the lowest perm ratings. They are rugged and, like butyl rubber sheeting, are less likely to be damaged by construction workers placing the concrete.