One of the most destructive of the forces that attack modern homes is a completely invisible and silent assailant called water vapor. Its ravages cost American families untold hundreds of millions of dollars every year. The real culprit in this whole picture has always been the area underneath the house. The ground area under the average home has been likened to an invisible spring which may produce as much as 20 gallons of water in the form of water vapor each 24 hours. This vapor passes though most building materials with complete ease, for it has only 1/205,000th of the density of water at 32 degrees F. The variety of materials available today as vapor barriers is a tribute to the inventive ingenuity of American manufacturers. Papers, films, felts, plastics, metals, asphalts, glass and mineral fibers, and even mineral aggregates, have been combined in every conceivable way to make effective an durable vapor barriers.