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Felt a Vapor Barrier To the Editor: Your staff report on vapor barriers, "The Last Word (We Hope) on Vapor Barriers" (8/93), is a fine summary report that addresses many questions on vapor barriers. However, it is not quite the last word. A table accompanying the staff report lists perm values for materials commonly used as vapor retarders (such as plastic films, foils, and building papers). The values listed correspond to perm values from ASHRAE based on the dry-cup method for all materials except for the 15-lb. asphalt impregnated felt, for which the perm value based on the wet-cup test is listed. Because the perm values for a given material vary greatly between the wet- and drycup methods, perm ratings