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