Vapour retarders in wood frame walls - do they have any effect on the drying capability?

Wood frame walls typically need a vapor barrier at the warm side to avoid interstitial condensation due to vapor diffusion and
air leakages from the interior. A more vapor open material than the traditional vapour barriers, here called vapor retarder,
could allow condensed moisture, built-in-moisture or moisture from minor leakages to dry to the interior in addition to the
outward drying. The application of permeable vapor retarders in wood frame walls have been investigated in this study by the
use of a hygrothermal simulation tool. A traditional wood frame wall usually has good drying possibilities to the exterior. If a
vapor retarder should have an effect on the total drying, it must not be too vapor tight. The purpose of this study was to find
some threshold value for the maximum vapor resistance of a vapor retarder – when a requirement is that it should have a
relatively large effect of the total drying of the wall. The increased risk for condensation as the vapor resistance decreases has
however not been investigated in this study. In general it was found that permeable vapor barriers have relatively little effect
on the total drying of ordinary wood frame walls in a Nordic climate.