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Vapor retarders are materials with a perm rating of 0.1 or less and are typically sheet materials such as 6-mil polyethylene, a two-ply built-up membrane or a one-ply modified bitumen sheet. Housewrap should not be used for a vapor retarder because it has inadequate vapor flow resistance.
The presence of a vapor retarder can make it difficult to find leaks, as they can carry water great distances from the source of the leak. However, as discussed above under Roof Decks, vapor retarders must be used on all new concrete decks and on steel decks wherever there is a high humidity occupancy below, especially in cold climates. To be effective, vapor retarders must be airtight at all penetrations. Vapor retarders may also be required to prevent condensation under white or light-colored membranes (cool roofs) in cold climates because the temperatures of such membranes may be so low that even occupancies with low or average interior humidity can cause condensation in such circumstances.
As noted above, if the deck is a new concrete deck, a vapor retarder must be provided on top of the deck, to keep moisture inherent in new concrete decks from migrating into the roof system. This is true, regardless of the building occupancy’s interior relative humidity.