The location of a vapor retarder is often a compromise between minimizing water vapor movement through the slab and providing the desired short- and long-term concrete properties. Some specifiers require concrete to be placed directly on the vapor retarder, and others require a granular blotter layer that's placed between the concrete and the vapor retarder. Advocates of each option argue that their preference results in a better concrete slab.
Finishers prefer concrete placed on a granular base because the base absorbs mix water, shortens the bleeding period and allows floating to start earlier. Floor-covering contractors prefer to install their products on concrete slabs placed directly on a vapor retarder. They believe the vapor retarder effectively reduces moisture inflow from external sources, so only water in the concrete pores must exit the slab. They also believe the uncovered vapor retarder acts as a slip sheet, reducing slab restraint and thus reducing random cracking.
Floor designers must weigh the alternatives and consider the benefits and liabilities of the choice they make. A flow chart is provided to help designers determine if a vapor retarder is required and, if so, where to place it.