Is a vapor barrier required by code under a concrete floor slab?
Monday, November 1, 2021
A vapor barrier (also called a vapor retarder) is required by the International Residential Code (IRC R506.2.3) and the Residential Edition of the Florida Building Code for concrete floor slabs on grade at the heated areas of a home. It is not required at driveways, patios, and other areas that are not likely to be enclosed in the future. But a vapor barrier is required under the floor slab of an attached garage because it has the potential for conversion to living space. See our article Where is a 6-mil polyethylene (Visqueen) vapor retarder not required under a concrete floor slab on grade? for a full list.
The vapor barrier is necessary because, contrary to what the average person thinks, concrete itself is not a waterproof material. Water can move upward through a concrete slab by capillary action, the same way that a sponge will suck water up out of a puddle, just much slower.
For more on this subject, see our articles Why does my concrete floor slab sweat and get slippery?
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