A minimum 10-mil (0.010 inch; 0.254 mm) vapor retarder conforming to ASTM E1745 Class A requirements with joints lapped not less than 6 inches (152 mm) shall be placed between the concrete floor slab and the base course or the prepared subgrade where no base course exists.

Exception: The vapor retarder is not required for the following:

1. 1. Utility buildings and other unheated accessory structures.
2. 2. For unheated storage rooms having an area of less than 70 square feet (6.5 m2) and carports.
3. 3. Driveways, walks, patios and other flatwork not likely to be enclosed and heated at a later date.
4. 4. Where approved by the building official, based on local site conditions.