International Building Code 1910.1
General. The thickness of concrete floor slabs supported
directly on the ground shall not be less than 31/2 inches
(89 mm). A 6-mil (0.006 inch; 0.15 mm) polyethylene vapor
retarder with joints lapped not less than 6 inches (152 mm)
shall be placed between the base course or subgrade and the
concrete floor slab, or other approved equivalent methods or
materials shall be used to retard vapor transmission through the
Exception: A vapor retarder is not required:
1. For detached structures accessory to occupancies in
Group R-3, such as garages, utility buildings or other
2. For unheated storage rooms having an area of less
than 70 square feet (6.5 m2) and carports attached to
occupancies in Group R-3.
3. For buildings of other occupancies where migration
of moisture through the slab from below will not be
detrimental to the intended occupancy of the building.
4. For driveways, walks, patios and other flatwork
which will not be enclosed at a later date.
5. Where approved based on local site conditions.