A vapor barrier must be installed between concrete and hardwood floors, which can be a rubberized elastomeric membrane. However, polyfilm and roofing felt are the two most versatile and commonly used materials for vapor barriers.
Concrete can absorb and retain moisture, which causes hardwood flooring to warp and rot if a proper vapor barrier is not present. Test the concrete floor's moisture content by covering it with a plastic tarp for 24 hours. If the underneath side of the plastic sheet is wet, the concrete is retaining moisture and a calcium chloride test is necessary. If the test results are greater than 3 pounds per 24 hours per 1,000 square feet, a sealant should be applied to the concrete before any type of vapor barrier is installed.