Above-Grade

• Any concrete slab installation that is above the surface level of the surrounding soil is termed an above-grade concrete slab. As a general rule, these types of slabs have an excellent evaporation rate because the concrete is exposed to air and sunlight that allow it to breathe more than in-ground installations, which are surrounded by dirt and cooler temperatures.
  
  

Below-Grade

• Typical concrete slabs in basement areas are known as below-grade slabs because they are located below the surface and surrounded by and resting on top of some form of dirt that traps moisture. You always need to use a moisture barrier in hardwood floors installed below grade because the surrounding soil does not allow the concrete superior evaporation, so the slab retains moisture.
  
  

Climate

• The climate in your area can also affect whether or not you need to use a moisture barrier on an above-grade slab or not. If you live in a humid, tropical environment where there is constant moisture in the air, regardless of the time of year, you need to use a moisture barrier just to be on the safe side. While the concrete can breathe above the surface, there are still condensation issues with humid environments.
  
  

Moisture Tests

• If you want to be absolutely sure whether or not you need to use a moisture barrier, you can hire a professional to come out to your house and run a series of tests to determine the moisture retention level of your slab. You can also run a rough test yourself by putting a 2-foot square of polyethylene film onto the slab and taping it in place then checking for moisture after 48 hours. If there is none, you should be good to go.