Do I Lay the Moisture Barrier or Underlayment for Laminate Floors?

Flooring

• Concrete floors generally have some moisture content because the material wicks the moisture from the ground under the floor and from the air. Wood subfloors over crawl spaces can be damp due to the heat exchange between the house and the crawl space. If you are laying laminate on concrete or a subfloor over a crawl space, you need a moisture test and the correct underlayment.

Moisture Test

• A moisture test is done on any concrete floor on the first floor or below ground level and any subfloors over crawl spaces. Subfloors made of wood can have a maximum moisture level of 14 percent. Concrete floors can only have a 4.5 percent moisture level. Moisture test kits are available at your local flooring dealers and some home improvement stores. The test must be done before considering the installation of laminate on these types of floors. If the moisture test results are above the maximum levels, you cannot install laminate on the floor, even if you use a moisture barrier underlayment.

Combination Underlayment

• The combination underlayment is used on concrete floors or floors over a crawl space that will have moisture levels under 4.5 percent for concrete and 14 percent for wood subfloors over a crawl space. The underlayment has a polyethylene film 6 mm thick, which provides the moisture barrier, attached to the standard foam underlayment. If the test reveals that the floor has an acceptable moisture level, you would need to use this type of underlayment on the subfloor before laying the laminate.

Other Types of Underlayment

• Aside from the ground level or basement concrete subfloor and the subfloor over crawl spaces, other subfloors such as plywood don’t need a moisture barrier if the floor is installed in a well ventilated building and the wood has had time to acclimate for at least 48 to 96 hours, but this also depends on the climate in your area.

  You can use an underlayment such as standard foam, cork or modified foam. Standard foam underlayment is about one-eighth inch thick. It is used anywhere a moisture barrier is not needed. Besides the standard foam, there is the modified underlayment, which varies is density between manufacturers . It will act as a sound barrier to the floors below the laminate.

  The other type of underlayment is the cork underlayment, which is about one-quarter inch thick and used for raising the subfloor height or providing a higher level of sound barrier. None of these are used as a moisture barrier underlayment.

Geographical Area

• The moisture content in plywood subfloors is generally lower than 10 percent throughout the United States. If you live in the coastal regions where the air is damp and warm, moisture content can be as high as 13 percent. If this is the case, you may need a moisture barrier on plywood subfloors. If you are in doubt about the moisture content of all of the floors you plan to laminate, do a moisture test.