Preparing the Wood

Acclimating Hardwood is often an essential task for ensuring the wood doesn't undergo any unforeseen dimensional shifts after or even during the installation. While acclimating, make sure not to store hardwood in a location where the temperature and humidity is not that of the living space within the house, such as a garage or exterior patio. Preferably, the house should have all of the heating, ventilation, and air conditioning systems running for at least five days before the hardwood arrives for acclimation.

Checking Boards' Moisture Content

The relationship between the moisture content of the hardwood boards and that of the subflooring determines whether or not the wood is acceptable for installation. In order to ascertain these moisture values, check various boards' moisture content, approximately 4 boards for every 100 square feet. If the wood is solid and wider than 3", then there should be no more than a 2% difference between moisture content of the wood after it has acclimated and the subflooring. With hardwood that is narrower than 3", the difference between the moisture content of the hardwood and the subfloor should be no more than 4%. As a general rule, wood floor maintains dimensional stability best within a temperature range of 60-80° Fahrenheit and a humidity range of 35-65%. The two exceptions to this rule apply to imported and exotic wood species, as they may react differently to these circumstances, and extreme geographical circumstances.

Processes of Moisture Testing

Subfloor Moisture Testing

In order to get an accurate reading of the sub-floor's moisture level, test approximately 2 locations per every 100 square feet and take the mean average of these values. If there is an unusually high value in one particular region, this probably indicates a problem which should be attended to prior to installation. There are various moisture tests that can be performed to determine whether moisture levels are acceptable for installation. The acceptable moisture conditions for installation are:

  • Moisture readings of less than 14% when using an equivalent moisture meter on wood substrates.
  • Calcium chloride test producing results of less than 3 pounds/1000 square feet/24 hours.
  • A reading of less than 5.0 on a Concrete Moisture Counter.


Vapor Barriers

Vapor barriers for hardwood and wood sub-floors are sometimes referred to as vapor retarders. They can be a membrane, vapor resistant material, or covering with a vapor resistance rating at or above .7 perms, or at or below 50 perms. They are effective at alleviating moisture problems by protecting the hardwood from ground moisture and condensation. They also provide a number of other benefits, including noise reduction, dust reduction, and the elimination of wood-on-wood contact.

However, different vapor barriers work more effectively than others in given circumstances. In the case of a wood subfloor, an impermeable vapor barrier with a rating of .7 or less shouldn't be used. With this level of impermeability, the vapor barrier could possibly trap moisture in or on the wood subfloor.

In the case of a concrete subfloor, there are a variety of methods to test its moisture, including calcium carbide testing, calcium chloride testing, or relative-humidity testing. Before testing, the concrete is required to be at least 30 days old. Various retailers provide concrete moisture meters and relative humidity kits. You can contact the NWFA to find a retailer near you at 1-800-422-4556. Unfortunately, these tests cannot guarantee a concrete slab that is completely free of moisture all year-round.

The vapor barrier specifications for a concrete subfloor diverge from the wood subfloor specifications by requiring a very high level of impermeability at .15 perms or lower, allowing for little or no moisture movement.