Moisture Requirements:

• Moisture testing for wood subfloors should be done at several locations in the room - a minimum of 20 per 1,000 square feet. A high reading in one area indicates a problem that must be corrected.

• An acceptable vapor retarder is a vapor resistant material, membrane or covering with a vapor permeance (perm rating) of greater than or equal to .7.

• Install a vapor retarder over wood panel or board sub-floors prior to installing nail down solid strip or plank flooring. Over-lap seams a minimum of 4 inches or more as required by manufacturer or specifier and local building codes.

• Some examples of acceptable vapor retarders over wood subfloors include:

  • An asphalt laminated paper.
  • Asphalt-saturated kraft paper.

• A vapor retarder has some extra benefits in that it eliminates wood-on-wood contact, wood strips slide more easily when positioned, minimizes the impact of seasonal humidity change and may reduce dust and noise levels.

• Over a wood subfloor, do not use an impermeable vapor retarder material with a perm rating of .7 or less, such as 6 mil polyethylene film or other polymer materials, as it may trap moisture on or in the wood subfloor.

• Do not use common red rosin or building paper which is not asphalt saturated. They are not vapor retarders.

• Before moisture testing begins, the concrete slab must be a MINIMUM of 30 days old.

• If the moisture meters indicate the presence of excessive moisture, as per wood flooring or meter manufacturer's recommendations, further testing is required.

• For slabs on grade and below grade, include a test location within three feet of each exterior wall.

• In on-grade and below grade applications, adding a vapor retarder is always recommended.

• The NWFA recommends an "impermeable" vapor retarder with a perm rating of less than or equal to .15, thereby limiting the passage of moisture to near zero.

• A minimum 6 mil construction grade polyethylene film, with perm of .13, or other impermeable material with a perm of .15 or less is recommended. A premium polymer material for concrete with higher tensile, tear and puncture resistance is highly desirable.

• A chemical retarder or urethane membrane, as recommended by the adhesive or wood flooring manufacturer. These are usually in the form of a liquid-applied or trowel-applied membrane dispensed from a bucket following manufacturer recommendations.

• For solid strip flooring (less than 3" wide), there should be no more than 4 percent moisture content difference between properly acclimated wood flooring and subflooring materials.

• For wide solid flooring (3" or wider), there should be no more than 2 percent difference in moisture content between properly acclimated wood flooring and subflooring materials.

• Make sure the concrete slab is flat to the wood flooring manufacturer's specification.


Equilibrium Moisture Content of North American Wood Species
at Various Temperatures and Relative Humidity Readings

Wood Flooring Has a Comfort Level Too: Wood flooring will perform best when the interior environment is controlled to stay within a relative humidity range of 30 to 50 percent and a temperature range of 60° to 80° Fahrenheit. Fortunately, that's about the same comfort range most humans enjoy. The charts below indicate the equilibrium moisture content of North American species of wood under various temperature and humidity conditions. These values do not necessarily apply to imported species.

The left column indicates temperature in degrees Fahrenheit and Celsius. The bottom row indicates percent relative humidity. The values in the chart indicate the equilibrium moisture content (EMC) for any given combination of temperature and humidity. For example, at 70° Fahrenheit and 40% relative humidity, the equilibrium moisture content is 7.7%.
The shaded area indicates the generally recommended range for wood flooring — 6-9% EMC, which occurs when temperature is 60-80° Fahrenheit or 15-26° Celsius and 30-50% relative humidity.



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