Subfloor Focus: Minimizing Moisture


pg51_editedAs an installer, it’s critical to understand the different types of underlayments for moisture control and how each can affect installation. Part one of this series will address the guidelines for moisture control underlayments over wood subfloors. Many flooring manufacturers recommend specific underlayment materials to be used below their flooring. Follow manufacturer recommendations whenever applicable.

A heavy moisture invasion can migrate up through a wood subfloor. It may occur slowly, but its effects are damaging. Proper installation of flooring calls for testing subfloors for moisture. In any case, the moisture content of solid strip flooring, material < 3” should be within 4 percentage points of the wood subfloor. (That is, if the subfloor is measured at 10 percent moisture content, the strip flooring should have no less than 6 percent moisture content and no more than 14 percent.) For solid plank flooring, material 3” or wider, the difference should be no more than 2 percentage points.

When installing wood flooring over a wood subfloor, an acceptable underlayment is a vapor retarding material, membrane or covering with a vapor permeance (perm rating) of greater than or equal to 0.7, and less than or equal to 10, when tested in accordance with ASTM E96. This is the essential element determining whether or not a material is an acceptable vapor retarder.

NWFA Guidelines follows the IRC definition for vapor retarders as: Class I < 0.10, Impermeable; Class II 0.10 < 1.0, Semi- Permeable; Class III 1.0 <10.0, Permeable. Materials with a perm rating of 10 or greater are not considered to be vapor retarders
by the IRC.

Installation of a vapor retarder reduces the rate of moisture vapor migration from below the wood floor. Do not use an impermeable membrane as this will trap moisture, which can lead to subfloor rot, mold or mildew. It is not a good idea to place a material with a lower or nearly equal perm rating over a wood subfloor with a higher perm rating. 23/32” plywood is generally recognized to have a perm rating of .55. This why the NWFA limits the low end of the perm rating to 0.70.

In situations where unconditioned crawl spaces or unfinished basements are directly below the wood flooring, it is always important to check and record the temperature, relative humidity and moisture levels below and above the subfloor to ensure the gradient between the two floors is not too drastic and also that the facility is able to be maintained at appropriate levels.

A vapor retarder will not stop the moisture below the floor from migrating into the installed flooring; it will only slow the process allowing the wood flooring to adjust gradually to seasonal changes. When applicable look for vapor retarders that have passed California indoor air quality regulations.

Other benefits of these types of vapor retarders may include:

  • Elimination of wood on wood contact, making flooring strips or planks slide more easily when positioned during installation
  • Reduces dust and airflow from below solid wood subfloor systems
  • Minimizes the impact of seasonal moisture changes from below the flooring system

Install a vapor retarder over wood subfloors before installing nail down solid or engineered flooring (unless otherwise directed by the flooring manufacturer). The vapor retarder should cover the entire flooring surface. Overlap seams a minimum of 4 inches or more as required by the manufacturer.

Some examples of acceptable vapor retarders may include:

  • Asphalt laminated paper
  • Asphalt saturated Kraft paper
  • ASTM rated roofing felt
  • Some foam padding may be acceptable vapor retarders under floating wood floors, when meeting the above perm rating requirements (.7<10), but are never recommended under nail down applications
This chart provides an overview of acceptable vapor retarders, based on flooring type and installation method. Remember that these are presented as a guideline and that flooring manufacturer recommendations should always be followed when provided.

Proper use of an underlayment material will help you control moisture migrations from below the installed flooring. As always, check with your manufacturer for proper use, capability and installation of the product.

Roger Barker is Market Manager Flooring at Fortifiber Building Systems Group in Fernley, Nevada. He can be reached at

One thought

  1. We had engineered hardwood floors installed in our new construction home. The plank ends started lifting about 1 1/2 months later. I had a wood floor inspector check the floors. The 3mm 5 inch oak veneer measured about 10% MC. The subfloor at two locations were 18% &20%. The floors were installed(nailed) about a month before the HVAC was running. The inspector said the planks could lay flat again maybe. I don’t think that will happen. There are 2 coats of polyurethane on top. Any thoughts?

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