Hardwood Floors February/March 2017

AT THE SITE

By Roger Barker

Subfloor Focus: Minimizing Moisture

PART 1: WOOD SUBFLOORS

As 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 < 0.01, Impermeable; Class II 0.01 < 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.

Student lays a vapor retardant underlayment during an NWFA training event.

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