What causes the greatest number of problems with any wood floor? Environmental moisture, whether introduced as a one-time occurrence or as a continuing condition, is the most often identified cause of wood flooring problems. For site-related excessive moisture conditions, the primary objective is to identify and remove the moisture source. Even a recommended vapor retarder will not prevent an excessive ongoing moisture condition from affecting wood flooring. The purpose of a vapor retarder is to prevent the occasional abnormal elevated moisture condition from affecting the flooring. Since this is a one-time event, the wood is protected long enough for the elevated moisture to be removed and thus does not result in a problem.
The NOFMA recommendation for solid wood flooring (strip, plank, or parquet) installed over an on grade concrete slab is to apply a low permeance vapor retarder on top of the slab. The vapor retarder can be: 4-mil or 6-mil polyethylene; 2 layers of #15 asphalt felt (conforming to ASTM D4869 or equivalent) sandwiched by 2 troweled applications of cut-back mastic, apply mastic first; or 4 or 6 mil poly film glued to the slab with cut-back mastic. The more protective retarders, the asphalt and cut-back or poly and cut-back, are recommended where the potential for moisture intrusion is high. Places such as coastal areas or other low-lying locations require the higher protection. There are other materials marketed as vapor retarders such as - adhesive systems, liquid systems, special plastic films, rubberized films, etc.- that can be used as retarders; however, the manufacturer should be contacted for their suitability and expressed warranty with wood flooring.
With multi-story slab construction, a retarder is not always required. Always use the retarder on the first floor, over a below grade area such as a basement, or unconditioned area such as a parking lot or mechanical room. With upper floors, 2nd floor and higher, if the space below is always conditioned the retarder is not required.
A high-permeance retarder (more than 1 perm and up to 7 or 8 perms) such as felt is recommended over the plywood subflooring. Place the felt in the direction of the flooring and lap 2 to 4 inches. This will slow or buffer moisture movement through the system, not stop it, so the effects are reduced as the seasons change. The felt also acts as a noise reducing membrane during the dry heating season. It is also a good clean slip-sheet during installation. With radiant heating, the asphalt is not recommended as odors may be noticeable and adversely affect sensitive people.
To this point, all these systems have dealt with solid wood. For engineered flooring and installation over a slab, some manufacturers recommend placing a vapor retarder over the slab before installation and others do not. Often the recommendation is a proprietary system or materials such as sheet vinyl. This is where the repeated directive occurs-FOLLOW MANUFACTURER'S DIRECTIONS. This also applies to wood joist systems as the high permeance felt is often not recommended. For joist and crawl spaces, poly on the earth in a crawl space is recommended with ANY wood flooring application.
The general recommendations-
• Apply a low permeance vapor retarder (4-6 mil poly) over slabs where 1/2-inch and thicker solid wood flooring is to be installed.
• Place a high permeance retarder (#15 felt) over the subflooring whether slab or wood joist systems for solid wood installations.
• Place poly on the earth in all crawl spaces.
• Follow manufacturer's directions with respect to placing vapor retarders for engineered flooring.