A vapor retarder for a wood flooring system is a material that inhibits the flow of moisture. The rate of moisture transmission through a material is measured in "perms." Lower perm numbers show greater resistance to moisture transmission. A properly placed vapor retarder can help prohibit problem moisture from adversely affecting wood flooring.

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.

NOFMA installation instructions call for some type of retarder in most solid wood flooring installations. A low permeance retarder similar to 4-mil or 6-mil polyethylene plastic sheathing is most often required with slab construction. A high permeance material such as #15 asphalt felt is sufficient for wood joist construction. Low permeance materials have a perm rating of less than 1.0 perm. Plastic 4-mil or 6-mil sheathing will typically have a perm rating of less than .4 perms.

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.

For an installation on a slab with plywood subflooring, place the vapor retarder directly on top of the slab. For on slab sleeper systems (2 -by-4s on 9-inch, 12-inch and 16-inch center spacing), the poly can be placed over the sleepers only if the sleepers are preservative treated; otherwise, place the poly or felt system directly on the slab. Retarders should be lapped 4 inches to 6 inches along edges to provide continuous coverage. Do not place a poly film over the subflooring. This can stop normal moisture movement and cause moisture accumulation on top resulting in cupping of the flooring. It can also cause deterioration of the subflooring if the moisture is trapped on the bottom.

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.

The NOFMA recommendation for solid wood flooring installed over a wood joist system with plywood, OSB, or 1-by- 6 solid board subflooring is to apply a high permeance membrane (#15 felt or equivalent) over the subflooring. Do not place poly over the plywood, on top of joists, or attached to the bottom of joists. This can place the flooring system at risk of deterioration from excessive moisture. The moisture can collect as condensation on the poly surface and cause mildew and mold growth and associated rot. The only place for poly in a wood joist system is directly on the earth in the crawl space to stop the moisture from entering the crawl space. The earth, rat slab, etc. should be covered 100 percent to protect the floor system.

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.

For adhesive applied engineered flooring where vinyl is recommended as a vapor retarder, be aware that the plasticizers can adversely affect the adhesive. A vinyl blocker may be required to stop this migration into the adhesive. One more time, check with the adhesive manufacturer.

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.