Managing the Migration of Moisture

It is probably the biggest issue all flooring professionals, builders, and homeowners face – moisture migration. Within the home, moisture migration is the transmission of moisture, as water vapor, through surfaces such as walls, ceilings, and floors.

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Protection against moisture involves utilizing moisture control systems through the entire building construction design, from the exterior shell to the interior of the building envelope.

Building scientists study this movement of water vapor through the structure. They help builders to institute adequate building practices to control this moisture flow. A properly designed home and quality construction practices will protect against the infiltration and condensation of moisture-laden air, the effects of seasonal humidity, and the effects of temperature fluctuations between the inside and outside of the structure.

The following are a couple of the fundamentals that help explain how water vapor migrates through the home:

  • Air transportation accounts for more than 98 percent of all water vapor movement throughout the building. Air naturally moves from a high-pressure area to lower-pressure areas by the easiest path possible – generally through any available holes, gaps, cracks, separations, etc. In an effort to control air movement, most homes have these air pathways sealed with air barriers.
  • Vapor diffusion is the physical process of water vapor moving through a material. Vapor diffusion is water vapor moving through a material due to a difference in pressure (pressure gradient) or a difference in temperature (thermal gradient). Diffusion through materials is a much slower process than air transportation. Most common building practices involve the installation of vapor retarders to help slow the migration of moisture through the home.

Water vapor moves through materials naturally. The more porous a material is, the easier it is for water vapor to move through it. The rate at which water vapor moves through a material is measured in “perms.” Higher perms mean higher moisture vapor flow rates.

One of the ways the wood flooring industry has always attempted to mitigate the potential for moisture migration is by using a vapor retarder over the subfloor. The function of a vapor retarder is to control the entry of water vapor in and out of building assemblies.

The International Residential Code describes three classes of vapor retarders (Class I, Class II, and Class III when tested in accordance with ASTM E96 Test Procedure A – the desiccant or dry cup method):

A. Class I vapor retarder ≤0.1 perm. Class I vapor retarders also are considered vapor impermeable membranes or vapor barriers. Examples of Class I vapor retarders include glass, sheet metal, aluminum, polyethylene sheet, and luxury vinyl.

B. Class II vapor retarder >0.1 perm and ≤1.0 perm. Class II vapor retarders also are considered vapor semi-impermeable membranes. Examples of Class II vapor retarders include unfaced expanded or extruded polystyrene, asphalt coated paper, plywood or OSB, and asphalt-saturated kraft paper.

C. Class III vapor retarder >1.0 perm. Class III vapor retarders also are considered vapor semi-permeable membranes. Examples of Class III vapor retarders include gypsum board, fiberglass insulation (unfaced), cellulose insulation, board lumber, concrete blocks, and bricks.

When and where should vapor retarders be used?

In order to determine which vapor retarder should be used, identify whether the space directly below the flooring is a conditioned space or unconditioned space.

Conditioned space is an area or room within the building that is heated or cooled intentionally, and humidified or dehumidified, to be maintained at the same conditions as the living/interior space. These conditioned spaces are used either for the comfort of occupants, or for preserving temperature and humidity-sensitive goods.

Unconditioned space refers to exterior spaces, or spaces within the shell of a building, that are neither directly nor indirectly heated, cooled, humidified, nor dehumidified.

Never use a vapor retarder over a wood subfloor to remedy a known moisture condition, and never install a wood floor over a known moisture condition. A known moisture condition is one that you are aware of, and could pose future damage to the flooring, the building, or the occupants. It is a compulsory practice to always test for moisture regardless of conditions so that any unknown conditions can become known conditions, which then can be handled appropriately.

When installing a wood floor over a conditioned space, no vapor retarder is necessary over the wood subfloor, unless otherwise directed by the flooring manufacturer. The reason is simple – the amount of water vapor in the air below the subfloor system, and above the subfloor system should be nearly the same. As long as those conditions fall within the parameters of the wood flooring requirements, you should not have concerns with moisture migrating from one space to another, adversely affecting the floor.

When installing a wood floor over unconditioned spaces, such as some crawlspaces, garages, or unfinished basements, a Class II vapor retarder (sheet-good or liquid-applied) may be used on wood subfloors to help slow the rate at which potential moisture-laden air could move through the assembly and into the wood flooring. Where holes, gaps, cracks, or separations exist in the subfloor, a sheet-good vapor retarder will slow the rate water vapor can travel into the subfloor by air transportation as well.

Liquid-applied vapor retarders are becoming more and more common within our industry due to the influx of wider flooring options requiring glue-assisted installations. These liquid-applied products should be approved by the adhesive manufacturer, and should contain information confirming that they fall within the appropriate IRC Class II or Class III designation.

In the unique situation where a Class I or Class II vapor retarder has been installed on the underside of the floor joists, no vapor retarder should be installed. Doing so could potentially trap moisture within the floor assembly cavity.

In a perfect world, the home has been appropriately designed and built, and there should be no need to be concerned about moisture migration affecting the wood floor. However, as we have become accustomed to, and are very cognizant of, moisture in buildings is the primary cause for wood floor failure. As retailers, installers, and sales professionals of wood flooring, our role is to try to minimize the negative effects, and manage the migration of this potential moisture through the installation methods and products we use and specify.

Brett Miller is the vice president of technical standards, training, and certification for the National Wood Flooring Association. He can be reached at

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