Subfloor Focus: Minimizing Moisture – Part 2: Use of Vapor Retarders over Concrete Subfloors

As an installer, it’s critical to understand the different types of moisture control membranes and the advantages of each. Part Two of this series will address installation methods utilizing the NWFA guidelines for moisture control membranes over concrete subfloors. Many flooring manufacturers recommend specific underlayment materials to be used below their flooring. Follow manufacturer recommendations whenever applicable.

The installation of flooring over concrete is not without its intricacies. Concrete appears to the untrained eye to be a solid, impermeable foundation, yet it’s actually a source in itself for moisture. Water is a critical part of the concrete mixture. Even after it has been placed, concrete absorbs moisture in humid weather or by exposure to rain. Concrete does not release this moisture easily and may never dry out enough to accept flooring without a vapor barrier. A vapor barrier is a Class I membrane with a perm rating of <.10.

All concrete surfaces regardless of age or grade level will emit or conduct some degree of moisture, usually in the form of vapor. This is a very natural and necessary function of  healthy concrete — it’s like continual “breathing.” However, too much moisture emission without proper moisture control can result in flooring failures. Many times the blame for this is placed on a faulty product, improper specification, or faulty workmanship when the real reason lies with slab vapor emission conditions and inadequate moisture controls in place.

Moisture conditions at the time of installation are the flooring contractor’s responsibility. When these conditions fall outside the necessary requirements, the contractor should take the initiative to determine the sources and advise the customer of adequate solutions and costs involved before the start of installation. Then let the client choose based on potential risks of each method you have presented and funding available to address the problems. Finally, have them agree to those choices in the contract.

Because moisture can rise through concrete by capillary action, moisture-vapor barriers and moisture retarders need to be part of the installation process. Laid between the base of gravel or crushed stone and the slab, this barrier is usually in the form of heavy plastic, uninterrupted film. This film prevents the penetration of moisture through the slab to the interior surface, where it can ultimately damage hardwood floors. Moisture vapor also rises from the ground; a moisture barrier is required by IRC¹ and IBC² codes beneath new concrete slabs to prevent this. The IRC and IBC require a Class I vapor retarder³ (aka barrier), minimum 6-mil poly under new concrete. Be sure to check local codes which may vary.

You should know, however, that common construction grade poly film tears and punctures easily, is easily perforated, and may deteriorate or dissolve due to bacteria in moist soil which eats it. These issues can result in ineffective moisture control. From my perspective, a better choice would be a more durable 6-mil or thicker 100 percent virgin resin plastic. A membrane with high tensile, tear, puncture, and bacteria resistance that meets ASTM E1745 for plastic vapor retarders in contact with the soil. It only costs a few cents more per square foot and may eliminate future moisture issues.

It’s important to note that beneath old concrete there may never have been a barrier or it may have deteriorated. Even if you know a moisture barrier is beneath the slab, test the concrete for moisture. A vapor retarder is always required when the moisture tests are >3 lb. by the Calcium Chloride test (ASTM 1869) method or 75 percent by the relative humidity (ASTM 2170) method. Simply using the testing method you are most familiar with or is most convenient will not necessarily coincide with the products you are using and may get you in trouble if the floor fails. Even when readings fall within the suggested parameters, it is always suggested to use a moisture control system to minimize the risk of future moisture infiltration.

The NWFA Guidelines define an acceptable vapor retarder over concrete as an “impermeable” vapor retarder. A Class I membrane according to building codes. A few options for an adequate vapor retarder over concrete include:

  1. Screed system over elastomeric peel-n-stick membrane. Photo courtesy of Virginia Hardwood.

    A minimum 6-mil poly, or better, more durable E1745 membrane over the slab or screeds. Usually used with a nail down floated plywood or screed system. It offers an economical, effective, Class I membrane allowing immediate installation of floors.

  2. Poly film/mastic method — 6-mil polyethylene film in 48-inch wide rolls or wider— applied over a “skim” coat of asphalt mastic. The mastic should be troweled with a straight-edge trowel to skim coat the slab, with coverage of about 80-100-square-feet per gallon. After about 30 minutes, during which the solvents flash or evaporate, the polyfilm can be rolled over the mastic by walking over the film, embedding it in the mastic. Air bubbles that form under the film can be forced out toward the seams with a push broom. Again I recommend using the more durable E1745 membrane. This is a tried and true method still employed by many installers today.
  3. Asphalt roofing felt, or asphalt saturated paper in asphalt mastic applied with a notched trowel at the rate of 50-square-feet per gallon.

    Asphalt/Mastic Method — Asphalt roofing felt, or asphalt saturated paper in asphalt mastic applied with a notched trowel at the rate of 50-square-feet per gallon. A second similar coating of mastic and asphalt felt should be added, with overlaps staggered to achieve a more even thickness. Another well-known and tested method still used today.

  4. Adhesive Methods — it is extremely important to use the method specified by the adhesive and vapor retarder and/or flooring manufacturer for glue-down methods. Be sure to follow recommended dry times. Always check with the flooring, adhesive, and vapor retarder membrane manufacturer to ensure compatibility before installation. This method is usually warranted by a manufacturer.
  5. There are several other manufacturer-specific  moisture retarding systems available, including:
    1. Other liquid or semi-viscous materials which typically work in conjunction with or are a part of a specific adhesive system, and even multi-purpose adhesives.
    2. Two-part epoxy, very moisture resistant.
    3. Rubberized elastomeric membrane, peel-and- stick method. Quick and easy install.
Liquid moisture control system.

Some of the above may include manufacturer-warranted systems. The manufacturer’s installation details must be read, followed, and strictly adhered to for the warranty to be honored in case of failure.

In all cases, installers must verify the acceptability of a particular system with the flooring manufacturer, and also verify adhesive compatibility when planning to glue down over any moisture retarding membrane. If using an adhesive method and you are not familiar with the system, check with the adhesive manufacturer, the flooring manufacturer, and the manufacturer of the moisture retarding system before attempting an installation.

Proper selection and installation of a vapor retarder material will help you control moisture migration from below the installed flooring. As always, check with your manufacturer for proper use, capability, and installation of the product.

Stay tuned for my third and final article in the next edition of Hardwood Floors Magazine where I will cover a few specific examples of vapor retarders as they should be used in different scenarios within NWFA Guidelines. Since it is important for you to be aware of alternate methods, I will focus on a few that may be unfamiliar.

Roger Barker is an NWFA Craftsman, Master Craftsman, Advanced Master Craftsman, Vanguard and Ambassador Member and Market Manager Flooring at Fortifiber Building Systems Group in Fernley, Nevada. He can be reached at

1. 2015 Section R506.2.3, Page 149.
2. 2015 Section 1805.2.1, Page 420.
3. IRC 2015 Definitions, Vapor Retarder Class, Page25 and IBC 2015  Definitions, Vapor Retarder Class, Page 39.

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