Subfloor Focus: Minimizing Moisture – Part 3: Vapor Barriers and Pads for Floating Floors

As an installer, it’s critical to understand the different types of underlayments for moisture control and how each can affect installation. In my first two articles in this series, I addressed the guidelines for moisture control membranes over wood and concrete subfloors. In this edition, I will address some 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.

Many in the industry believe that a floated subfloor system with plywood, minimum two layers 3/8” or one layer 3/4” plywood kerfed for flatness, is the most effective way to deal with concrete. The thickness of the system can be a drawback in some situations however.

A vapor retarder is always required over the slab when the moisture tests are >3 lb. by the calcium chloride method or 75 percent by the relative humidity method. Which moisture test you use typically will be dictated by the flooring product selection. In order to have a full understanding of what the slab is doing, it is not a bad idea to use both tests as the calcium chloride method indicates the level of moisture vapor migrating out of the slab, while the relative humidity test indicates the level of moisture within the slab. Use of a non-destructive electronic moisture meter in conjunction with these tests will allow you to identify potential areas of concern as well. Even when test results are within industry acceptability, it is still a good idea to use a vapor retarder in all cases.

NWFA Guidelines recommend a minimum 6 mil construction grade polyethelene film (C & A film) or other impermeable material with a perm of .15 or less over concrete, under a floating wood panel subfloor, or over crawl space ground. This follows the IRC and IBC recommendation as a minimum Class I vapor retarder under poured concrete or over crawl space ground.

NWFA Guidelines¹ offer a better choice: a premium polymer material meeting ASTM E1745 for concrete with higher tensile, tear and puncture resistance is highly desirable. An E1745 membrane meets test standards that far exceed C & A film at a minor increase in cost per square foot. This type of tougher, more-durable material virtually eliminates the potential for pinholes at the time of manufacture or during installation, polymer degradation² of tensile, tear, and puncture resistance properties which may occur due to the effects of chemicals, alkalis, some salts, and biological degradation from micro-organisms in the soil. Some floor manufacturers require an E1745 membrane, which they refer to as a 100 percent virgin resin plastic.

Nail down subfloors always require a vapor barrier with 3/4” plywood nailed to the concrete. The mastic/poly system is a good choice. Another option is to use a manufacturer’s warranted system. With a manufacturer system, all installation requirements must be closely followed for the warranty to be valid.

The 3/4” plywood may be kerfed as needed to allow the plywood to relax and lay flat. There are various methods to do this. The plywood may be fastened with power actuated fasteners, pins, screws, or other fasteners recommended by the manufacturer. Follow fastener manufacturer’s recommendations for corrosion resistance, fastener size, spacing, and layout. Some manufacturers recommend as few as 15 pins spaced in three rows of five fasteners and 8” from ends and edges and all obstructions. Others require a minimum 32 fasteners per 4”x8” panel.

Follow the flooring manufacturer’s requirements for which vapor control systems are recommended.

A minimum 1/8” thick cushion sometimes composed of foam, rubber, or other resilient material is usually required by the flooring manufacturer. Some cushions are also moisture barriers with a film laminated to the pad, or the pad itself is made from a closed-cell foam or resilient material that itself functions as a moisture barrier. These are known as 2-in-1 pads. Other pads are 3-in-1 pads providing a moisture barrier, cushion, and rated sound deadener all in one. Over concrete, a minimum 6 mil poly film or a superior 100 percent virgin resin film meeting ASTM E1745 may be required as well.

Proper selection and use of an underlayment material will help you control moisture migrations from below the installed flooring. Always remember that many flooring manufacturers recommend specific underlayment materials to be used below their flooring. Follow manufacturer recommendations whenever applicable.

Foot Notes: 1) Sec. I, CH. 3, Part IV, E-1; 2) Polymer Degradation – Wikipedia

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

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