Subfloors and Underlayments

The market for flooring materials is vast and has been expanding by leaps and bounds ever since mankind decided that bare dirt underfoot just didn’t jibe with the interior décor. From plush carpet to contemporary tile to classic hardwoods … From linoleum to laminates to cork—even natural fiber—the choices and materials for flooring seem endless. Yet, to keep any material looking new and performing longer, you have to start with a strong foundation, the subfloor.

Superior Subfloors

When most homeowners think of flooring, they think of finished floors. However, it’s foolish to overlook what’s hidden beneath a flooring system. For example, consider the case of finishing a basement on a concrete slab. Concrete notoriously transfers moisture, so you should never install carpet or wood directly over a bare slab; they’ll ruin in no time. Flooring experts generally recommend against installing traditional plank hardwoods below grade, because the changes in moisture can shrink, swell, warp and generally ruin the floor. Engineered or laminate flooring is a better choice in this case.

There are two typical methods to create the subflooring to protect from this dampness: Plywood-on-slab and “Sleepers.” The plywood should be 5/8-inch or thicker panels. “Sleepers” are 2-by-4 lengths of wood on the slab spaced 16 inches on center. But first, begin either method by covering the slab with a vapor barrier. You can cover the entire slab with 4- to 6-mil polyethylene film, overlapping the edges 4 to 6 inches, and allowing the sheeting to extend under the baseboard on all sides. Install either the plywood or sleepers. Before installing the finished floor, cover the plywood with either building paper or asphalt paper; cover the sleepers with an extra layer of polyethylene.

To make installing the subfloor situation easier to accomplish, you can opt for a floating subfloor system. These newer systems consist of sturdy 2-by-2-foot OSB panels manufactured with a raised, rigid moisture barrier bonded to the underside of the panels. Depending on the manufacturer, the moisture barrier is made of polyethylene or closed cell polystyrene (Styrofoam). The raised channels created by the cleats of the barrier allow air to flow underneath the subfloor system keeping floors warm and dry. The airspace also functions as a layer of insulation between the cold, hard concrete and the finished floor. The panels interlock easily in a tongue-and-groove fashion and require no fastening or gluing. Plus, the panels are engineered to allow uniform expansion and contraction with changes in temperature and humidity.


The Barricade system from OvrX includes not only modular subfloor panels, but also wall panels.

In fact one company, called OvrX, takes this idea a step further. Not only does OvrX offer Barricade modular subfloor panels, but also insulated wall panels for moisture protection on below-grade walls.

A similar concept comes from Cosella-Dorken’s Delta-FL product for laminate floors in basements. However, instead of tongue-and-groove panels, the Delta-FL product comes in rolls; it’s a gray, heavyweight plastic moisture barrier with “dimples” to create the insulating air pocket. Just unroll the barrier on top of the concrete floor and overlap at the seams for protection against dampness. Then apply a layer of foam overtop, followed by interlocking laminate panels. It creates a slightly resilient floor surface, comfortable for walking, but strong enough to support weights of up to 5,200 pounds per square foot. The finished floor measures about 1 inch high.

Cosella-Dorken’s Delta-FL product comes in easy-to-install rolls to protect interlocking laminate floors installed over concrete slabs.

Softer Solution

For carpeted floors a new type of sound-absorbing carpet underlayment, developed in-house by the Acoustics Division of American Micro Industries, addresses two common sound control challenges. Each type of noise requires different types of sound control methods. The product, dubbed the Step Above Impact Barrier, consists of two different materials—a cotton blend and a vinyl content sound barrier. One component of the underlayment reduces the airborne sound transmission between the floors of a building, such as voices, radios and conversation. The other component of the underlayment reduces impact noise transmission, like footsteps, percussion instruments and vibration from appliances or machinery—noise that radiates through the structural parts of buildings.

And, “green” builders take note: By minimizing impact, the barrier also increases the life expectancy of carpet flooring, which in turn reduces the burden on landfills. Furthermore, preliminary testing indicates Step Above can be re-used when carpets are replaced, and its cotton component is made with a 60-percent recycled blend. The 4-by-8-foot sheets are easily cut to fit with basic hand tools.

The DTRA product from Schluter Systems creates an uncoupling layer that allows the tile floors to move independently of the subfloor, preventing cracks in the tiles and grout.

Dependable Tile

To preserve the structural integrity of ceramic tile and natural stone floors, DITRA from Schluter Systems is an uncoupling membrane that is only 1/8-inch thick and weighs 2 ounces per square foot. The uncoupling layer allows the tiled surface to move independently of, or be ‘uncoupled’ from, the subfloor to prevent cracks in the tiles and grout.

The theory behind the system is ages old; the secret of large tile and marble floors that have lasted for centuries lies in the fact that there was no direct bond between the floor covering and the building structure. The uncoupling layer used in the past was a bed of sand between the mortar bed of the tile and the subfloor. Much easier to install than sand, DITRA is a polyethylene membrane with a grid structure of square cavities, each cut back in a dovetail shape, with an anchoring fleece laminated to its underside. The underlayment is rolled in place over a bed of thinset, with the tile installed on top. The membrane allows construction materials beneath the tile to breathe, but is impervious to water so floors and ceilings won’t be damaged by spills and leaks. The system is designed for use on the most commonly used subfloor materials—plywood, OSB, concrete and gypsum-based screed.

Material Resources

American Micro Industries
Schluter Systems


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