Since the 1980s, "floating floors" have become standard in new construction, and they are common in remodeling projects as well. These floors are not attached directly to the subflooring. Instead, they are attached to a flooring underlayment that rests on the subflooring. A good underlayment reduces sound, provides insulation, makes the finish flooring more resilient, smooths slight imperfections in the subflooring, and increases the service surface life of the finish floor. A quality underlayment is essential for the success of the finish flooring.
Some flooring underlayment also provides a vapor barrier, keeping the top floor completely dry. A moisture barrier is needed for any floor that is below ground level, and is also useful over a plywood subfloor above a crawl space, which can be quite damp. If there is none, a thin plastic layer should be spread over the subfloor before the underlayment is installed.
Wood products such as plywood or oriented strand board were once standard for underlayment. However, wood tends to shrink and expand as the temperature and humidity change, which stresses the finish flooring and may damage it. Many contemporary flooring underlayment materials do not have this problem.
Gypsum fiber panel is a good choice for flooring underlayment. It has a smooth surface, is quite resistant to moisture, and does not contain dyes or resins that could seep through the top floor and ruin its appearance. It is compatible with a wide range of flooring and with most kinds of adhesives.
Foam underlayment is available under many brand names. It is sold in rolls, and the foam is about 1/8 inch (3.2 mm)thick. Combination underlayment adds a moisture barrier to the foam.
Rubber underlayment provides better noise reduction than standard foam flooring underlayments. So do modified foams such as high-density foam and closed-cell foam. These kinds of underlayments are available in many different thicknesses, and they may include a moisture barrier.
Cork reduces sound better than any other flooring underlayment. The thicker the cork layer, the greater the sound reduction, up to about 7/16 inch (1.1 cm)layers of cork. Many building codes require cork underlayment for floors in upper stories. A separate moisture barrier is required for damp conditions. Cork also has the advantage of being a renewable resource.
When choosing a flooring underlayment, be sure to consider its suitability for use with the finish flooring and the adhesive. Many laminate floors recommend the use of particular types of underlayment, to avoid potential problems. The adhesive should work with both the finish flooring and the underlayment.