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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.
@Vincenzo -- that is good advice, but it may be a bit cynical. If you have a humidity problem, for example, your installer may know the best underlayment available to help prevent that. A good installer should have some alternatives available at lower prices.
The same is true of your home improvement store. Yes, they do try to push some types of underlayment on customers, but they also like repeat business. For that reason, employees are usually glad to discuss your options with you.
Still, some research is a very good idea. Unless you are an expert in underlayment, you will probably need to know the types available, their prices and the advantages of the different kinds before you talk to someone who has a profit motive in suggesting different types of underlayment to you.
Do your research on underlayment before you pay an installer to put in a floor or you choose to do it yourself. Armed with good research, you can determine what kind of underlayment you need instead of the expensive stuff an installer or your local home improvement store may try to sell you.
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