One important reason for using underlayment under a wood floor is to even out imperfections in the subfloor, but that isn't the only reason. Perhaps the most important reason for using it is that underlayment provides a moisture barrier and prevents the flooring from warping. Underlayment also provides a sound barrier, which is useful if the floor is in an upper story, and makes the floor more comfortable for walking. One more thing: Underlayment provides thermal insulation, which is particularly important on the ground floor. Given all these benefits, it would be a mistake to skip the underlay. Professional flooring installers never do, and you shouldn't either.
Manufacturers Require a Vapor Barrier
If you check the warranty on the box of new flooring you just purchased, you'll probably find that installing the product without a vapor barrier voids it. Your plywood subfloor may look dry, but after you install the flooring, moisture can migrate up through the subfloor and penetrate the flooring, and the results are usually disastrous. The boards may cup or crown, or they may separate. This is most likely to happen if the floor is directly above a crawl space, but it can also happen on floors in upper stories, especially in humid climates.
You can meet the vapor barrier requirement by stapling 6 mil plastic sheeting or tar paper to the subfloor, but there are better underlay options. You can purchase foam or vinyl underlays with built-in vapor protection. They provide sound and thermal insulation as well as moisture protection. You can also install plastic or tar paper on the subfloor and cover it with a resilient layer of cork or foam.
Installing a Floating Floor
If you're installing laminate or engineered flooring that snaps together and floats over the subfloor without nails, you need underlayment even more. Besides all the other benefits, underlay helps support a floating floor. Without this support, the boards can buckle when you walk on them, and the seams may separate. Underlays for floating floors are usually made of plastic foam. The only time you would skip the underlay would be if you purchase flooring with a built-in underlay, which is rare. If you purchase this type of flooring, check the manufacturer's specifications before you install it. You may find that the flooring still requires a vapor barrier.
When installing wood flooring on a plywood subfloor, there are almost no good reasons to glue it instead of using nails. However, if you do choose this installation method, you still need an underlayment – but you can't use a conventional one. Enter the spreadable moisture barrier. You apply this product, which you'll find at flooring outlets, with a trowel. Once it dries, it provides a suitable substrate for the adhesive you use to secure the floor.
How to Install Underlayment
Before installing underlayment, it's important to level the subfloor, sanding down any ridges and filling depressions with leveling compound. After leveling the floor in this way, it doesn't hurt to go over it lightly with a floor sander to make sure it's flat. Vacuum well, and you're ready for the underlay.
Whether you use polyethylene foam, vinyl, plastic sheeting or tar paper, you'll want to lay it in full lengths from wall to wall, overlapping the seams by 1 to 2 inches. Staple the underlayment down, especially along the seams and around the perimeter of the room. This ensures the material stays in place while you install the flooring.