A vapor barrier is exactly what it sounds like: a thin, plastic layer that works to keep moisture from seeping between the subfloor and hardwood floor. These barriers are usually sold in rolls and can easily be cut down to size.
You’ll find several different types on the market, from those designed for tile (not an issue here), laminate, and even hardwood flooring. Since vapor barriers are so thin, they are easily combined with other types of underlay.
Speaking of combinations, this particular type of underlay consists of both foam and a vapor barrier all in one. It often has the vapor barrier attached on one side of the foam, making it clear which side needs to be placed down facing the subfloor.
If you want a solid underlay that does many things, from reducing moisture and preventing mold to dampening sound and providing insulation, you want a combination underlay.
THE ROLE OF YOUR SUBFLOOR
Your underlay needs to be compatible with your subfloor. Since there are three main types of subflooring—concrete, plywood, and hardwood—you need to know which of the many varieties of underlayment will work best with them. Otherwise, you could end up with quite a problem on your hands.
If you have a concrete subfloor, you’ll need an underlay that prevents moisture from seeping up underneath. A vapor barrier will work well, especially when paired with a lightweight and thin form underlay. In addition, depending on your location (some parts of the country are much more humid than others), you may need to use a spreadable moisture layer.
PLYWOOD AND HARDWOOD
With plywood and hardwood subfloors, you have many more options to choose from. For example, you can go with foam, cork, orcombination underlayments, or even a vapor barrier. In addition, you can use a spreadable moisture barrier if you plan on gluing your wood flooring to your plywood subfloor.