When it comes to wood floor installation, moisture prevention is of ultimate importance. The National Wood Flooring Association publishes specific guidelines for acceptable moisture levels in plywood and concrete subfloors, and failure to adhere to these can result in cupping, crowning and gapping of hardwood floorboards. In addition , the NWFA specifies a vapor barrier with a permeability between 0.7 and 10 perm(A perm rating is the unit telling us the mass rate of water vapor flowing through one square foot of material. To put it simply, the higher the perm rating, the more water vapor can travel through a material). Among the materials that satisfy its criteria are asphalt laminated paper (tar paper) and asphalt-saturated kraft paper. Plastic sheeting is not a recommended material because it can trap moisture and accelerate the aging of floor.
Tar paper can also be applied as great underlay besides being put on the roof.
You'll also find wood floor underlayment made from materials such as cork, rubber and felt. These are all suitable as long as they satisfy NWFA's permeability requirements, although they may be more expensive than tar paper or kraft paper. Their main advantage is that they provide a cushion, as well as a limited amount of thermal and sound insulation. However, these materials are generally higher in price than the tar paper. In particular, cork is becoming increasingly popular because of its sound deadening property.