Environmental moisture is the leading cause for failure in wood floors. Never mind spilling water onto your floors, moisture that is already in your crawlspace or basement can and will attack your floors.
Wood expands and contracts as the temperature and moisture content of the air that surrounds it change. Excessive moisture, rapid changes in moisture content, and rapid temperature changes can cause a wood floor to buckle or crack. Managing the environment in the room the floor is installed in is the first step to eliminating failures. Keeping the room at a temperature comfortable for habitation takes care of this.
The second step is managing moisture that travels through the subfloor to the underside of the wood floor. This is done with a vapor retarder (commonly mischaracterized as a vapor barrier). Heating and cooling the house has little effect on this type of vapor migration.
The vapor retarder's job is to limit water vapor migrating to the underside of the wood floor. To do this properly the vapor retarder must be semi-permeable to vapor.
Permeability measures how much water vapor can pass through a material in a given time span. It is measured in perms. A perm rating of 0.1 or less makes the material truly impassible for water vapor. The liner of swimming pool, for example, has a very low perm rating. Materials with a rating of more than 0.1 perms let water vapor pass, but slow down its migration speed to some degree.
A perm rating of 0.5-50 is desirable for installation of wood above a wood subfloor. The retarder will slow down episode moisture spikes, such as that produced by a storm, until the event passes, and it will not trap water below the floor.
Roofing felt is not rated as a vapor retarder
Roofing felt's permeability properties put it into contention as a wood floor vapor retarder. There are, however, two main problems with using roofing felt as a water retarder below a wood floor:
Firstly, roofing felt is not manufactured to the federal standard UU-B-790, the standard for building paper. The testing methods used to rate roofing felt are much less stringent than those used for building paper.
Secondly, roofing felt (both in the #15 and #30 variety) has a considerable thickness. Overlapping runs as required for a correct installation will cause ridges. The final floor will mask these ridges, but they will remain present none the less. Roofing felt's thickness can, in fact, be used to fill in low spots. Layers of felt are quick to cut and install.
If you have a floor installed over roofing felt, there is no need for concern. If you are installing a few floor, however, skip the felt and choose a proper vapor retarder.
Plastic vapor barriers trap water
True vapor barriers can trap water below a floor. Having a perm rating of 0.1 or less, plastic vapor barriers make existing conditions worse by collecting vapor from different incidents until enough water is present to cause the floor to fail. They are probably the worst choice.
Red rosin paper is a poor choice
Rosin paper has perm rating of 100 or more. It offers little protection against water vapor. It should be avoided in favor of rated materials.
Proper vapor retarders
The best choice for the vapor retarder is a rated building paper designed for use under wood floors that meets UU-B-790. Such a paper will retard vapor movement through the subfloor without trapping moisture below the floor. Temporary spikes in moisture levels, like after a storm, don't create enough moisture to penetrate the vapor retarder. Moisture that finds its way below the floor can escape through the paper given enough time.
Fortifier Aquabar "B" is one such paper. It is easy to layer, does no cause ridges, and performs flawlessly. Best of all, it is priced similarly to roofing felt.
The vapor retarder below a wood floor is an important component of the flooring system. Without a proper vapor retarder wild swings in moisture levels can cause failures in the floor. The only correct choice for a vapor retarder is a building paper that meets UU-B-790. Roofing felt, rosin paper, and plastic vapor barriers are poor substitutes. For a long lasting wood floor installation, a vapor retarder is an essential component.