Vapor retarder on slab is 6-mil poly in cut-back mastic at 75 ft/gal; 3/4-inch plywood planks are glued with cut-back at 30 sq ft/gal. This system used where the moisture potential is high.

Poly on slab ready for plywood subflooring; plywood can be shot or screwed down or floated.
Most common wood flooring installations require use of a vapor retarder to ensure optimal performance over the life of the floor. These vapor retarders can be placed within the flooring system itself or can be associated with the surrounding structure. They are most often a requirement for solid wood flooring and can be an option for engineered flooring or parquet flooring. They also may afford protection from seasonal moisture changes or it can be an absolute necessity to protect and assure proper long term flooring performance. The primary purpose of vapor retarders is to protect against moisture related problems such as cupping. By "vapor retarder" we mean a material that inhibits the movement of moisture. Since most of the materials allow passage of some moisture, the commonly used term "vapor barrier" is not really accurate.

Sleepers embedded in hot melt mastic; there should be 100 percent mastic to sleeper contact. Sleepers are generally cut in short lengths in order to lay flat. A vapor retarder is required, either hot melt between sleepers or poly over the sleepers.
NOFMA defines a "vapor retarder" as "a material which has a permeance of less than 1 perm. 6-mil polyethylene (.06 perm rating) or equivalent. Ordinary asphalt felt or building papers are not good vapor retarders."(See NOFMA Technical Information Publication "Vapor Retarder is Necessary")

Spreading cut-back on poly for gluing plywood, as in Photo 1 at 30 sq-ft/gal.
In the 1960s and 1970s plywood became the subflooring of choice and plastic sheeting became available to the building trade. NOFMA installation recommendations were changed to include using the 4-mil polyethylene under plywood instead of the 2-layer felt system. This worked well in many instances but where high moisture conditions were prevalent such as coastal areas and slabs at or near grade level, some cupping was a frequent occurrence. To address this cupping in moisture prone areas, the recommendation was changed to use cut-back to glue down the plastic and provide the necessary extra protection. One of the modifications to this system has been to glue the plywood to the poly with cut-back. This provides an even greater level of moisture protection since fastening the plywood and puncturing the poly is eliminated. Since the early 1980s the most often used subfloor system over a slab is poly with or without mastic and 3/4-inch plywood.

2-by-4 floor trusses over crawl space with kraft-faced insulation on the bottom truss cord. The space above creates a high potential for moisture problems as noted by the significant stains already on the plywood.
This 6-mil polyethylene membrane is basically used for thicker, 1/2-inch and 3/4-inch, solid nail down wood flooring over on-grade slabs with earth fill below, slabs over basements, or suspended slabs open to the elements or with no continuing environmental control below. This is required over the on-grade in ground contact slab even though there is normally a vapor retarder already placed under it. A vapor retarder over the slab protects the flooring from normal moisture emission through the slab and the short term temporary elevated moisture condition.

Another material recommended for use with solid wood flooring is # 15 felt that is placed between the flooring and subflooring. Since the perm rating of felt is high, 3-7 perms, it does not effectively stop moisture movement but acts as a seasonal buffer to ease the effects of seasonal extremes. The felt is not expected to stop continuous moisture or the temporary elevated moisture condition.

Poly attached to the bottom of floor joists over a crawl space. The result can be mildew, mold, and rot if moisture gets trapped above the poly.
There are other products used as vapor retarders for wood flooring over slabs. They include concrete sealers, builder grade vinyl flooring, poured hot melt asphalt, some adhesive products, rubberized membranes, and the 2-layer felt with cut-back asphalt adhesive. These different materials should be evaluated to assure their perm rating is near that of the 6-mil poly films. Some issues that should be addressed when evaluating these materials:
• Epoxy type sealers were the first most widely used slab sealer. Cupping sometimes was reported with these materials. The most often related issue to the cupping was the epoxy cracked as the slab cracked and allowed moisture to affect the flooring. Concrete sealers that are deigned to stop moisture movement do not typically seal where cracks in the slab occur. Most all slabs will develop cracks.

Faced insulation between I joists with significant space. Note the evidence of moisture damage and mold to wood members.
• Vinyl may also be an effective vapor retarder. Its use has been most often associated with the direct glue down thinner solid wood and some engineered flooring. Manufacturers of these wood products should be consulted for their specific requirements with this vinyl underlayment. One issue is that plasticizing chemicals which keep the vinyl pliable can affect adhesive used to attach the flooring and may even migrate thru the flooring system and affect the finish. This is especially true with site finished flooring. Another issue that may develop is where moisture conditions in the slab are great enough to accumulate under the vinyl. In this instance the vinyl adhesive is affected and the vinyl "floats" off the slab.
• Poured hot melt asphalt has been used particularly on the gulf coast for many years and if properly applied has performed well with sleeper systems. The poured hot melt asphalt was used even before the cold cut-back. The asphalt is heated, melts, and is poured in rivers on the slab. While hot the sleepers are placed into the rivers. The sleepers become firmly positioned as the asphalt cools. Typical practice was to then pour additional asphalt into the space between the sleepers to about 1/4-inch depth sealing the entire slab with a very effective vapor retarder and an elastic material that can bridge most cracks. It is generally applied by a roofing company and does require the melting kettle.
• The 2 membrane felt and cut-back system has been used as a vapor retarder for over 40 years. It is described in the NOFMA publication, "Installing Hardwood Flooring," as a recommended system over slabs. It was first specified in the 1950s as residential slab construction became a building option. The system is cut-back mastic, #15 felt, cut-back mastic, and #15 felt. No perm rating tests we know of have ever been conducted on this system; but solid wood flooring installed over it has a long history of good performance. One benefit is that the system provides a cushioning thickness making it easier to walk and work over than the poly and plywood shot to the slab. The cut-back mastic may be difficult to obtain in some areas.
• Adhesives are now used as vapor retarders. The earlier epoxy compounds have generally given way to the new urethanes. These can require special application and instructions can contain caveats that require extensive slab preparation and testing to assure performance. These moisture curing urethanes are elastic and can be trowel applied as a continuous film. If cracks develop in the slab a crack does not form in the urethane. NOFMA has done no studies on the effectiveness of these materials so the manufacturers should be contacted to relate their performance as vapor retarders.
• Rubberized membranes generally have very low perm ratings and are elastic, but cost considerations and specialty application can influence their use.

This is the same condition as Photo 7 except before likely damage. Note the insulation is probably sized for solid 2-by-4 wood and not for I-joists. Even if placed against the subfloor, it would likely fall to the bottom flange.
All these vapor retarders are placed directly in the flooring system typically on the slab. The primary rule with thicker solid wood and slab construction is a vapor retarder is absolutely required unless the slab is an upper floor slab and a conditioned space is directly below it. The #15 felt goes between the flooring and the subflooring and is not considered adequate by itself to protect flooring from normal moisture emitted by a slab.

With wood joist systems over a crawl space the only recommended vapor reducing material to be used is the #15 felt which we have already said helps reduce normal seasonal extremes. Using a plastic film within this system either attached to the supporting joists or placed over the plywood can cause the floor system to fail. What can happen is that normal moisture can be stopped and accumulate on one side of the plastic, causing rot. The condition can go unnoticed until significant sag of the system occurs as the joists are weakened. Also, if the plastic is on top of the subfloor, the subfloor can rot as the moisture accumulates. The flooring on the other side of the plastic bears the load until someone actually steps through the floor. NOFMA does not recommend this type of vapor retarder in the wood joist system.

A vapor retarder associated with the surrounding structure and wood flooring is the 6-mil poly used as a ground cover over the earth in a crawl space. NOFMA recommends this as the first line of moisture defense with crawl space construction. The earth should be covered 100 percent with the poly and turned up the foundation wall 6 inches. The poly should be placed as soon as possible after the structure is dried-in, not after the flooring is installed.

Vapor retarders such as kraft or foil faced insulation can also affect flooring performance if not properly placed. Know your area building practices for placement and contact an HVAC engineer if placement problems are suspected. The facing on insulation acts as a vapor retarder and if moisture gets trapped or comes into contact with the flooring system, cupping and or buckling can result. With kraft faced insulation used in a wood joist system, the facing should typically be in direct contact with the subflooring. Any area or space where this doesn't occur such as at rim joists, pipe penetrations, 2-inch-by-4-inch truss joist systems and or even I-joist systems have the potential to allow moisture laden air to affect the subfloor and flooring. Foil faced insulation is even more effective at stopping moisture and should raise a yellow flag to make sure that it is placed as specified by the engineer.

Be sure site conditions are correct and the vapor retarder is properly placed. For the other retarders, ground covers, insulation facing, etc., check to see that they are placed as recommended. When in doubt contact the manufacturer for their recommendation, particularly on special systems.