Vapor Retarders and Wood Flooring
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.
• 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.
• 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.
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.