By Mary Uher
Understanding How High Moisture Levels Affect Long-Term Floor Performance
Some of the most common pitfalls of flooring installation center around excessive moisture levels. Moisture at job sites is difficult to control and is particularly problematic when present at the time of flooring installation. Minimize the risk of future problems with wood flooring by following these recommendations, developed with input from the National Wood Flooring Association (NWFA).
NWFA Recommendations
Installation of the flooring material onto the subfloor/substrate requires significant consideration of the job site, including the ambient temperature and relative humidity at the time of installation. The complete list of variables that may affect a wood floor is noted in NWFA’s Wood Flooring Installation Guidelines and Methods. In short, the following criteria should be met (see NWFA guidelines for more specific requirements):
1. The building should be conditioned to the level of moisture/humidity expected during service life.
2. The subfloor must be properly prepared prior to wood flooring delivery or installation.
3. The moisture content of the wood flooring and the subfloor should be within 2 percent for plank flooring (boards 3 inches or wider) and 4 percent for strip flooring (boards less than 3 inches wide).
4. The subfloor and floor system must be flat, with all components adequately attached, allowing no independent movements in the system that may create floor noise.
Typical Conditions Caused by High Moisture Conditions
1. Flooring boards expand as they pick up moisture from the air or from floor sheathing with elevated moisture levels. This expansion forces the flooring boards more tightly together. Specifically:
• When expansion is restricted, flooring board edges can ridge upward, applying greater pressure to the fasteners located along the tongue of the boards.
• Tight joints along tongue-and-groove and end-joints of flooring boards can create noises when rubbing against adjacent boards as a result of foot traffic.
• Expansion areas around the perimeter of a room and vertical penetrations, such as columns or built-in cabinets, leave no space to permit the full expansion of flooring boards.
• A common cause of floorboard cupping is excessive moisture on the underside of flooring that results from insufficient conditioning of the subfloor panels prior to flooring installation.
• Expansion can also cause flooring to lift off the subfloor, which can lead to floor noise.
2. Noise from the flooring can occur any time the flooring is inadequately attached to the subfloor sheathing. This is usually caused by improper fastening schedules, improper fasteners, exposure to moisture, improperly selected subfloor material, or movement in the floor system beneath the flooring.
3. Subfloor sheathing panels shrink as they decrease in moisture content, pulling flooring boards apart as the floor system dries out. This can lead to irregular, wide gaps between flooring boards along joints in the subfloor sheathing, and most commonly, noisy flooring systems.
Minimize the Risk
In new construction, moisture conditions are seldom optimal. In order to compensate for installations that may pose concern, consider some of these simple ways to minimize your risk and create a floor that will perform well for years to come.
1. Check the crawlspace to see if it is conditioned, or that there is a vapor barrier membrane in place on the crawlspace ground, and that it is adequately ventilated prior to wood floor installation.
2. Consider methods to remove excess moisture from the framing/floor sheathing and the rest of the interior of the building prior to installing the flooring (such as using an auxiliary dehumidifier in the short-term), in order to get everything to a moisture content that coincides with the flooring manufacturer’s requirements.
3. Follow the recommended fastener schedule or consider augmenting the attachment by spacing the fasteners at closer intervals. For example, if the recommendations are fastening 8 to 10 inches apart, keep fasteners no further than 8 inches apart. (According the NWFA Guidelines, tighter fastener schedules are acceptable as long as the tongue and core material of the flooring is not compromised or split during installation.)
4. Make sure that the length and diameter of the flooring fasteners meets or exceeds flooring industry and the manufacturer’s recommendations. Flooring manufacturers typically recommend a minimum length of fastener. The greater the length of the fasteners’ embedment in the wood subfloor or the larger shank diameter of the fasteners, the greater the fastener withdrawal capacity.
5. The recommendation for fastening of solid wood flooring boards is often within 1 to 3 inches of end joints. Keep fasteners within 1 to 2 inches of end joints, when possible, to better stabilize movement.
6. Consider using an approved wood flooring adhesive in conjunction with the recommended fastener schedule (glue-assist) for flooring being installed over conditioned spaces or with an approved liquid vapor-retarding system to control moisture mitigation.
7. In some situations, it may be necessary to build in expansion rows within the flooring system. This may include built-in gaps within the floor, t-molding transitions, cork, or other methods.
For additional information, visit https://www.apawood.org/high-moisture-and-wood-flooring
Mary Uher is the Region Manager, Field Services Division, at the Engineered Wood Association (APA) in Tacoma, Washington. She can be reached at mary.uher@apawood.org or 253.620.7400