OSB such as Weyerhaeuser EdgeTM and Edge GoldTM is a proven substrate material underneath hardwood flooring. But like any wood product, OSB is susceptible to moisture. If panels are installed improperly, if their moisture content is too high or if their moisture content differs too much from the hardwood above, problems can arise ranging from squeaks and nail pops to cupping, warping and gapping of the finished hardwood.
Fortunately, ensuring that the hardwood flooring performs well for years to come isn’t difficult. Follow these best practices to ensure proper preparation and installation.
• The basement floor should be poured, all concrete and drywall should be cured, and all doors and windows should be in place prior to installing the hardwood strip flooring. For homes with crawlspace floors, APA – The Engineered Wood Association recommends the crawlspace be well drained and dry when hardwood flooring is installed. Also, says APA, a minimum 6-mil polyethylene sheeting should be installed as a vapor retarder on the ground in the crawlspace prior to installation of the hardwood flooring.
• If installing during warm months, ensure the building is well ventilated; during winter months, heating should be maintained near occupancy levels.
Installing the Subfloor
• National Wood Flooring Association (NWFA) guidelines specify at least 23/32”-thick panels for floor joists spaced 19.2” o.c. or less and 7/8”-thick panels for floor joists spaced 24” o.c. Because of the increased deflection of a 24” o.c. floor system, upgrading to 7/8”-thick panels contributes to improved performance of the floor, at least 20% better nail retention and a better feel underfoot.
• To increase floor stiffness and help prevent squeaks, vibrations and bounce, glue the panels to the joists in addition to nailing or screwing. Use only solvent-based subfloor adhesives that meet ASTM D 3498 (AFG-01) performance standards. When latex subfloor adhesive is required, choose carefully as there is a wide range of performance between brands.
• Because the OSB will naturally expand and contract following installation, it’s important to leave a 1/8″ gap at all panel ends and edges to prevent buckling. The tongue-and-groove profile on Edge and Edge Gold panels automatically gaps the long edge of the panel. For improved performance, glue tongue-and-groove edges together.
• For panels 1″ thick or less, use 8d nails to completely fasten the panel to the joists. For panels thicker than 1″, use 10d nails. Screws of an equivalent root diameter and length may be used instead; use screws from a manufacturer that can provide an ICC-ES Evaluation Report. Do not use drywall screws.
• Keep all fasteners at least 3/8″ from panel edges, with maximum fastener spacing of 6″ o.c. along supported panel edges and 12″ o.c. along intermediate supports (joists) in the panel field, unless specified otherwise.
Prep for Flooring
• As the subfloor is installed, make sure it stays level, and sand any raised joints flat. Lightly countersink protruding nail heads and re-nail areas as needed. With screws, tighten heads slightly into panels to ensure panels are securely fastened against framing.
• Clear subfloor panels of debris and dust.
• Subfloor panels should be dry, with a moisture content of not more than 4% over the expected in-use average moisture content of the flooring (8-12% in coastal regions, 6-10% in inland regions, 5-9% in drier climates).
Installing Hardwood Strips
• Hardwood is generally manufactured to a moisture content (MC) of 6%–9%, but it can absorb moisture from humid air when in storage and during shipping; therefore, schedule delivery of the hardwood only after the house has dried. Unwrap flooring and allow it to acclimate in the home with the HVAC system operating prior to installation, if local codes allow.
• Use a moisture meter to check the moisture content of the subfloor and hardwood. During installation, follow the hardwood flooring manufacturer’s recommendation for acceptable differences in moisture content between the two surfaces. Typically, the difference should be no more than 4% for solid strips less than 3″ wide and no more than 2% for planks 3″ and wider.
• Once the subfloor is properly prepared, the work area is enclosed and the hardwood strip flooring is acclimated, install the flooring following the NWFA’s Installation Guidelines and/or those of the flooring manufacturer.
Along with these steps to control moisture related to hardwood flooring installation, there are a number of additional best practices for the rest of the floor framing to help reduce the chance for squeaks and deflection underfoot. For those strategies, download “Prevention and Repair of Floor Squeaks” here.