Excess moisture in a plywood or wood substrate is a probably just as damaging to a floor installation as high moisture content in a concrete slab. According to industry standards, the moisture content of a plywood substrate should be in the range of 10% to 15%. Dryer than that risks extreme adhesive absorption and subsequently poor adhesive transfer to the flooring. Wetter than that (such as through exposure to rain before installation) and the adhesives will take longer to dry; then when you have installed the flooring and the plywood has dried out, you will see the result of the plywood panel shrinkage through the surface of the flooring. A critical success factor for any floor installation is to make sure that the substrate you are working on is stable and acclimated to the working environment. Anytime you step outside that playing field, you will find yourself in trouble.
Traditional OSB is a problem for resilient and wood flooring installations because of the nature of its construction. Traditional OSB is made from large chips of wood that will swell when exposed to humidity and moisture, causing the texture of the chip structure to show through on resilient floors. According to the Engineered Wood Association, OSB is only suitable as a substrate for stretch-in carpet, which isolates the texture of the OSB from the flooring with a layer of carpet pad. The association goes on to recommend applying a 1/4" layer of underlayment over the OSB for installation of resilient and wood flooring. The main thing to remember about traditional OSB is that it is very absorptive and swells when exposed to moisture (such as rain). This swelling is not good for floor installation. Having said that, there are new OSB products out there that are “engineered” materials, making them different from traditional chip/strand OSB. Some of these new engineered OSB boards have much better moisture resistance and can handle water contact without a change in dimensional properties.
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