Hardwood Floors August/September 2019

AT THE SITE

thewood is sourced, and the variety of nonwood resins and adhesives used tomanufacture them, it is difficult to get an accuratemoisture reading of thesematerials. The OSB and plywood manufacturing process can also affect their ability to absorb moisture readily in comparison to their solid wood counterpart. The manufacturing process fundamentally alters the wood cell wall chemistry. When the sites that water used to be able to enter through and bind to are now occupied by adhesives and/or if the wood is so altered that those sites are not accessible, EMC will be lower because fewer water molecules can get into the product at the same RH and temperature conditions. The accompanying chart providedby theAPA(theEngineeredWoodAssociation) indicates thedifferences inEMCof eachproduct category (solidwood, plywood, andOSB) at 70°F (21°C).Thedataprovided for plywoodandOSBhas been scrutinized for accuracyby the wood science community, but ingeneral,mostwouldagree that EMCfor solidwood is higher thanEMCfor plywood/engineeredwood, which is higher thanOSB.

Unusually high MC readings must be identified, documented, and addressed to establish the size and magnitude of the problem area. Installation should not proceed until the origin of the moisture has been identified and remedied. **IMPORTANT** Never install a wood floor over a known moisture condition. A known moisture condition is one that you are aware of and could pose future damage to the flooring, the building, or the occupants. It is best practice to always test for moisture regardless of conditions so that any unknown conditions can become known conditions, which then can be handled appropriately. 2 Percent/4 Percent NWFA Guidelines state that solid- strip wood flooring should be within 4 percent MC of the subfloor and solid-plank wood flooring should be within 2 percent MC of the subfloor. This is a general guideline that has proven to be a useful and acceptable tool for determining when it is OK to install wood floors and when it is not. When taking into consideration the many variables that can affect an accurate moisture reading, including the functionality of the tool itself; the temperature of the material being tested; the +/- 1 to 2 percent accuracy of the meter itself; the differences in each species or sub-species; the non-wood components of engineered flooring, plywood, and OSB; and the overall “skill of the operator” factor, it is necessary to understand what your readings really mean. With any of

Fortunately, many of our moisture meter manufacturers work closely with some of the OSB manufacturers to determine proper settings, testing methods, and correction values for their specific subfloor material. It is your responsibility to know your meter when testing wood subfloor materials. It is your meter manufacturer’s prerogative to help you determine the best setting for the material you are testing. When in doubt, check the moisture content of other properly conditioned wood materials (2x4s, newel posts, wood beams, etc.) within the structure to get an idea of where the EMC is in comparison to where it should be, and then compare this value with the EMC chart as a baseline for testing the subfloor. After calibrating your meter for the subfloor materials being tested, check the MC in a minimum of 20 areas per 1,000 square feet and average the results. Pay special attention to exterior walls and plumbing. In general, more readings will result in a more accurate average. The MC of the subfloor should coincide with the expected in-use (e.g., normal living) conditions of the facility, based on the EMC chart. Anything outside of this range would be considered unusually high. Record, date, photograph, and document all of your results.

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