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Craig Drumheller
January 31, 2013

How Wet is Too Wet?

You've heard the saying, "it's all relative." That adage certainly holds true when considering moisture content in wood-based building materials. Over the past five years the NAHB Research Center has been conducting an ongoing field investigation monitoring the moisture performance of various common wall assemblies — you can check out the report about the first couple years of this study. Over this period, wall systems have been changed and the indoor temperature and humidity conditions have been varied while the moisture conditions within the wall are constantly monitored.

Each wall system reacted differently to its ambient conditions and orientation. The current configuration has peak sheathing moisture levels between 15% and 35%. It's commonly accepted that a 15% peak moisture content of wood is acceptable and a sustained 35% moisture content will ultimately result in decay and/or fungal growth. The wall sections that experienced 35% moisture content had to be opened up because the high moisture level actually ruined our testing instrumentation. While the walls were open, a mold-like substance was visible over a large portion of the sheathing after being in place for less than a year.

There are industry-established moisture thresholds for wood products at which durability or performance problems may occur. The threshold levels are driven by concern related to rated structural performance, decay, and mold. Wood building materials and assemblies constructed of wood and wood-based products perform best when they remain dry.

Prominent organizations in the wood industry have defined a dry condition for engineered wood products (e.g., OSB, plywood) as having a moisture content of less than 16%; for solid wood it would be less than 19%. These levels relate to structural performance. When wood products are used in conditions where moisture contents exceed these levels, the National Design Specification requires that wet service factors are applied.

The dry design threshold conditions are roughly equivalent to a relative humidity just below 90% which corresponds to a moisture level just below the long-recognized 20% moisture content (MC) threshold for wood, which will prevent propagation of decay. In fresh, uninfected wood and wood products, decay is only likely to be established when moisture content exceeds the fiber saturation point (around 26-29% MC in most lumber species) at temperatures between 50°F and 95°F. The 20% threshold provides a margin of safety with regard to preventing decay propagation.

Conditions necessary to minimize mold growth have been outlined in the first edition of ANSI/ASHRAE Standard 160-2009: Criteria for Moisture-Control Design Analysis in Buildings. The values in the standard translate to lower maximum allowable in-service moisture contents for engineered wood products than the value assumed by the building industry. The mold growth prevention criteria outlined in ASHRAE Standard 160 consist of three parameters — time, temperature, and surface relative humidity. The criterion with the longest time duration element (30-day running average) specifies that surface relative humidity not exceed 80%; this corresponds with a solid wood moisture content of 16%, which approximately equates to a plywood MC of 14% and OSB MC of 13%.

The Research Center continues to install and monitor the moisture performance of wall assemblies for both public and private clients in our outdoor test huts. The next available window for those interested in having a proprietary wall assembly be installed and compared to commonly constructed wall types will occur later this spring. If you are interested, please call 800.638.8556 or contact us online.

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