Moisture testing is used as an indicator of moisture intrusion or water damage. Wood is generally the most critical of all water-damaged materials as elevated moisture content can result in microbial growth and wood decomposition. Moisture content (MC) is a measure of how much water is in a piece of wood relative to the weight of the wood itself. MC is expressed as a percentage and is calculated by dividing the weight of water in the wood by the weight of that wood if it were oven dry.
The above approach is impractical for real-time monitoring of moisture content in building materials. Instead extrapolations of percent moisture are performed using electronic meters that measure the direct-current electrical resistance between pins driven into the wood. As moisture content increases, electrical resistance decreases.
Normal moisture levels for wood framing and sheathing typically range from 8% to 14%. It is widely recognized that significant decomposition by wood-rotting fungi will occur above the fiber saturation point at 28-30% moisture content. Depending on the type fungi present, pre-conditioning and limited decomposition may occur at moisture levels as low as 20% to 25%. The minimum moisture required for a particular fungal species is defined by its water activity (aW), which is equal to 1/100th of the equilibrium relative humidity when expressed as a percent. Equilibrium Relative Humidity (ERH) is the humidity of a tested material while at the same vapor pressure as the ambient environment. In other words, moisture is not migrating to or away from the tested material. Several fungi, including certain species of Aspergillus and Penicillium, grow at water activities as low as 0.70-0.80, which correspond to wood moisture contents of approximately 16%.
When interpreting moisture test results, it is important to understand that moisture is subject to temporal and spatial variations. Moisture readings for given day or test area do not necessarily represent prior or future conditions within all building materials. Therefore, more than one survey event may be necessary. For best resolution, testing should be performed under suitable environmental conditions, preferably after recent precipitation or immediately following a water damage event.
It is a common misconception that mold will not grow on wood if moisture content is maintained below 20%. The use of 20% as a minimum limit is based on moisture requirements for growth of “dry rot” fungi, not surface mold. Extensive wood rot does require higher moisture levels (typically 28-30% or greater), but surface growth by species such as Aspergillus and Penicillium can occur at 16% and higher.
A moisture content of 16% is well-recognized as the limit for surface mold.
-Lstiburek, J. 2002. Moisture control for buildings. ASHRAE Journal. February 2002.
-Andrews, S. 2002. Mold: a growing concern. Professional Builder. April 2002.
“Wetted materials are presumed dry when their moisture content readings are less than or equal to 15 percent when taken with an intrusive/penetrating moisture meter.”
-The Federal Emergency Management Agency recommends 15% moisture content as a minimum standard for drying and remediation of flood-damaged buildings.
“16-20%: readings indicate a possible elevated level of wood moisture. Such readings should alert the homeowner to look for a source of excess moisture”
-The Clemson University Cooperative Extension Service recommends 16% moisture content as a determinant of elevated wood moisture.
Another misconception is that wood moisture contents less than 20% reflect levels found in new lumber. New lumber often contains elevated moisture due to conditions present during processing and storage. But to avoid excess shrinkage, moisture content of new lumber should in fact be below 14% and preferably below 12%. Various terms are used to reference “dry” wood. For example, the Forest Products Laboratory Division of the U.S. Department of Agriculture uses the term “shipping dry” as the recommended moisture content “to prevent decay in transit”. Similarly, the Timber Harvesting and Forest Engineering Glossary defines “shipping dry” as:
“Having a moisture content (oven dry basis) of 14 to 20 percent. Results in reduced shipping weight and less susceptibility to decay. Used in the international lumber trade.”
-The Timber Harvesting and Forest Engineering Glossary
Lastly, when interpreting your test result it should be noted that recognized industry standards recommend that background moisture levels in unaffected areas should be used as a reference for determining relative dryness.
“Restorers should establish moisture content or drying goals for affected building materials and contents near the beginning of the restoration process, and it is recommended, if possible, that agreement with materially interested parties to the appropriateness of these goals be reached and documented. This can be achieved by determining a dry standard, which is a reasonable approximation of conditions prior to the moisture intrusion, or by comparing moisture content conditions in unaffected areas of the building.”
-IICRC (Institute of Inspection, Cleaning, and Restoration Certification). 2006. S500 Standard and Reference Guide for Professional Water Damage Restoration.