Wood Moisture Content (WMC) is often used as an indicator of decay problems in homes. The moisture content of the wooden substructure members is routinely being disclosed in the inspection phase of real estate transactions. These readings are usually provided by pest control operators and are included in the “termite letter” (form CL-100 Official South Carolina Wood Infestation Report). Moisture damage occurs in residences across the state of South Carolina and elsewhere in the Southeast due to the region’s warm, humid climate.
Wood Moisture Content is the weight of water in a piece of wood expressed as a percentage of oven dry weight of wood. Fresh cut trees can have a wood moisture content over 200%, while completely dried wood will have a wood moisture content of 0%. Wood in buildings generally has a wood moisture content ranging from 5% to 15% in ideal situations.
Commonly accepted Wood Moisture Content levels, and their indications include:
Below 12% – Readings in this range are common to kiln or oven dried woods and furniture grades of wood and represent dry conditions. Most interior wood is in this range.
12% – 16% – Readings in this range are common to lumber during construction, air dried lumber and “healthy” residential substructures (beneath first floor in crawl spaces). These are typical readings for exterior wood.
16% – 20% – Readings in this range indicate a possible elevated level of wood moisture. Such readings should alert the homeowner to look for a source of excess moisture. The excess moisture source should be corrected if found.
20% – 28% – Readings in this range indicate that conditions are borderline for decay. Surface molds are likely to develop given time. The excess moisture source should be corrected, and monitored until the homeowner can achieve consistent readings below 20%, with an ideal goal of 12-16%
28% and above – Readings in this range are often accompanied by decay damage. Substructures with WMC in this range may show decay or rot in floor joist, sills, and subflooring. Repairs are often required when WMC readings are in this range.
Wood moisture content between 0% and about 28% is dependent upon the relative humidity (RH) of the air. As the air’s RH increases, so does the moisture content of any wood exposed to the air. Wood exposed to air with a RH of about 90% will reach a Wood Moisture Content of about 20%. Above 90% RH or 20% WMC, mold can grow on the wood.
We often have Realtors ask us to wait on a CL100 since it is raining. It is important to understand that this relationship between Relative Humidity and Wood Moisture Content is not an immediate reaction. Rather, it occurs over time. A couple of years ago we had a fall and spring that had average rainfall amounts far above normal. This year we have had nearly 20 inches of rain above normal. Each of these years are examples of the time when prolonged wood exposure to high humidity will affect wood moisture content levels. During periods of normal rainfall activity, the common ebb and flow of the natural environment, precipitation should not affect the wood moisture content in a significant way.
As for comments regarding decay in CL100’s, decay fungi needs liquid water to grow. Once wood is dried below about 28% WMC, enough water is not available to support decay, unless the wood is exposed to liquid water. This water may come from condensation, roof leaks, plumbing leaks, or direct contact with the soil. If decay or WMC readings over 28% are present, find and fix the sources of liquid water quickly.
WMC of framing members in a crawl space will usually be lowest in late winter and highest in late summer, as these periods demonstrate the longest periods of lower relative humidity, thus demonstrating the wood moisture content levels are more of a long term measurement rather than increasing with the presence of rain. If low WMC readings were obtained during the winter, and other signs of high moisture levels are present, obtain additional readings during the summer.
Other signs of high moisture readings include surface mold, evidence of water in the crawl space, evidence of water stains or evaporation from foundation walls and columns, evidence of condensation on ducts and evidence of water drops impacting the soil under ducts.
In conclusion, the CL100 is a tool for the Buyer or Realtor, and it is important to understand the scope of the measurements and what the information you receive is telling you.