One type of wood moisture meter. The pins are pressed into the wood to obtain a reading.

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Wood Moisture Content (WMC) is often used as an indicator of decay problems in houses. This document describes the meaning and use of readings from a wood moisture meter.

The moisture content of wooden substructure members is routinely being disclosed in the closing process of real estate transactions. These WMC readings are usually provided by pest control operators, and are included in the “termite letter” (form CL-100 Official South Carolina Wood Infestation Report). Wide-spread and often extensive 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 usually has a wood moisture content of 5% to 15%.

WMC Scale’s Meanings:

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 border-line for decay. Surface molds may develop. The excess moisture source should be corrected immediately, and monitored until the WMC returns to the 12-16 range.

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. Repair is 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.

Decay fungi need liquid water to grow. Once wood is dried below about 28% WMC, 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 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. 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 flows 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.

Moisture affects wood in several ways. As air becomes damper, the moisture content of wood exposed to the air changes. The following drawings represent several situation that can occur with hardwood floors as a result of MAD Wood™.

In a proper installation, the boards stay flat and uniformly tight together all year long, as shown in the diagram below.




In the next example. the boards were installed too wet for the house’s long term environment. The result is boards that shrank as they dried, creating excessively large gaps between the boards.




The third situation shows boards that were installed too dry for the house’s environment. The result is boards that expanded and pressed together hard enough to actually compress into ridges.




The forth example shows boards that have cupped because the air on the upper side is dryer (lower relative humidity) than that on the under side. This can often happen in the summer in houses with vented crawl space foundations.




The last example shows boards that have reverse cupped, or humped, because the air on the upper side was wetter (higher relative humidity ) than that on the lower side. This quite often happens happens in the winter in houses with vented crawl space foundations.

Moisture affects wood in several ways. As air becomes damper, the moisture content of wood exposed to the air changes. Due to these changes in wood moisture content, the wood’s dimensions, strength properties and potential for fungal activity change. The following chart summarizes the properties of wood as a function of moisture content of the air. This chart represents a generic species of wood: A particular species or sample of wood may exhibit different property changes, but the pattern is similar.

One of the more visible changes that occur in wood in a home occur with hardwood floors. See examples of problems encountered with hardwood floors as a result of MAD wood relationships.


Bed bugs are insects that feed on people’s blood, like mosquitos do. Bed bugs live inside and like to feed at night. Therefore they are usually found near beds. Hence the name “bed bugs”.

Bed bugs are on the comeback, primarily because of an apparent resistance to pesticides, and because of more travel by people around the world. Hotel and motel rooms become infested, and guests inadvertently spread the bugs.

Treatment with pesticides and chemicals isn’t as effective as in the past. Treatments that do seem to work well involve freezing or heating the bugs. Freezing involves spraying the area with dry ice (or frozen carbon dioxide.) Dry ice is near -110 degrees Fahrenheit. This method appears to work well for treating cracks and crevices where bed bugs like to hid.

Heating bed bugs is another effective treatment option. This method works well for treating large areas such as whole rooms with furniture, bedding and clothes. Temperatures of 110-130 degrees Fahrenheit are necessary to kill adults and eggs. These high temperatures need to be held for 20 minutes or so.

What does this mean for hardwood floors?

Since wood changes size based on temperature and relative humidity of the surrounding air, and relative humidity is affected by temperature, heating or cooling a space can cause wood to expand or shrink. Hardwood floors that get or too cold could warp, cup, crack or check.

In most treatments, the heat or cold is applied relatively quickly. So minor, if any, issues with the floor will occur. In cases where the treatment takes a long time or something goes wrong with the process, some damage could occur. When things go right, floor temperatures won’t get higher than what you could find in front of a large sunny window.

To help keep treatment times to a minimum, it would be helpful to get as much stuff out of the space as possible. If you have a closet slam full of clothes, heating time will take longer. So bag up extra clothes and heat treat them in a tent or PODs or something like that. Don’t just move them from room to room or take them to your neighbors, or you’ll just spread the bed bugs.

If you have questions about treatment effects of your hardwood floors, contact us. For more information about bed bugs in general, contact your local Extension Service or pest control company.