Lab · Technology

Moisture meters and thermal imaging

Where it fits

• Putting a number on moist wood, one of the main conducive conditions. Mississippi State extension lists moist wood from plumbing leaks, roof leaks or condensation as a major condition that makes a building more likely to be attacked. The USDA Wood Handbook describes electric meters as fast and able to measure wood already installed in a building, without cutting it beyond the pin holes of a resistance meter.
• Screening walls for subterranean termite moisture. Subterranean termites, especially Formosan, build galleries and carton with wet mud and water. That wood ends up at a different temperature from dry framing, which an infrared camera can pick up. The University of Mississippi patent treats infrared as the first screen, with a second method used to confirm.
• Confirming activity in one suspect drywood board. After the California field trials, the researchers' next-best option to opening the wall was an acoustic emission sensor screwed into the board, then a microwave motion sensor on a tripod to locate the galleries. In the test structure, every board both methods called active was confirmed active when it was cut open.
• Checking behind drywall for movement. In laboratory work, a low-energy microwave device produced detectable signals from a single drywood termite through drywall. The maximum reliable wood depth was 5 cm.

Where it does not

This is the part the brochure leaves out, so it is the part worth reading.

• An infrared image as a termite finding. The patent background calls infrared a nonspecific method that produces many false positives. It lists uneven insulation, air-conditioning ducts, leaks, air moving through wall cracks and moisture problems as other causes of temperature differences. It says results still need walls opened to verify, and that the method needs a high degree of operator skill. A warm or cool patch is a reason to probe, nothing more.
• Drywood termites, with infrared. In the California trials, images at room temperature showed no pattern of damage. After boards were heated for 5-10 seconds at 540 °C, the patterns turned out to be knots, peeling paint and chips, not galleries. Behind paneling, drywall or stucco, no damage could be seen at all. Drywood termites do not use water to build, so there is no moisture gradient to image.
• Reading a moisture meter above about 30%. The Wood Handbook gives a reliable range of about 6-30% for resistance meters and 4-30% for dielectric meters, and says readings above 30% are qualitative only. Resistance readings need a temperature correction of about 0.5% for every 10 °F away from calibration, plus a species correction. Uninsulated pins read the wettest wood along their length, so rain-wetted surfaces can read high. Pinless readings are driven mostly by the surface layers.
• Microwave or acoustic tools as a whole-structure clearance. In the field trials the microwave device was reliable over an area of only 5 by 6.5 cm, 3.5 cm deep. Wind and foot traffic produced background motion that could cause false positives. Wall coverings got in the way of both microwave and acoustic tools. In the laboratory study, termite numbers did not predict signal strength, so a strong signal does not mean a large infestation.
• Replacing an experienced inspector. The California report, which was not peer reviewed, could not recommend any single device that reliably shows termites are active in walls, especially up high or behind coverings. Its best option was a trained inspector with 20 years of field experience and permission to open walls.

Sources

• Assessment of Devices and Techniques for Improving Inspection and Evaluation of Treatments for Inaccessible Drywood Termite Infestations, final report — California Structural Pest Control Board / UC Berkeley (c. 2009, not peer reviewed)
• Wood Handbook, Chapter 13: Drying and Control of Moisture Content and Dimensional Changes — USDA Forest Products Laboratory, FPL-GTR-282 (2021)
• US Patent 7,434,990 — Method to detect termite infestation in a structure (documents infrared false positives and the need to verify) (2008)
• Evaluation of Low-Energy Microwaves Technology (Termatrac) for Detecting Western Drywood Termite in a Simulated Drywall System — Journal of Economic Entomology (2018)
• Conducive Conditions (Common Termite Risk Factors) — Mississippi State University Extension