False Readings From Moisture Meters

A: I am familiar with the phenomenonyou have observed in utilizing the nondestructive type moisturemeters in buildings where sheet rock interior walls exist. Whileit is always a good practice to be cautious when a moisture meterindicates an increased level of moisture within walls, I believein the case you have described that another explanation for theincreased reading is likely.

When drywall contractors applygypsum or sheet rock to interior walls they often utilize agalvanized metal corner bead to give a more finished and uniformappearance to corners and edges. A quick trip to a homeimprovement center with a visit to the drywall section will provevery educational and very likely to explain why you are seeingincreased moisture meter readings in these locations. Thesecorner beads are essentially long galvanized metal strips with a90-degree bend along their entire length. Drywall contractorstypically nail these corner beads to all edges before plasteringover them. This not only provides a more finished look but alsoprotects the corners from damage. These metal strips act toreduce the resistance which is normally seen in a dry wall. Thisreduced resistance increases conductivity, which of course yourmoisture meter is fully capable of detecting.

You may want to demonstrate thisto yourself by taking a reading on a metal file cabinet with yourmoisture meter. I think you will see the needle go completelyover to the corner of the red scale. Then try placing severalsheets of paper or even a sheet of cardboard between the meterand the metal file cabinet. As the thickness of the insulatingmaterial increases, you will notice less reaction on the part ofthe moisture meter scale. Since the corner bead lies just belowthe plaster and paint at the corner of the wall it is notdifficult for the moisture meter to detect its presence and reactaccordingly. The lesson to be learned here is to be very cautiousabout what may lie beneath the wall you are testing. Falsereadings can result in poor recommendations to customers.

Jeffrey Tucker is president ofEntomology Associates, Houston. Questions can be sent toQuestions & Answers, c/o Entomology Associates, P.O. Box70375, Houston TX 77270, or faxed to 713/681-9069.