This first picture is of a hot water pipe behind a wall taken with an Infrared thermometer.
At lower left in the picture is my finger pointing at the line that shows in the pictures below. Any pipe in a wall with hot or cold water in it can show up like this if the pipe is close enough to the wall and the temperature difference is great enough.
Two of the several types of moisture meters that I have can also find this pipe. In this case it is a “false positive” due to the pipe being metallic type pipe. If it was a plastic pipe it would not be a false positive but will tell you there is water behind the wall surface.
What it won’t necessarily tell you is if the water is harmlessly contained within the pipe.
The first moisture meter (Tramex) is a “sounding type moisture meter.” In “sounding” mode, the meter sends a signal into the material and depending on how the signal bounces back to the meter a “moisture content” reading is determined. But the signal is not very smart—it will bounce off all kinds of things and result in “false positive” readings.
This next picture shows a “normal” moisture reading for drywall at 0% with the Tramex Meter.
This next picture shows a “positive” (albeit false) reading of about 12% with the meter placed over the location of the pipe.
This next picture, with a second type of meter (Protimeter) shows a “normal” moisture reading for drywall at about 8-9 percent.
As you can see, with this meter placed over the pipe, we once again get a false positive reading of about 14%.
So if the inspector is curious as to whether there is “actually” moisture behind the wall, the next level of evaluation (without going Holmesallistic) would be to use the “pins” on the Protimeter type of meter to see if there is actually a positive reading at the surface of the material itself.
As you can see in this next picture the pin-probe reading is the same as the surface reading obtained with the same meter away from the location of the pipe.
Of course without physical signs of water damage to the wall, even if the pin-probe reading indicated moisture, it “could” still be a false positive.
For example the surface might be contaminated or painted with conductive materials that would fool the meter. In the real world this happens when areas have been flooded with salt water or pets have marked their territory etc. Some wall papers and paneling materials are somewhat conductive.
When I use a pin-probe type moisture meter on drywall and do not get a positive reading, I am confident that I have a higher level of certainty as to there not being a moisture issue than I will ever have with Infrared or pin-less type moisture meters.
So there are many levels of evaluation that the inspector will use to not only find issues but to verify issues and sometimes it is wise to qualify our conclusions, but in the end, the inspector must use his or her brain—and not solely rely on the tools as their disposal.
By Charles Buell, Real Estate Inspections in Seattle
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