How Accurate is Exterior Moisture Testing?

This is a guest blog post by Barry Eliason, of Structure Tech Home Inspections and Private Eye Moisture Testing.

I received a call a few weeks ago that was typical of calls I get several times a year. It was the real estate agent for a client of mine that had recently hired me to do a moisture test on a stucco home they were buying. The moisture testing had found several areas of high moisture and even some soft or missing sheathing, indicating some structural damage. The sellers of the home questioned the accuracy of my report and hired another company to re-test the house. This other company did their testing from the interior rather than from the exterior as I had done. Their report to the home owners? "Every place we tested was dry".

The agent on the phone was politely asking me if I thought I had gotten it wrong. First I summed up the situation. "Well, you now have two different opinions and we need to know which one of them is correct." He agreed and wondered how to resolve the situation. My answer was to do what I had recommended in my initial report: remove some small pieces of stucco in the areas that tested high and see what's going on.

The pin probe testing that we do involves drilling two small holes, using a 3/16" drill bit, about one inch apart through the stucco at each test site. We then insert the probes of a moisture meter into these holes and pound them into the sheathing material that is just behind the stucco. The sides of these probes are insulated to protect them from contacting the stucco or the metal lath fastened to the exterior of the sheathing. The moisture meter passes a small electrical charge between the probes and calculates the percentage of moisture, by weight, of the material being tested based on the level of conductivity. If a material is wet, it's a better conductor and results in a higher reading.

Any reading of 15% or less is considered normal in an exterior wall. A reading of 20% or more indicates excessive moisture and the possibility of structural damage; if not now, certainly over time. A reading of 15-20% is higher than normal and indicates some amount of leakage. Wood materials won't start to rot until their moisture level reaches about 28%, but the decay process will continue until the material dries back down to 20% or less. If it never goes above 28% there may never be any damage. Wood and wood materials are considered to be saturated at about 40%.

Sometimes, as the drill bit passes through the back side of the stucco and encounters the wall sheathing there is little or no resistance and the drill bit pops right into the wall cavity or wooden framing member. This indicates structural damage- usually caused by a moisture problem. This is indicated in the report as "Soft" or "No sheathing detected". In some cases, when the sheathing is completely rotted away, there is nothing left to hold moisture anymore. In that situation it is possible that the moisture meter will not detect any high readings, but the lack of any sheathing indicates a problem and possible structural damage.

I'll be the first to admit that this testing method is not perfect and there is always the possibility that in spite of my best efforts the probe touches something unexpected that produces a false high reading. Usually these "false positives" are isolated high readings surrounded by one or more normal readings. Two or more high readings in an area reinforce their accuracy. I always try to take multiple readings when I encounter areas of high moisture. Contractors that have removed stucco for repairs based on my testing tell me that I usually "nail" it. Most reliable contractors insist that a comprehensive moisture test be done before they start ripping stucco off of the house. When the stucco is finally removed the extent of the damage is often much more than the home owner ever imagined.

Because there are seldom any visual clues as to the extent of the damage, or what is causing it, I recommend that a small area of stucco be removed - about a 6" x 6" area - to verify the accuracy of the test. If this small square is done at the bottom corner of a window it can also give us a clue as to what is causing the failure and what it will require to correct it. Once the stucco is removed (we call this a stucco cut) the sheathing can be re-tested with a moisture meter to verify the accuracy of the original reading. Remember, if the sheathing has never gone over 28% it will not appear to be damaged. In fact, it may appear to be just fine, but the sheathing is in a part of the assembly that by design should always be dry. Even after a driving rain, or during days of high humidity, this part of the wall should be dry. If it is above 20% there is uncontrolled moisture entering the system. This is a problem and steps need to be taken to stop it.

It's extremely rare to have a stucco cut reveal that our initial probe test got it wrong. This is usually the result of the probes touching something like a metal flashing, aluminum tape, a cluster of staples or the wire stucco lath. This is what is referred to as a "false reading". Yet, this is what those with a vested interest in the testing being wrong hang their hat on. Our testing has about a 95% accuracy rate but the party that stands to lose big bucks if we're right will sometimes try to get the entire test thrown out based on a small percentage of false positives. This can be real estate agents, home builders and their insurance companies, and sometimes homeowners. Everyone is entitled to their opinion as the the accuracy of a moisture test, but they are not entitled to their own facts. The results of a probe test are a fact. The accuracy of that test is an opinion. The only way to prove or disprove the accuracy is to start peeling back the layers and see what is going on with our own eyes.

The real estate agent took my advice and had a stucco contractor come out to make some exploratory holes in the stucco to get a look at the sheathing. Can you guess what was found?

The sheathing was wet and damaged enough so that the exterior surface could be scraped off with your fingers! It's entirely possible that the interior side tested by the other company was still dry and undamaged; for now. This house was a ticking time bomb and was going to cost somebody a lot of money to fix. By spending under $500 for a moisture test, my client saved themselves tens of thousands of dollars and a lot of heartache.

The sellers, on the other hand, wasted whatever they paid for the second test rather than go straight to the stucco cut. Once again, the exterior probe testing method proved to be the most accurate, minimally invasive and low in cost. After doing thousands of exterior probe moisture tests I am convinced that it is the only way to go.

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Author: Barry Eliason, Private Eye Moisture Testing

Barry has been a home inspector for nearly the past 30 years, and has been the "go-to" guy for moisture testing inspections in the Twin Cities since 1998.