Mold in Wall Cavities

The mold industry largely ignores the subject of mold in wall cavities, or in ceiling cavities, except if there is an active leak or visible mold from a past leak or if a basement has been flooded.

A story

I was called out to a house, because medical tests showed homeowner exposure to high levels of Penicillium mold. The house had a history of multiple leaks, from roofing, from plumbing, and from older skylight installations.

Extensive air and surface (tape) testing was conducted. Room air sample results were within satisfactory levels. Tape testing revealed a few things but nothing noteworthy. However, the air sampling results from testing through /4″ drill holes into walls and ceilings at areas of known leaks were high for Aspergillus and Penicillium.

What do these high results mean?

All we can say is that there is a footprint of mold in these cavities. Since a little bit of mold can give off a high number of spores, we can’t be sure that any mold will even be visible once the walls and ceilings are opened up… but even invisible mold can be present in high counts and can be troublesome for health.

On the other hand, when commonsense is applied, we know that water had to have been trapped in wall and ceiling cavities from past leakage. If water was trapped there, why wouldn’t mold grow? (…a useful question!) The area couldn’t dry out quickly, and there was food and water.

High levels of mold in wall or ceiling cavities may not show up in any room air testing. Some sort of access holes have to be made for testing inside the cavities.

What are the implications for mold in wall cavities for our homeowner’s health?
  • This is a good question. Even though particulates may not be an issue, since solid particles can’t travel through drywall (remember that room air sample results were satisfactory), mold also gives off gases as it grows, and gases can penetrate into room air. These gases are biologically active. That means that they can affect health. One homeowner told me that he got a headache as soon as he walked in his front door. The mold was in the basement, significant residuals from inadequate drying by a company referred by his homeowners insurance company.
  • Why would gases continue to be produced even though a leak was fixed? The answer is because common molds, such as Aspergillus and Penicillium, can continue to grow just from the relative humidity in an unconditioned wall or ceiling space.
  • What health effects come from exposure to mold gases, which, by the way, are known as MVOCs, or volatile organic compounds from mold? Creative research was starting in this field in the early 2000s, but when insurance companies pulled back from paying for these lab tests, the research dried up, from what I was told. Symptoms from mold exposure are typically inflammation, respiratory and asthmatic issues, neurological and sleep issues, etc.
  • I had one client who could locate Stachybotrys mold behind finished walls by walking around the perimeter of the room and pointing out where she felt dizzy.

A core question about mold in wall cavities relates to the medical tests. What do the test results mean? What are they measuring? spores? other particles? gases?

  • Could the medical test done on this homeowner by his physician (whatever that test was) be picking up on exposure to mold gases coming from contaminated wall and ceiling cavities rather than from spores and other particulates in room air (since the room air sample results were good)?
  • We are back in Cutting Edge Information-!! Follow-up is needed here, to try and get an answer from the lab. What aspect of Penicillium is being tested for? What can the lab tell us about the exposure from the test results?
Implications for the mold industry of mold in wall cavities (and ceiling cavities)
  • That’s a tricky question. The mold industry works mainly with spore trap sampling of room air (1 test per floor, with an exterior sample), with a moisture meter, and with a visual examination. A mold inspector could do a conventional mold inspection and completely miss mold in cavities. For that matter, an unconventional mold inspector (like myself) could also miss mold in wall cavities. We don’t have crystal balls. How could we pick up on mold in cavities?
  • If homeowners know the history of the house, then sampling in wall and ceiling cavities would be possible – but for a pre-purchase inspection, the history of the house may not be known, and we wouldn’t be able to drill holes in someone else’s wall or ceiling cavities, anyway.
  • Is there NO way to find out if there is hidden mold in wall or ceiling cavities? The good news is that there is testing, known as MVOC testing, which can pick up on mold growth in a house, including in wall cavities. The bad news is that you can get a positive result but may be clueless about where the mold gases are coming from. A positive result may be from mold growth on basement ceiling joists and subflooring rather than from inside wall cavities.
  • Question: Doesn’t that make buying a pre-owned home risky? And what about buying an apartment in a multi-dwelling unit, where a toilet in the apartment above may have overflowed or some other type of leakage happened? You bet. You get the picture now. The BEST inspection can’t guarantee that there is no hidden mold. The best a home buyer can do is to reduce the risk of buying a house with some hidden problem – but no guarantee that there won’t be some unpleasant surprise (assuming the hidden mold is ever found by accident).
  • The mold industry should not be so fast to pronounce a house OK based on spore trap tests, moisture meters, and visual inspections.

Tip: In some states (such as NY, from what a colleague told me), structure home inspectors aren’t required to work with a moisture meter. If true, this is … perplexing (which wasn’t the first word I wrote down!), to my thinking. You may have to buy your own moisture meter and look for leaks yourself – under windows, bathroom floors around fixtures, inside sink cabinets, lower walls if land slopes towards house, water-stains on ceilings, etc. Moisture meters are available on-line. I have a Tramex Plus, purchased from, but there are less costly models, too. Buy yourself a gas detector, too, while you’re at it, because that may not be part of the inspection kit, either. I work with the TIF combustible gas detector.

What can you do to reduce your chance of buying a house with mold in wall cavities or in ceiling cavities?
  • Buy a house with plaster walls. Mold doesn’t like plaster.
  • Be cautious about buying a house with newer stucco walls. Check out EIFS on-line.  Some contractors are careless with installations.
  • Learn what you can about the history of leaks and extrapolate from there.  Ask how clean up was done. The current homeowner may not know much about the history.
  • Buy a house that has had good upkeep. For a house that has had a neglected, leaking roof at one time, the risks are high for hidden mold in wall and ceiling cavities. You have to follow the path of the water to find the mold, and who knows where the path of the water went? There is a small compartment between each set of studs. Will you have a drill hole done between each set of studs all around the house? That’s not practical.
  • If there is a new roof, how do you gauge whether the previous roof might have been a disaster? Do some math. How old is the house? If the average economic lifespan of a roof is about 20 years and if you can learn how many times the house was re-roofed, what is your judgment about whether roofs were replaced in a timely manner? If a house is 40 years old, and the only new roof was put on recently, for re-sale, that’s a long stretch for one roof. What shape was it in but the time it re-roofed?! …especially if there is only one layer of roofing. Maybe the original roof was so bad that a new roof couldn’t be put down over it, and a tear-off had to be done. You get the picture.
  • Be cautious about buying a house with at-risk wall cavities, that is, with finished, below-grade spaces (finished basements, bi-levels, split levels, walk-out basements).  Concrete isn’t waterproof, so if moisture migrates regularly into wall cavities, there would be risk of mold in the below-grade cavities.
Suppose you suspect that there might be hidden mold in your wall cavities and in ceiling cavities, what can you do short of costly investigations and remediation?
  • For your health’s sake, you may need to proceed with investigations and removal. Judgment calls based on commonsense can be useful. Often, I can pick up on mold in wall cavities by covering a kitchen knife with clear tape and sliding the knife under base molding. Then, I look under the microscope to see if mold is present. You could send me some tape samples from suspect areas. If the tape is negative, there could still be hidden mold, though. Sometimes the knife won’t fit under base molding, or into the crack between carpeting and base molding.
  • When I was in training as an environmental home inspector 20 years ago, one recommendation was that “if you can’t immediately remove a toxin, dilute it.” The air in our houses is often recycled. Even though air conditioning makes the air feel better, it is still the same recycled air, with the same build-up of toxins. Look into a ventilation system for your home, where fresh air would be brought in. One such system is an ERV (energy recovery ventilator). If you have a neighbor who burns wood in his outdoor stove at night, you could turn the ERV off until he stops.
  • A judicious use of an exhaust fan can also help. Let’s say that you suspected mold gases in your bedroom. Put an exhaust fan in the window of a neighboring room and crack open your bedroom window. Stale air from your bedroom will be pulled out by the exhaust fan, with fresh air being the make-up air in your bedroom.

A bad wall cavity story

By the time I was called to this Cape Cod brick house, a test hole had already been opened in a wall cavity. Looking inside, you could see black mold on the sheathing. A microscope was set up, and examination of a tape sample confirmed: Stachybotrys. These homeowners had recently moved into the house and several family members were experiencing symptoms which prompted more investigation. It turned out that the brick used in construction was an inferior quality. Every time it rained, water was soaking into the brick and wetting the plywood sheathing in the wall cavity. What to do? There was no way to prove (or to know) if the previous owners suspected this problem and failed to disclose it. The new homeowners were stuck. Insurance wouldn’t cover it. Could the sheathing even be removed without taking down the brick siding, too? A tight budget led the homeowners to do what they could: put vinyl siding over the brick to stop water infiltration and hope that Stachybotrys would go dormant. They had thought that buying a brick house was a good thing to do…which it can be.

A bad wall, ceiling, floor cavity story

My client had previously called in an Industrial Hygienist to do a pre-purchase mold inspection. He did the best he could, with spore trap tests, a moisture meter, and a visual examination. He even did the ERMI test (DNA-level testing – see Cutting Edge Information). Based on the test results, his words to her were, “It doesn’t get any better than this.”

She bought the apartment. She started reacting in it, and then she noticed that the wall by the bathroom/closet was bulging out. I was called in. I took a tape sample under the base molding at this wall and, bingo, Stachybotrys. There had been multiple leaks from the apartment above, and mold was in the ceiling and wall cavities, plus under the wood flooring along the bathroom and closet walls. This a huge remediation job, followed by new leaks from above, and she lived for more than a year in a rental apartment. The neighbor above refused entrance to his apartment, and a second issue became how to seal off mold gases and water in his apartment from infiltrating into hers. What a nightmare she went through and whether she ever would be able to live in the apartment hung in the balance for a long time. She was very mold-sensitive.

Who knows, maybe I would have missed the bulge, too – but this woman’s dire experience has made me more aware of checking walls around bathrooms. When I owned a pre-purchase franchise years ago, the inspectors were told, “There’s water leakage in the home. Find it.” And now, they don’t even carry a moisture meter, or a gas detector…unbelievable. Maybe they do in your area, if you’re lucky.


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