Ask the Architect

Where’s the mold coming from?


Q. I have a home that was built in the 1920s, and I moved some shelves in my basement recently only to discover black mold on the wall. My basement is a little musty most of the time and I run a dehumidifier, so I was surprised to see such a buildup of mold. I keep the house in excellent condition, and there are no leaks anywhere that I know of. Can you tell me what to look for, since this mold is on the cinder block near the boiler, even though it’s dry around the boiler?

A. Mold is a living organism that, just like you and me, must eat and drink to stay alive. You’re looking for the source of moisture that keeps your mold alive, but it may be hidden in plain sight.

As I have often explained, when looking for the moisture source, follow gravity and look to the highest points above the area where the mold occurs. Even the most well kept exterior can have the tiniest of openings where trickles of water get in. If you’re satisfied that the roof, siding, gutters and window-to-wall connections aren’t the problem, then look at the places where wires, pipes and chimneys penetrate or join the house. You may find that a cable coming through the wall wasn’t sealed, an electric meter has a conduit hole behind it, etc.

After having explored and sealed any of those areas, if the problem still persists, it may not be in plain sight after all, and you may have to do some digging. Before that, however, look for internal issues, such as the humidity level in the basement. Determine how much of the time the dehumidifier has to run, and how much moisture the equipment is pulling out of the room. Are all of the basement windows closed?

Are cold-water pipes creating condensation, and do any of the joints of the pipes look corroded from a possible leak? I’ve even seen where the wrong kind of pipe hangers were used, causing a trade-off of electrons in dissimilar metals, called electrolytic action, which corrodes the side wall of the pipe, eventually opening up the pipe to leaking.

Then we come to the most extreme culprit, the hidden one that probably will take the most effort to resolve: the outside of the basement walls. Your home was built at a time when the masonry block industry was evolving from using real cinders from burned wood in manufacturing to becoming completely made of concrete, mainly because the consistency and strength were greater, and there was more control of the way the block was being made. Unfortunately, almost nothing lasts forever, and the coating on the exterior of the foundation walls was a tar-like product that has probably turned to dust. By digging in the area of the mold, you can confirm this, and then the exterior foundation walls would need to be “scarified” (cleaned) and re-coated. Good luck!

© 2021 Monte Leeper. Readers are encouraged to send questions to, with “Herald question” in the subject line, or to Herald Homes, 2 Endo Blvd., Garden City, NY 11530, Attn: Monte Leeper, architect.


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