Ask the Remodeler: Their guest house is full of mold

Plus, a reader’s concrete foundation is flaking. Should he be concerned?

Send your home improvement questions to [email protected].

By Mark Philben -- Globe Correspondent

November 15, 2023

Q. Dear Ask thee Remodeler: I have a one-room unheated guest house with a half bath. It has no foundation and sits on the ground with a 6-foot space beneath it. The guest house is in a lovely, landscaped spot, and my children enjoyed staying there when they were young. We did not use it last summer, and it developed a serious mold problem because of the damp weather. Black mold is creeping up the walls, and the paint is peeling. I should have put a dehumidifier in there, but now it’s too late. How can I fix this problem?

CAROLYN

A. Sounds like you have a project on your hands. Before you do anything, you should address the mold problem, which can become pretty involved. You should have a mold abatement company do this work and have them test it again when they are finished. After that, I would do two things: First, get the place insulated, if possible. Even if it means taking up the floor, try to get a vapor barrier between the floor system and ground. Second, I would make sure that if the bottom 6 inches between the floor and the ground are covered with siding or something, that there is some kind of venting so air can move through the space. Assuming you have tightened things up, I would make sure that the space gets ventilated even when not in use — either by opening windows regularly or installing a mechanical vent on a timer that will exchange the air inside periodically.

The foundation is crumbling in small areas in this reader’s foundation.

Q. We have a 26-year-old house with a poured-concrete foundation that we’ve owned since it was new. About 20 years ago, I painted the foundation with a Drylok waterproofer. We have a few spots on the interior where the foundation walls are flaking. I assume this is moisture-related, but we’ve had water in the basement only once, back in 2010. It does get humid down there in the summer. What kind of professional should I contact for help? Do I need a structural engineer? I don’t want this to affect the future resale of this property.

S.B.

A. Based on your pictures, I wouldn’t do much of anything. You definitely do not need an engineer; the foundation wall is not going to fail. It could be spalling or efflorescence. “Spalling” is the breaking away of the concrete surface. “Efflorescence” is when moisture pushes its way through and leaves a salt residue. Your damage looks minor, and given that it’s near the bottom, I would guess that the water table stays at that level outside the foundation for a good part of the year. Just clean up the flaking and monitor things. I think the moisture will continue to seep through slowly, but now it has a few openings where it can dissipate into the basement. There’s no harm when it is in such small amounts. If you try to seal it again, you will just be trapping the moisture and starting the spalling process over again.

Mark Philben is the project development manager at Charlie Allen Renovations in Cambridge. Send your questions to [email protected]. Questions are subject to editing.