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How do I stop a reoccurring mold problem?

Robert White | Posted in General Questions on

I have a client with a set of stairs leading to their finished basement. They have had a reoccurring issue with mold growth on the last few treads and risers. I have added a vapor barrier in the crawlspace behind, re-insulated the stairs with rigid foam and caulked or foamed every last crack. I’ve had these stairs apart twice now and after the last time (2 yrs ago) I thought I’d finally solved the problem. Unfortunately, I got the “it’s ba-ack” call last week. Any ideas?

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  1. GBA Editor
    Martin Holladay | | #1

    The only way that these stair treads and risers can be growing mold is if they are located in a humid environment. Since the problem occurs near the floor, I'm inclined to think that the slab is damp.

    Are you the contractor who finished the basement? It's always risky to finish a basement unless you are absolutely positively sure that the basement is dry.

    Ideally, the basement slab sits on top of a layer of polyethylene, and the polyethylene sits on top of a layer of rigid foam insulation, and the foam insulation sits on top of a well-drained 4-inch layer of crushed stone. If any of these layers are missing, then the basement may be damp.

    Of course, the moisture may also be entering through the basement walls, or through the crack between the slab and the walls.

    Your story might serve as an object lesson to contractors who are asked to finish basements.

    If you own the liability for this problem, you need to lower the humidity levels in the basement. You also need to talk to the customers to see what they are willing to accept. Maybe they are willing to purchase and use a dehumidifier. That's not a great solution, but it might solve the problem.

    Maybe you need to demolish the finish flooring and try to seal the floor. That's not fun.

    Only you know the specifications used for this project. (Assuming that you are the contractor who did the work.) Does the house have working footing drains? Is there a sump and a sump pump? Is there an interior French drain? Since we don't know the specifications, it's hard to guess where the problem is.

  2. Robert White | | #2

    Thanks Martin. We did a major remodel on the upper part of this house 4 yrs. ago but the basement slab and walls were left alone. Considering the age ('60s), I'd be surprised if they had a vapor barrier or insulation. And I have no idea about the drainage but the house was built on a hillside in the Pacific NW so it probably isn't adequate.
    Since we are the ones who replaced the stairs, I've been focused exclusively on that aspect, assuming it was a condensation issue. Your reply was very helpful in pointing out another possibility. Thanks a lot for that.

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