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Community and Q&A

Algae on Vinyl Siding and Crawlspace Moisture

josephny | Posted in General Questions on

I’ve read a whole bunch of old posts and articles, but, like so many other times, I still need some expert analysis and help.

I have a small cottage in the Catskill Mountains of NY, with some fiberglass batting between the 2×4 studs and some mineral wool between the studs and above the ceiling (on the floor of very low attic).

When I got the place, one external wall had some green mold/mildew/algae growing on the vinyl siding.  I removed some pieces and the wood underneath (which was a T-111 type siding) was dry and without any evidence of mold.

Lately I think I smell a mildew smell and have a light covering of dusty stuff that I’m worried is mold.  No other visible signs of mold or dampness.

The 2×6 flooring framing sits only several inches above dirt.  No vapor (or any other type of) barrier.

The perimeter of the house, which the framing sits on, appears to a several inch wide concrete footing (that is, several inches wide and running the entire perimeter).  The footing comes up several inches above the dirt line and has the framing members sitting directly on top of it.

There’s no room for even a skinny guy to crawl underneath.

I’m attaching some pictures.

What would you all do?


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  1. Expert Member
    BILL WICHERS | | #1

    What is the "dusty stuff" that you think is mold? The usual mold you get on moist parts of structures isn't dusty, it's a big stain that looks fuzzy, typically black or very dark gray.

    Mold and mildew are very different from algae. Algae are photosynthetic and somewhat plant-like, so they need light to grow. Mold and mildew are fungi and are fine in the dark. Algae won't invade the interior of your walls or other concealed parts of the structure since it needs light to survive. Mold likes moist and warm, and is usually found in concealed spaces.

    A borescope would help to check in that super small crawlspace. You might want to check behind the T111 in the area of the exterior stain too, just to see if there isn't any hidden leak in that area. Finding concealed "smelly stuff" can be very tedious, but I'd start with places you think might have leaked water at some point, and other places that might be air leaks.


    1. josephny | | #2


      Thank you so much for the explanation.

      I don't believe there is or have been any internal leaks (either from plumbing or weather).

      My concern is that the natural moisture from the ground combined with the minimal air flow between the ground (dirt) and the flooring is a problem.

      I'll keep an eye on it and if I see any more suspicious dust, I'll take some good close up pics.

      I am attaching a couple of close up pics of the mold (I believe it is) growing on the outside of the vinyl siding. This has been here for a long time and I have very recently opened up the area in front of where this is to more sun and air.


      1. Expert Member
        BILL WICHERS | | #3

        That doesn't look like mold. It's green, so it's probably photosynthetic (some kind of algae, maybe moss). It's hard to be sure from the pics. Scrub it off with some household cleaner, I'd probably use one of the Clorox cleaners in the spray bottle. Not a big deal, just don't use anything too abrasive that will scratch up your siding.

        Is your current crawl space vented? If it's not, it could be building up high levels of moisture in the summer season.


  2. Jon_R | | #4

    "Wet and forget" works well on algae, moss, mold and mildew.

    Dehumidification of the interior is easy and usually reduces odor.

  3. GBA Editor
    Kiley Jacques | | #5

    I have the same type of algae on my vinyl siding, and based on a recommendation from one of my Fine Homebuilding colleagues, I use Mr. Clean Magic Erasers to take it off. They work like a charm and the algae stays away for at least a year. The other product I know people have success with is Wet & Forget.

    1. Expert Member
      BILL WICHERS | | #6

      I never thought to try those magic erasers on stuff like this. I always think of them as "get whatever my kid wrote on the wall off the wall" :-) I'll have to be more adventurous with those things now!


  4. user-5946022 | | #7

    An application of Wet & Forget will be MUCH quicker than the magic eraser. Just spray it on and move on. Algae will be gone in a month. I'm betting this is on a north facing side of the cabin?

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