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green stuff on wood siding

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  • green stuff on wood siding

    It appears that I have a green type mold growing on my oil stained lap cedar siding on the north side of my home ... it is near a kitchen window .... The house wall is 2x6 / 24"oc / 1" sprayed foam w/ R-19 fiberglass batt w/ 6 mill vapor barrier over interior stud wall with 5/8" sheetrock. There is 1" dow foam brd on the exterior part of the wall with 15# felt between the cedar siding (the siding was back primed w/oil stain) and the foam. This part of the home is over a conditioned crawl space with a concrete floor.

    1. What may be causing this? (does not appear on any other location on the house)

    2. How can I clean it off without messing up the stain on the wood? It is directly above a planting bed ... flowers, bushes.

    Thanks for any help.
    richard miller
    northern indiana

  • #2
    Re: green stuff on wood siding

    It is fairly common to have mold and mildew grow on the north sides of homes. The north side does not get warmed by the sun, stays closer to the dew point temperature, and doesn’t dry our as much as the other sides. It also sounds like you have plantings in this area. These plantings get watered, so there is an added source of water and if they are tall, they may be restricting air flow around the building. Mold and mildew only need a relative humidity of ~ 70% to grow. If it is mold and mildew you see, it should be fairly easy to remove. Try this in an area of the wall that is not a primary area of focus to see how it looks before treating the entire side of the house: Mix 1/3 cup of household detergent, 1 quart of household bleach, and 2 quarts of warm water. Scrub the surface with a bristle brush and/or sponge. Leave the solution on the surface for 5 or 10 minutes and then rinse it off. This should kill the growth. When you refinish you should use a stain that has a mildewcide in it. Also, you may be able to purchase a water repellent that contains mildewcide in it. If this is possible, then treating the wall with this may help retard future growth.


    • #3
      Re: green stuff on wood siding

      What about the felt? Could the fact that felt has a perm value of about 5 be a problem? Could the fact that felt absorbs water after time, and the excess moisture is allowing mold to grow? The temperature conditions are right, the mold spores are there, the nutrients are there, and the moisture is there. All four conditions are met for mold growth.

      To the owner, where is the home located - area of the country?


      Bill Chapman


      • #4
        Re: green stuff on wood siding

        It is my opinion that the felt is not to blame. Sure, the requirements for growth are present. This is obvious by the fact that there is growth. Let’s start by looking at the source of moisture. It is either liquid or vapor. I think is most likely deposited by rain or excessively high humidity. Since the home’s wall has (from what I can gather) has exterior grade sheathing and sprayed-in-place foam over the fiberglass insulation in the wall cavity (both materials are less permeable than felt), it is probable that drying toward the inside is limited. And since the sprayed foam is usually airtight and the level of insulation adequate, it is unlikely we have condensation from the inside air. Maybe the crawlspace generates moisture, but a clear path from the crawlspace to the siding isn’t apparent.
        The siding is back primed, but the siding is treated with stain, which does not have high moisture exclusion. The siding can easily get wet, especially as the siding ages. When the siding gets wet, drying to the outside is largely unrestricted by the way the wall is constructed – which makes me think that the wall does not get proper air circulation or exposure to the sun.
        The perm of felt is about 5 when it is dry, but its perm rating rises as it gains moisture. The perm of felt is around 60 when the relative humidity is at 95%. One of the things that I like about felt is that it is dynamic: it retards liquid (and vapor to a degree), but when moisture gets to be a problem (too high), you have a check valve in place to store moisture temporarily and then release it. The felt paper is made of cellulose, the same material that wood is made of. The wood stores more water than the felt.