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How does high indoor humidity in summer cause mold in a house?

1869farmhouse | Posted in General Questions on

I understand in attics that lower temperature air at night is not able to hold the moisture, so moisture condenses, which can cause mold.

But in the conditioned space, when outside air is warm and even a poorly dehumidified home has less moisture than outside air, how does say, 65-75% rh lead to mold?

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  1. Expert Member
    Michael Maines | | #1

    It doesn't, at least not directly. This may be somewhat simplified, but it's how I understand it:

    Mold needs three things to grow: temperatures in the range that humans are also comfortable; food, which they can find nearly everywhere; and water, which can be in the form of moisture adsorbed onto surfaces.

    There's usually not much you can do about the first two, so the only thing remaining is moisture. If every surface in your home is above the dewpoint temperature, moisture won't condense and mold won't grow.

    Any surfaces that are below the dewpoint temperature can condense moisture. It doesn't need to be a liquid pool of water; the moisture can be in a very thin layer attached to a surface where you can't see it, but mold can find it. Those surfaces are likely appear at basements and slabs, at window interiors (and other thermal bridges) when the temperature drops at night, around air-conditioning grilles, near toilet tanks filled with cold water, etc..

  2. walta100 | | #2

    I could be wrong but I believe moisture is the required but not necessary in liquid form.
    Growth is possible any time you allow the humidity to exceed 55%.


    1. 1869farmhouse | | #3

      This would make sense if true. Enough moisture, even if not condensed into liquid form, would be that necessary third ingredient.

  3. maine_tyler | | #4

    >"But in the conditioned space, when outside air is warm and even a poorly dehumidified home has less moisture than outside air"

    Are you talking about an air conditioned house?

    Where are you seeing mold (or hypothetically seeing it?)

    Wood and other cellulosic materials are hygroscopic. Condensation doesn't need to occur to change wood moisture content. Look up sorption isotherms.

    1. 1869farmhouse | | #8

      I did intend to imply the house in question has air conditioning, but the question was completely hypothetical. Just trying to wrap my head around the concept.

  4. Expert Member
    DCcontrarian | | #5

    The general rule is that wood will support mold growth if its moisture content is above 20%. It can reach that level just by absorbing moisture from the air. Here:

    is a chart that shows equilibrium moisture content relative to temperature and humidity. Note that the relative humidity has to be 90% or higher for the equilibrium moisture content to get above 20%.

    Where you typically see mold is where you have a part of the house that is cooler. For example, 90% RH at 60F is the same moisture content as 50% RH at 77F. If you have a basement that is cool it can be quite comfortable upstairs and a mold risk downstairs.

    Note that wood is the opposite of air, the cooler it is the more moisture it absorbs. In humid places wood has trouble in cool spots.

    1. 1869farmhouse | | #9

      Thank you. So how to well ventilated attics not mold? Like in a zone 3/4 house in the summer where outdoor humidity is so high, is it just the air movement that keeps things dry enough to not mold?

      1. Expert Member
        DCcontrarian | | #10

        Attics tend to be hot and wood absorbs less moisture the warmer it is.

        Let's say it's a typical summer day, which where I live might be 80F and 60% RH. That's a dew point of 65F. Let's say in the attic it's 100F. At a dewpoint of 65F at 100F the RH is 31%. Looking at the equilibrium moisture content chart, at 100F and 30% RH the EMC is 5.8%. That's really dry, less than half the EMC of kiln dried lumber. That's why the wood in attics is usually really, really dry.

        On the flip side, let's say it's 65F in the basement. If outside air gets in, it's at dew point, you're getting condensation. Where I live it's really common to pick up a 2x4 that's been in a basement for a while and even if it's not wet it feels like it weighs a million pounds.

  5. rockies63 | | #6

    In the summer, the greater danger for mold growth is inward vapor drive. The hot summer sun drives moisture (vapor) through any small hole in the exterior building envelope, where it can condense on the coldest surface - usually the surface of the interior drywall. This problem is especially compounded when the building's interior is air-conditioned.
    Dr. Joe Lstiburek has written on inward vapor drive several times.
    There is also this article from Fine Homebuilding by Martin Holladay.

    1. maine_tyler | | #7

      "the greater danger for mold growth is inward vapor drive."

      Greater than what? Vapor drive is just movement of vapor. If it's going from outside to inside it means there is higher vapor pressure on the outside, but it doesn't really mean much as far as whether mold will grow. If there is interior poly damming up the inward drive and cold surfaces from AC, to the point that moisture content gets too high... then there could be problems. But note that making interior conditions drier (as a dehumidifier would) actually increases inward vapor drive, but that certainly does not increase mold risk.

      Ultimately mold growth is affected by the factors outlined above by Michael, and of which temperature can affect moisture content as DC spells out. He cites 20% MC, though some sources cite slightly lower numbers (19 is common, some even claim as low as 16% wood moisture content).

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