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Building science – sudden seasonal change in basement humidity

bsawers | Posted in General Questions on

All winter long, my basement was extremely dry, even drier than the rest of the house. Very suddenly, my basement became very humid and the dehumidifier is running again. Same thing happened last year around this time.

There hasn’t been enough rain to justify a change in the soil moisture. So, I think it must be a change in temperature.

(I live in Maryland, just outside DC.)

What’s the explanation?

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  1. Expert Member
    DCcontrarian | | #1

    What kind of heat do you have?

    I'm going to guess you're burning gas. All winter long the furnace is taking in air, burning gas, and exhausting it to the outside. The replacement air is coming into the house through leaks. All winter long the humidity of the outside air is much lower than the inside air, so it dehumidifies the inside air. The furnace is in the basement so a lot of the air exchange is happening there.

    Now it's April, the heat is off, the outside humidity is a lot higher. It's warmer outside than in the basement, so when air infiltrates into the basement it's quite humid.

    How's that?

    1. bsawers | | #4

      I think the end of heating may be a driver, but the particular week that the basement went from dry to humid was very dry here. Relative humidity below 30 percent.

      Could changes in the temperature difference between the ground and the basement account for part or all of the change?

      1. Expert Member
        DCcontrarian | | #5

        Yeah, but it was 80F yesterday. That air has a ton more moisture at 30% RH than winter air at a higher RH.

        Most likely there is some moisture wicking through your basement walls. During the heating season the forced air provides enough dehumidification to remove it. Once the heat is off that goes away.

        A leaky house dehumidifies significantly in the winter. If your house was built in 1955 it is almost certainly quite leaky by modern standards. Forced air exacerbates the problem because it relies on pressure differential between the supply and return to circulate the air, which makes the house leak more.

  2. canada_deck | | #2

    More details would be helpful. How is the space heated/cooled? Is there any ventilation into the space? What type of a basement (concrete walls with no windows? walk-out with a full wall of windows? etc.)

  3. bsawers | | #3

    The basement is finished. A single air handler serves the basement and main floor, although the basement is a separate zone. Heat is provided by natural gas, but the air supply is supposed to be external.

    There are four windows, three of which face window wells. A door leads to a garage, which is below grade on one side and half-below on the other.

    The walls are cinderblocks. The house was built in 1955, but I don't know what was done in 2014 during the renovation.

    1. Expert Member
      BILL WICHERS | | #6

      By "the air supply is supposed to be external", I assume you mean you believe you have a "sealed combustion" furnace, with a seperate air intake for combustion air? It's easy to double check that: you should see TWO PVC pipes from the furnace to the outdoors, one is the combustion air supply, the other is the exhaust. If this is what you have, the combustion air shouldn't be contributing to any air leakage within the home. It's still possible you have some combination of leaky ductwork and leaking exterior walls that is making the furnace draw in outdoor air, or force out indoor air, in which case "furnace running" means more air leaking outdoors.

      My guess is the ground has thawed enough to contribute more moisture, and probably some of what DC is saying in regards to outdoor moisture conditions is also at fault here. I used to do a lot of work in your area, so I'm pretty familiar with the generally high outdoor humidity levels there -- which is what makes it so brutal in the summer. High outdoor moisture content means even relatively small air leaks can bring in a sizeable amount of moisture to your home. Since you had similar conditions last year, it makes sense that this is the "normal" state for your area and home.


      1. Expert Member
        DCcontrarian | | #7

        We don't get frozen ground here. I would say right now the ground is more dry than usual, as the trees come out of dormancy they suck enormous amounts of water out of the soil.

  4. Expert Member
    DCcontrarian | | #8

    It would be helpful if the original poster could take temperature and humidity readings in the basement, other floors of the house, and outside, and then convert them to dew points. Normally the dew point is about the same throughout the house. It's usually higher inside than out, occupant behavior like bathing, cooking and breathing introduces moisture into the air.

    If the dewpoint is higher in the basement that means that moisture is entering the basement. If it's about the same throughout the house, it's normal for basements to be cooler which would result in higher relative humidity at the same dewpoint.

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