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Community and Q&A

Do dehumidifiers bring moisture in through the concrete walls in a basement?

SLF52 | Posted in Energy Efficiency and Durability on

My husband and I are debating over if a dehumidifier will do harm/good in our basement. We are in Pittsburgh, PA and have a lot of rain and humidity. We just refinished our basement and have a full bathroom (with no ventilation) and part of the basement stands a laundry room with outside concrete walls as well as drywall. I smell a bit of a musty smell when going downstairs; however, there is no mold/mildew on the walls that can be seen. I want to use a dehumidifier but my husband believes that the dehumidifier is bringing in the air from outside through the concrete walls. There is also a sump pump present in the laundry room. Please advise to whom is correct.

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  1. GBA Editor
    Martin Holladay | | #1

    If your wall has air leaks, they should be sealed. Most concrete walls don't leak air -- typically, air leaks are found between the concrete foundation wall and the mudsill. Use caulk or canned spray foam to seal air leaks.

    A dehumidifier removes moisture from the interior air. It will not affect the rate of air leakage through cracks in your wall or mudsill. Running a dehumidifier will tend to lower indoor humidity levels, not raise them. It helps. (The only disadvantage of running the dehumidifier is that it raises your electricity bill.)

    During the summer, air leaks can admit warm, humid air into your house, raising the indoor relative humidity -- which is why it's a good idea to seal air leaks.

    Most of the moisture entering a house through a concrete wall has nothing to do with air leaks, however. The moisture enters the house through diffusion and evaporation. The concrete wall is in contact with damp soil, so the concrete is damp. The interior face of the concrete allows evaporation.

    For a thorough discussion of ways to address these problems, see these two articles:

    Fixing a Wet Basement

    Preventing Water Entry Into a Home

  2. Anon3 | | #2

    Dehumidifier does increase the rate of moisture entering the house, but that's due to the house now having lower humidity than the outside. Also, it heats up the air, so the air moves up, drawing air outside in.

  3. GBA Editor
    Martin Holladay | | #3

    To the extent that a dehumidifier raises the temperature in the basement, the higher temperature could (in theory) increase stack-effect air flow, especially in winter (when the outdoor temperature is cold). But of course infiltration in winter helps lower the indoor humidity levels.

    It's also true that dryer indoor air increases evaporation from damp concrete walls. That's why the recommendations in my "Fixing a Wet Basement" article focus on reducing moisture entry rather than depending on a dehumidifier.

    Note that the indoor air in the basement space will still be dryer, not more damp, with the dehumidifier operating, in spite of the increased rate of evaporation from the damp concrete walls.

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