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Community and Q&A

Dehumidifier in a basement

Alan B | Posted in Energy Efficiency and Durability on

I have frequently heard mold forms in basements at 65% humidity, hence a dehumidifier should be set at 65 or 60%
I have an unfinished basement with 1/3 crawlspace and openings to outside that i cannot seal at this time. I am in climate zone 6 iirc near Toronto Ontario, we get hot humid summers and cold dry winters.

While i was looking up something for dew point i came across this website

And at 10ºC it suggests mold risk comes at 80% humidity.

Two questions,

How accurate is the 80%?

I have an IR temp reader that reads the floor at ~20ºC and the basement ceiling at ~24ºC, assuming the mean air temp is 20ºC the calculator recommends a max if 68% humidity, is this accurate and should i assume the temp is above the 10ºC rule of thumb below ground for dehumidifier setting purposes?

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  1. User avatar GBA Editor
    Martin Holladay | | #1

    The most important issue here is that you have a crawlspace connected to your basement, and that crawlspace has openings to the outdoors which you cannot seal. Under these circumstances, operating a dehumidifier in your basement is foolish. You are trying to dehumidify the great outdoors, and you will never succeed -- even if you install a huge industrial dehumidifier, and even if you invest $1,000 a month in electricity to try to make it happen.

    The outdoors is very, very big.

    So you need to tell us more about your crawl space, and more about the reasons that you can't seal the crawlspace openings.

  2. Alan B | | #2

    I agree very foolish, but necessary.

    It removes about 10 pints a day in summer, a bit less in spring and fall and none in winter. The blower door test was close to 15ACH50, much of it coming from the crawlspace. Without the dehumidifier the basement humidity was at 100% and the furnace and everything else in the basement started to rust and the framing was developing surface mold, hence the dehumidifier is necessary (i lived here the first year without it but learned the hard way how needed it is).

    My house is a century old plus building which currently has 2/3 basement and 1/3 crawlspace. The crawlspace is uncovered and very shallow in places with apparent gaps to outside that are hard to locate and even harder to reach and seal. Also the house is balloon framed and there are what i call chimneys from the basement to attic in the basement where the basement ends and crawlspace begins (i suspect part of the house where the crawlspace now is was added later. Attic insulation and sealing this gap is a question i was going to ask in another question later.

    Also the house was built incorrectly where the mudsill is at or below ground level on one side so at some point someone added a concrete wall on the outside on that side to build up the side and it extends underground but i'm not sure how it is connected to the brick foundation but there is a gap i can see with a boroscope camera but can't reach (but i suspect this is where mice enter that i occasionally catch).

  3. User avatar
    Jon R | | #3

    As you know, one can dehumidify any reasonably closed off space - similar are air conditioned storefronts where the entire front is open to the outdoors. It works, just not efficiently.

    You may not need to dehumidify 24x7. A few hours per day may be enough to prevent mold.

    I suspect that spray foam would effectively seal the crawlspace.

  4. User avatar GBA Editor
    Martin Holladay | | #4

    According to a paper by Mark Willians (“Developing Innovative Drainage and Drying Solutions for the Building Enclosure”), "Most water-induced deterioration of wood-based products and mold formation requires wood moisture content above 20% (Morris, 1998). Readings from 20% to 28% [moisture content] are typically considered moderate and indicate that damage may occur if moisture levels are sustained. Readings of 30% and above indicate that wood-based components are saturated and damage is likely if sustained (Morris and Winandy, 2002)."

    According to another source (, "As the air’s humidity increases, so does the moisture content of wood exposed to air.  Wood exposed to 90% ambient relative humidity will reach a Wood Moisture Content (WMC) of about 20%."

  5. Alan B | | #5

    Adding new paragraphs seems to not be working for the software, my apologies that this is all stuck in one paragraph. @Jon Spray foaming the walls is tough because of access. Also the walls are brick and i'm no sure how they would handle being cut off from indoor heat in the winter, not a road i want to go down if the foundation starts crumbling. If i had the means i would insulate them from the outside which can be done for 2 sides of the house but is not currently in the cards. @Martin So your suggesting humidity at 90% is where mold starts? That is rather hard to believe.

  6. User avatar
    Dana Dorsett | | #6

    Dry rot can get started at 10C/80% RH but it takes a long time to become a structural risk. At 20C/90% RH it'll happen a lot quicker.

    Vented crawlspaces are a universally bad idea in that climate. The vents are there to manage ground moisture, but become a moisture problem in summer, and in a heated & air-conditioned building adds more moisture to the crawlspace than it removes. Air sealing the vents to the crawlspace would substantially reduce the summertime load on the dehumidifier, as well as reduce the wintertime heat load. The crawlspace (and bricks) would be warmer and drier during winter than they have been, and the exposed wood in the crawlspace & basement would be drier year-round. I'm sure it DOES it pull 10 pints out during the summer with the vents open. Sealing them (and any other vents to the basement or crawlspace) could cut that in half.

    So, why is it that the crawlspace vents cannot be sealed at this time?

    Putting down a ground vapor barrier in the crawlspace floor would be secondary to just stopping the outdoor air infiltration. The dehumidifier is clearly managing the ground moisture as well as the air infiltration. In the winter air infiltration is taking care of the ground moisture (and not adding moisture) but the duty cycle on the dehumidifier would be almost vanishingly small even with the vents sealed, even without a ground vapor barrier.

    1. Alan B | | #7

      There are no vents, there are gaps to outside that would cost a pretty penny to find because of the shallowness of the crawlspace in parts and the concrete secondary wall in the back. Believe me i would love to stop the air infiltration, assuming Hot2000 is accurate its not only causing the dehumidifier load but eats almost 1/3 of the heating consumption in winter.

  7. User avatar GBA Editor
    Martin Holladay | | #8

    The solution to your problem involves air sealing at the perimeter of your crawlspace foundation.

    The work can be accomplished from the exterior, or -- if you have an entrenching tool and a bucket, and lots of time, from the interior (after you lower the grade of the crawlspace).

    A thorough job might involve foundation repairs or even replacement of sections of the foundation.

    A home performance contractor or weatherization contractor can provide you with advice. The work won't be cheap -- but remember, your energy bills are now high, and you posted your comment because you are looking for a good solution.

    1. Alan B | | #9

      I agree but i don't have the means for this unfortunately (plus concrete barrier complications on all 3 sides of the crawlspace on the outside now that i think about it, the supplemental wall on one side, a concrete patio on another and and a sidewalk on the third). So thats why i asked about the dehumidifier, if i can safely turn it up a bit then i can save a bit of energy. Its not a great solution only a very partial one but if possible a more achievable one for my situation at present.

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