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Sealed and ventilated crawlspace, yet moisture is up in high 70s

agurkas | Posted in General Questions on

I have about 1K sq ft full basement and 500 sq ft fully encapsulated crawlspace.

Since I was mitigating radon problem too, I have reinforced thick radon mitigation poly floor to ceiling in the crawlspace taped and sealed. Rim joist is spray foamed, so everything is sealed as well as it could be. There is also a fan constantly doing 150CFM between basement and crawlspace. Two, what used to be, windows are wide open between the spaces.

I also have an 80 gallon heat pump water heater in the basement.

My conundrum is why the heck moisture in the crawlspace is still 70% right now. During spring winter I would have about 50F and mid-40s% moisture. Now it is 65F down there yet moisture is in mid-70s%

(ignore blocked windows in the picture, they are open and that is where crawlspace connects with basement)

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  1. Expert Member
    Dana Dorsett | | #1

    The outdoor air dew points in much of the eastern half of the US (zones 3 & up) are well above deep subsoil temperatures in the summer, but below the subsoil temps in the shoulder seasons, and MUCH below the subsoil temps in winter. The indoor air of a reasonably ventilated fully conditioned space will track that of the outdoor air unless controlled by air conditioning, and it has to be conditioned quite a bit to keep it below 70% RH @ 65F.

    Actively moving 75F, 50% RH air from conditioned space into a 65F crawlspace, at the lower temperature the RH becomes 70%. If you condition the air to 73F, 40% RH, putting it into the 65F crawlspace results in 53% RH.

  2. agurkas | | #2

    I AC only part of the house. Crawlspace is under decently insulated North side of the house where I have run my mini splits very occasionally.

    Did forget to add that I run HRV at lowest speed and it takes in the air from the basement and brings fresh air to the 2nd floor, though I can open a port to dump some air into the basement too.

  3. Expert Member
    Dana Dorsett | | #3

    So, what is the outdoor dew point today? (Where are you?) If you're only air conditioning part of the house the dew point of the indoor air is going to more or less track the dew point of the outdoor air.

    The dew point of 70% RH 65F air is 55F. In most of my region (New England) right now today the outdoor dew points are ranging between 57F - 68F. Putting that air directly into a 65F crawlspace would raise the crawlspace RH to 100%. Lowering the ventilation rate on the HRV & mechanically dehumidifying (or air-conditioning) is the only way to lower the RH in a 65F crawlspace. You will only be able to lower the RH below 70% in the crawl by ventilation if the dew point of the ventilation air is under 55F.

  4. agurkas | | #4

    I am in Boston.
    HRV is mostly to cut down radon bellow 0.5 (without HRV it is about 0.7 in the basement).

    So sounds like I need to cut down on the ventilation to basement

  5. Expert Member
    Dana Dorsett | | #5

    The outdoor dew point in Boston right now is 66F at the airport:

    That's pretty close to the mid-summer average. It's been a drier than usual summer so far- most years the outdoor dew point averages are below 55F from roughly the last week of September through the first week of June. See the dew point averages chart about 3/4 of the way down this page:

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