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Crawlspace moisture

stevedavis | Posted in General Questions on

Single family home in coastal California climate zones 3C (95062). The home is 1263 square feet with a small crawlspace at 18″ in height.

I’ve just completed my encapsulation project. The vents have been sealed shut with wood blocks and spray foam. A vapor barrier has been placed on the floor and fully taped to the all the posts and stem wall. I have a remote data logger in the crawlspace so I’ve been able to monitor the temperature and relative humidity.

After encapsulation, my relatively humidity is far more stable. However, it seems to enjoy staying at 60-65%. I’ve settled on putting in a dehumidifier as I do not like mixing the crawlspace air with my house air for a variety of reasons.

My problem is finding a reasonable dehumidifier. With 1,900 cubic feet, I have a relatively small crawlspace with, I believe, relatively small dehumidification needs. All the purpose-built crawlspace dehumidifiers I see seem way overkill (and wallet killing as well!) Other dehumidifiers, however, won’t physically fit as their designs are usually about 24″ tall.

Am I missing something here or am I going to have to purchase a $1,000 dehumidifier for my relatively small problem? Alternatively, am I fretting unnecessarily about my humidity levels?

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  1. jenniferz5 | | #1

    I am not an expert, by any means, but I had a similar issue here in CT that I addressed as you have. Ultimately, I did purchase a Santa Fe dehumidifier and have been running it for three years with no further issues.

  2. GBA Editor
    Brian Pontolilo | | #2

    Hey Steve.

    I understand that you don't want to mix the crawlspace air with the air in your house, but have you considered the exhaust fan option?

    I believe that the current IRC calls for floor registers between the living space and conditioned crawlspaces with either a forced air register supplying conditioned air to the crawl or an exhaust fan installed in the crawlspace.

    The latter option depressurizes the crawlspace so, in theory, crawlspace air is not mixing with the air in the homes, it is only being exhausted and replaced with air from the home that would be dehumidified by your A/C.

    I wonder if this would cost less to purchase and operate and reduce the energy penalty of the heat created by a dehumidifier? I'm sure there are some GBA members who can answer these questions.

    Finally, if you haven't read it, you may find this article helpful: Building an Unvented Crawlspace.

    1. stevedavis | | #4

      Hi Brian,

      Thanks for the info. The house does not have AC. It is unusual for our area. Does that change your response at all? I should add that the home itself is not dehumidified in any way. I do also have a humidity sensor indoors. I averaged 55% over the last year with normal fluctuations. I would think that pulling this air into a cooler crawlspace would likely not be helpful?


  3. Jon_R | | #3

    With your climate, +1 on the exhaust fan. They work less well in places with colder ground and more humidity.

    > Other dehumidifiers, however, won’t physically fit

    You could dig to make a few square feet of the crawlspace taller. With good air and moisture sealing it will run so seldom that the heat/electricity will be insignificant.

    In all cases, install a humidity monitor so that when the device fails, you are notified.

    1. stevedavis | | #5

      Thanks, Jon. Not sure why I hadn't considered digging a spot for a taller dehumidifier. I'd be a bit concerned about airflow around the unit but this might be a solution to keep things simple and my wallet in check. This won't be fun in 18" of space and clay soil but it might be worth it to save around $800.

  4. _jt | | #6

    I had the same issue, mid 60 crawl space humidity in the summer. I put a "personal" dehumidifier in - brought the humidity to closer to 50 in about a day. Very small amount of actual moisture so I can probably run several weeks on the small tank. Not sure if this is the best solution but does seem simple. (The dehumidifier is about 10 inches tall)

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