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Thread: crawl space humidity

  1. #1
    DIY Junior Member LindaK's Avatar
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    Default crawl space humidity

    Hi, I'm new here and have a dampness problem in my crawl space.
    I have a 1.5 story house in British Columbia, Canada. When I bought it, it had the vents closed and a heater on full blast. It was dry but my hydro bill was huge. So, last year my son and I turned the heater off and insulated the ceiling with batts. (I'm a woman and don't know all the technical terms so please bare with me on that). Then, the toilet valve leaked slowly at the floor and ran down the pipe into the crawl space. This spring I got a big surprise when I checked under there. There was water and the insulation was dripping wet. So, I removed all the insulation and dried it in the sun, opened the vents,fixed the toilet leak, and I thought all was well. It was nice and dry so put the insulation back in. The floor of the crawl space is cement with one area where the sewer pipe leaves the crawl space and it is just gravel. It's maybe 2' X 2'.

    I just went to check the crawl space expecting it to be dry but it's dripping wet again. I closed the vents cause it is fall and raining and installed a dehumidifier.

    Now the question (finally, Ha Ha!!). Is the insulation we put in causing a problem? Should I remove it all and just keep the dehumidifier going with maybe the heater on low too? I don't see any leaks from pipes. Help, it's very frustrating and want it cleared up soon before winter is here. Thx. -L

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    DIY Senior Member Reach4's Avatar
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    Non-expert thoughts:
    Concrete will not stop humidity. Some heavy plastic sheet covering the crawl space may be a good idea.

    Also, how about adding some ventilation to the crawl space. I doubt that closing the vents was a good idea, despite the rain outside. Let that insulation dry out. Also, you should have put a vapor barrier above the crawl space bats to keep inside humidity from. Maybe your bats had a built-in vapor barrier on top. Maybe your flooring above serves as a vapor barrier.

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    DIY Senior Member Murphy625's Avatar
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    1. Find out where the moisture is coming from. Do you have a high water table there?

    2. As reach4 said, plastic on top of the cement is a good idea.

    3. Ventilate ventilate ventilate. A solar powered exhaust fan or other device which moves the air out and sucks in fresh air is a good idea.

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    DIYer, not in the trades LLigetfa's Avatar
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    Others are saying ventilate but let me tell you, the air you remove will be replaced with humid air most of the year and the coolness of the crawlspace will will condense it.

    Too much heat will drive the moisture into the air, so keeping the space cool and not exchange too much of the air is the way to go. Run the dehumidifier to get the humidity down.

    Insulation that is not closed cell has to have a vapor barrier on the warm side otherwise there will be condensation issues. It is very difficult to provide a good air seal with poly vapor barrier on the underside of floor joists.
    Last edited by LLigetfa; 09-29-2013 at 12:28 PM. Reason: as per below

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    DIY Senior Member Reach4's Avatar
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    Quote Originally Posted by LLigetfa View Post
    Insulation that is not closed cell has to have a vapor barrier on the cold side otherwise there will be condensation issues.
    I think you meant to say the vapor barrier should be on the warm side.

  6. #6
    DIYer, not in the trades LLigetfa's Avatar
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    DOH! Right you are!

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    DIY Junior Member LindaK's Avatar
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    Thanks for you're fast replies! One thing I forgot when replacing the dried out insulation was to check about the vapour barrier. Also, I don't want to open the vents cause it's totally pouring outside. The air outside will be saturated in water. I live in a very wet part of BC. I'll also turn off the heater and keep the dehumidifier on. Does that sound right? I really can't tell at this point where the water is coming from because it is all damp. The water table is high here. If I eventually get it all dry, could I use the spray can of "leak stop" meant for gutters, on the cement, or something else? (Okay, maybe a dumb question but I'm desperate.)

  8. #8
    DIY Senior Member Reach4's Avatar
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    Turning off the water heater will not help in any way that I can think of, unless it is overflowing into the crawl space (it should not do that in any case). That presumes that the water heater is not a gas water heater located in the crawl space; that would be odd.

    I think the heavy plastic will be cheaper, easier, and better for storing stuff on than a spray stop leak would be. You would not have to seal, but you would want a fair amount of overlap between sheets. The "heavy" part (maybe 10 mils) is just for durability rather than humidity stopping differences.

    I suggest you get a couple remote-reading thermometers and humidity meter (hygrometer) such as http://www.walmart.com/ip/La-Crosse-...ation/11162338 Set to different channels. Confirm that the readings for the remotes match when the sensors are side by side. Put one remote under the house. Put one outside where it will not get rained on.

    Compute the dew point under the house and outside. If the dew point outside is lower than the air under the house, then ventilation will help.

    http://www.dpcalc.org/ is one way to compute dew point.
    Last edited by Reach4; 09-29-2013 at 02:55 PM.

  9. #9
    DIY Senior Member Murphy625's Avatar
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    Quote Originally Posted by LLigetfa View Post
    Others are saying ventilate but let me tell you, the air you remove will be replaced with humid air most of the year and the coolness of the crawlspace will will condense it.
    While that would be true for a basement, I would tend to think a crawlspace would be somewhat different in that it would depend on the amount and source of the moisture as well as the ambient conditions.

    Too much heat will drive the moisture into the air, so keeping the space cool and not exchange too much of the air is the way to go. Run the dehumidifier to get the humidity down.
    I think that was the idea.. Get the moisture into the air and then send it out. Dehumidifiers are expensive to run! Its like having a 700+ watt appliance on all the time. Again, I think it depends on the source of the moisture.. (or is everyone just assuming its coming from the ground?)

  10. #10
    DIY Junior Member LindaK's Avatar
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    I put the dehumidifier down there and have moved it about and empied it twice. My goal is to dry it out and then try and see where the water is coming from. There are no leaking water pipes that I can see. There is a creek on 2 sides of the house but it was low when I noticed the problem. (It's risen since then with the rain.) It's drying quite nicely down there. I've had the place for 4 years and no problems until I added the insulating bats under the floor. I'm still wondering if the problem is connected to that?

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