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Community and Q&A

Bulk Water Management in Conditioned Crawlspace

newbiemike | Posted in Energy Efficiency and Durability on
We are 15 months into a new build and are having significant moisture issues in our 2 enclosed, crawl spaces with just R10 exterior/perimeter foam. Unfortunately I was talked out of insulating the floor of the 2 crawls so it is just a vapor barrier over dirt. We are located in fairly dry, Eastern Washington.  Fairly tight envelope with balanced ventilation in the house.
Rain got in just after the perimeter foundation was finished and before the floor and roof. The builder thought that the water would soak in and go away but it didn’t. Now we have a framed, insulated house on top of the crawl spaces. The floor above the crawls is not insulated and won’t be until we can solve the moisture issue.
Last summer we cut sizable holes in the sub floor to open both crawls and ran 6000 CFM fans running air through each during the hottest part of a very dry summer and after 6 weeks, it helped significantly but did not completely dry the spaces, especially in certain areas. Mold started and we have sprayed. After we removed the large fans and installed plastic over the dirt, commercial dehumidifiers pulled additional moisture out but still didn’t fully dry the framing lumber in the spaces. We will respray but need to rid the moisture once and for all.
I am trying to decide the best approach and am considering:
1. For one of the spaces, Code calls for a 75 CFM exhaust fan to run continuously whether or not outside conditions are better than inside.  This does not seem remotely adequate and with a 1.5 ACH house I am not sure where the air could come from. This approach also cools the space all winter.
2. Dehumidifiers. This seems like it might be good to maintain the RH% but according to AprilAire’s engineering, dehumidifiers requires adding significant amount of heat to maintain 70-75*F all year or add multiple dehumidifiers. That equals about $5000 for 3 dehumidifiers.
3. Dedicated Fantech HRV’s ducted to distribute the air throughout each space to ensure good air flow. From my limited understanding, balanced ventilation seems like a decent long-term option since, with ducting, it can exchange and distribute the air well and doesn’t require heating the crawl to maintain 70*F.
4. Other suggestions??
Does anyone have experience solving an issue like this in a drier climate?
Has anyone used a dedicated HRV to ventilate just their crawls? Was it successful?
Thank You,

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  1. Expert Member
    BILL WICHERS | | #1

    This sounds to me like a bulk water problem more than a moisture problem. Moisture is water vapor, and usually causes issues when it condenses on things below the dew point. This would give you things like damp walls, and mold problems, but no puddles unless it gets really, really out of hand. Moisture can come in through masonry walls that aren't sealed, or air leaks from moist areas (we have that in the summer here in Michigan since we often have very humid air outside).

    If you have water coming up under the vapor barrier on the floor of the crawl space, then you have drainage issues. This could be a footing drain installed above the footing (so basically a drain that is in the wrong spot, so water can pool under it). You may be able to "fix" this by grading the ground away from the house, putting up gutters and/or downspout extensions, etc, to direct water away from your foundation. You may have to dig down and rework the footing drain though for a complete fix is this is the problem.

    Missing damp proofing on the outside of the foundation wall can also let water get in under certain conditions. The easiest way to deal with this is the stuff mentioned above to redirect water away from the house. Excavating the entire foundation wall to fix damp proofing issues is a big project.

    I see though that it looks like rain "got in" while the house was under construction and then stayed put in the crawl space. Was that pooled on top of the vapor barrier? If it was on top of a dirt floor, I suppose it's possible you have a very high water table or something like that, but that sounds strange to me. My first thought would be to slope the dirt floor in the crawl space and put in a french drain to deal with any future bulk water problems, then put the vapor barrier over that, possibly with a drain tied in.

    All of this is geared towards dealing with bulk water that is causing the moisture issue, which is what it sounds like is going on here. Once you get the bulk water problem under control, the moisture problem will either clear up on its own, or at least will be a lot easier to deal with. If I'm missing something here, let me know and I can rethink things.

    You DO need some air flow through the crawl space to prevent moisture buildup though. Normally a small amount of coninuous air flow will keep the mositure levels under control. A dehumidifier running in a sealed crawl space will also keep things under control, but will use more energy to do so. Since your moisture problem isn't going away, that's the main reason I'm thinking you have some kind of bulk water problem and not just a water vapor problem. Something is replacing all the moisture your fan and dehumidifier are removing, and until you can find and fix that you're going to continue to have issues.


  2. walta100 | | #2

    After I read your question, I like Bill am still wondering if liquid water continues to entering your crawlspace?

    When I read words “encapsulated crawl space” “enclosed crawl space” “unvented crawl space” I read "Free Lunch here" we all know it is too good to be true. Seems to me only once you can bring yourself to call it a conditioned crawl space will you be able to except that you will spend energy and money conditioning the air in the crawl space so it is more or less the same temp and humidity as the rest of your conditioned spaces.


  3. newbiemike | | #3

    Thank you both for your input. I will explore if bulk water is somehow continuing to enter. One side of the footing drain has not been connected to daylight and that may be contributing to the issue.

    I discussed your point with another contractor this weekend who agrees with you and he will come by tomorrow to give his assessment and some possible remedies.

    Would you recommend installing a sump pump? I am not sure how we could put in an effective well and pump when the crawls are so flat and chopped up with full-length footings under the framed floor support walls. This only leaves long, 9 foot wide "alley's" between footings which make it difficult to slope the ground to one place.


  4. Expert Member
    BILL WICHERS | | #4

    I would put in a sump pump if it was needed for proper drainage of the footing drains. You'd have to dig out a sump somewhere and tie it into the perimeter drainage system, then you'd need to pipe the outflow from the pump to somewhere well away from the house. The sump pump will help if the perimeter drains are otherwise unable to fully drain, but otherwise it won't really do much for you.


    1. newbiemike | | #5

      Good to know. Thank you.
      First I think we need to get the existing perimeter footing drain downhill and to daylight. Then we can assess the need for a sump pump.

      1. Expert Member
        BILL WICHERS | | #6

        If you can gravity drain the footing drain, then you won't need a sump pump. You only need a sump pump when you have to pump the ground water up to get it out. If you have a walkout (for example), that allows the footing drains to gravity flow to daylight somewhere downslope, you really don't need a sump pump.


  5. Expert Member
    PETER Engle | | #7

    I think we need a little more information here, and a better explanation of some of the terminology. When you say that there is still too much moisture in the crawl space, what specifically do you mean? Is there standing water on top of the plastic? Is the soil under the plastic damp? Have your checked/tracked the temperature and humidity in the air of the crawl space? Have you checked/tracked the wood moisture content (WMC) of the framing above the crawl space? Each of these could point to different sources and solutions.

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