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How to condition an existing crawl space?

Cherylann128 | Posted in GBA Pro Help on

Hi. I haven’t been here for a while because I thought I had found the solution to my crawlspace problem but alas I haven’t. I had been dehumidifying the crawlspace at our beach house with a high capacity Santa Fe dehumidifier which unfortunately got destroyed when the crawlspace was flooded during Superstorm Sandy. Rather that buy a new one, I had a vapor barrier installed, replaced the open vents with insulated smart vents and had the underfloor of the house covered in 2-3″ of closed cell spray foam.

Now that the weather is hot and humid again, the crawl space smells horrible (like cat urine which is probably some kind of hydrolysis product from the spray foam….there are no cats down there!). Even though the duct work to the HVAC system is embedded in the foam, the smell does manage to enter the house is some places. Not only is that unpleasant, but having such a humid crawlspace just seems like a bad idea. So what I’d like to know is how best to remedy the situation. I could

1. Install an exhaust fan that pulls make up air from the house; downside fan has to run 24/7
2. Dehumidify the space again with a Santa Fe unity and pump; downside dehumidifier runs a lot!
3. Create a new HVAC zone and heat and cool the space. I don’t want to mix air between the house and the crawlspace so it would need it’s own unit; most expensive option

I have already spent a bundle on the insulation and would like to correct the problem as inexpensively as I can however I want to do it right. Any suggestions would really, really be appreciated. Most of the crawlspace and HVAC people I talk to have no understanding of building science. Thanks!

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  1. GBA Editor
    Martin Holladay | | #1

    Cheryl Ann,
    You really don't want your crawl space to have any vents -- even so-called "smart" vents. To learn more about the steps you need to take, start by reading this article: Building an Unvented Crawl Space.

  2. exeric | | #2

    I think Martin's advice is really good and is the usual answer to a damp crawlspace in humid southern locations. The smell of cat urine from the foam, if that is what it is, complicates things. To have a sealed crawl space requires a space that has conditioned air by code. This is usually done with some sort of connection to house air so that the house and crawl space act as one dry conditioned space.

    But if the smell is coming from the foam, and not the dampness, then that complicates things. You certainly wouldn't want that smell coming directly from the crawl space. On the other hand, if instead it is originating simply from the dampness down there then after an initial drying out it should disappear after the conditioned spaces are combined.

    I hope the smell is coming from dampness because otherwise it becomes a complicated problem that I'm not sure there would be an easy (read cheap) solution to.

  3. Cherylann128 | | #3

    Thank you Martin and Eric. Yes, no vents would be ideal and I have read that article very carefully however this is a beach house in a town that requires storm vents. We are not allowed to have a totally sealed crawlspace. And Eric, you may be right that the dampness alone is causing the smell. We had the same problem with the fiberglass insulation before we foamed and I had assumed it was hydrolysis of the glue. In any case, dehumidification took care of the smell back then.

    So, recognizing that I can't totally seal the space (the smart vents are solid not louvered and they are insulated), and I don't want to mix the air between the crawlspace and the living space what do you think my options are? Thanks again!

  4. GBA Editor
    Martin Holladay | | #4

    Cheryl Ann,
    Follow "option 1" as described in my article:

    "Option 1 requires “continuously operated mechanical exhaust ventilation at a rate equal to 1 cfm for each 50 square feet of crawl space floor area.” In other words, install an exhaust fan in the crawl space that blows through a hole in the rim joist or an exterior wall; make sure that the fan isn't too powerful. (The makeup air entering the crawl space is conditioned air from the house upstairs; since this conditioned air is drier than outdoor air, it doesn't lead to condensation problems.)"

  5. exeric | | #5

    Not that it's important, but I concur with Martin as long as you're sure it's only a dampness problem and not a result of an incorrect installation of the foam. (It happens, though rarely)

  6. Cherylann128 | | #6

    Thanks, Martin. That's what I was leaning toward. I will probably have to cut a hole in the floor for make up air because of the foam but I have several places where that would be feasible.

    I have leaned so much from this website and Building Science it makes me want to build a new house just to do it the right way!!! All this retro work is tedious and expensive! I don't understand why more builders don't educate themselves and build sounder, greener homes.

  7. jablo | | #7

    I bought home witch crawl space so humid be ause of fog I couldn't see opposite wall
    My solution
    To layers staggered styrofoam
    Around 2 -3 inch thick (altogether) on top I have poured 3/4" of fiberglass reinforced concrete to keep EPS down and to be able to walk (no need for fine finish
    Crawlspace is 4 ' high so I can Store some tools
    It was completely dry in 3 days
    Surprisingly nor even 1 crack developed
    No need for any forced ventilation of any kind furthermore no need for any kind of ventilation

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