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Crawl space in North Carolina

BarbinNC | Posted in General Questions on

Just had 15 year old 2300 SF crawl space treated for mild mold/fungus. We never had vapor barrier down. Having new HVAC R-8 flexible duct installed. Also putting down 8 mil reinforced poly on ground. Never had water intrusion problem; however two boot registers were vented under cabinets and over the years created a huge moisture issue at subfloor which went unnoticed because it was hidden by insulation between floor joists (luckily no rot). We are relocating those boot registers. Ground is pretty saturated now from mold remediation 5 days ago (pressure wash with Shockwave). Poly will go down this week after HVAC duct work complete.

We purchased dehumidifier (Santa Fe Advance 2) and are using that along with fans to aid drying following mold remediation. Crawl Space temp is 59-61 with 75% humidity (down from 90’s following remediation 5 days ago). Presently in NC the temp is 30’s at night with 90’s humidity and 50’s day temp with 50’s humidity. We are working towards a sealed crawl space by the spring as finances hopefully allow. Right now the vapor barrier will go down on ground and just lay against the wall until we can get started on the walls. Obviously encapsulation would be ideal but can’t swing $$$$$’s at this time. Also, we don’t want to go the HVAC supply air route. Is a dehumidifier sufficient or do we need to add exhaust fan in a few vents and keep others closed?

Any suggestions, comments, recommendations are most appreciated. Thanks

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  1. GBA Editor
    Martin Holladay | | #1

    Step one: seal your crawl space vents. However you seal them, strive to make these vents airtight.

    Step two: Install a dehumidifier in the crawl space temporarily and monitor the relative humidity (RH) levels down there. When you reach your RH goal -- I'll let someone who lives in North Carolina suggest a target -- you can unplug your dehumidifier.

    As soon as you can afford to do the work, perform the steps listed in this article: Building an Unvented Crawl Space.

  2. Brian Knight | | #2

    Good advice from Martin and would add that adding a well detailed vapor barrier is a top priority. I would read Martin's article and Closed Crawlspaces an introduction for the Southeast US from Advanced Energy's
    I think they mention keeping it 70% or lower down there.

    I would personally prefer less humidity depending on the details but also feel that new, vented crawlspaces should be illegal .

  3. BarbinNC | | #3

    Really appreciate your comments. When you said to install dehumidifier temporarily then unplug after reaching target, how would we maintain that environment. Since our house is 15 years old, I really didn't want to use supply air from HVAC to condition space. If we close vents, is the dehumidifier sufficient to keep space controlled or do we need to exhaust air or condition air? By the way we bought 8 mil white reinforced poly for vapor barrier on ground and will overlap 1 ft and tape seams. Have already wrapped piers.

  4. GBA Editor
    Martin Holladay | | #4

    I suggest that you read the article I linked to: Building an Unvented Crawl Space.

    The use of a dehumidifier should be temporary. Once the space is dried out, there should be no need for further dehumidification (in most cases), especially if you have done a good job sealing the vents and air sealing leaks.

    That said, the code requires (and building scientists recommend) that you use one of two techniques to condition the air in your crawlspace. As my article noted:

    “The code lists two options for conditioning unvented crawl spaces; both options require the installation of a duct or transfer grille connecting the crawl space with the conditioned space upstairs. 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.)

    “Option 2 requires that the crawl space have a forced-air register delivering 1 cfm of supply air from the furnace or air handler for each 50 square feet of crawl space area. (Assuming the house has air conditioning, this introduction of cool, dry air into the crawl space during the summer keeps the crawl space dry.)”

  5. BarbinNC | | #5

    I have a 2300 SF crawl with 3' to 4' height. Could I put exhaust fan in a few vents rather than seal them up? I am fearful of total encapsulation because of friends who months later get this gosh awful smell. I appears to be linked to a crawl which has been sealed and uses home HVAC to supply air down there. My home is 15 years old and knowing what I know now, I would NOT have a crawl space at least not in NC. I could have built a nice basement for what I have invested in mold remediation, new duct work & plenums, removal & install new insulation, dehumidifier (Santa Fe Advance 2) and materials to semi-seal crawl.

  6. GBA Editor
    Martin Holladay | | #6

    In a crawl space your size, all you need is a small exhaust fan with a rating of 46 cfm. You don't need multiple fans in multiple locations.

    Don't forget to install a grille in your floor to allow conditioned air from upstairs to enter your crawl space.

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