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Community and Q&A

Crawl space vapor barrier without encapsulation

SamuelH | Posted in General Questions on

I’m having issues with my crawl space, including standing water, sweating ducts, and minor mold. I’ve since installed a sump pump, covered the vents with plastic, and put in a temporary dehumidifier. I was going to have my crawl space encapsulated, but it’s quite expensive, and I don’t have time to do it myself. So now I’m considering doing the following instead:

1. keeping existing ground vapor barrier
2. keeping my existing fiberglass batts
3. re opening up the crawl space vents
4. adding a new vapor barrier to the walls
5. taping up all seams in the vapor barrier

I’m wondering if this will be enough to control the humidity in the crawl space? My rationale is that my vented attic’s ductwork is exposed to outdoor humidity, and I don’t have problems with sweating ductwork there. So if I keep ground moisture out of my crawl space then that could be enough.

I could keep the crawl space vents closed. I feel like it needs some ventilation though for indoor air quality purposes, especially with the fiberglass batts staying in place. My vents automatically close in high temperatures and close in cold temperatures. So there would be reduction of air movement on hot summer days.

I live in climate zone 4 in Nashville, TN. Also, I live about 30 feet from a drain easement, not sure if that proximity will also affect humidity levels.

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  1. Jon_R | | #1

    Hard to say - you could try it and then switch back to the dehumidifier (or some supply of air from the interior) if mold returns. With reduced moisture input, you will have less mold than when it was previously vented.

    1. SamuelH | | #2

      Ya, just concerned I'll waste a day of my time. Really, what I'm looking for is a reason the crawl space environment might be different than my attic's environment. The attic I know is hotter, but I would think that might be more conducive to condensation. Perhaps the extra heat heats up the duct foil/insulation to a point above the dew point. I dunno.

      1. Jon_R | | #7

        Air/vapor sealing is helpful in either case - only the small effort to open/close the vents and turn on/off the dehumidifier would be wasted.

        Yes, cooler and more moisture sources leads to higher relative humidity (which is what mold cares about).

  2. Expert Member
    Dana Dorsett | | #3

    >"2. keeping my existing fiberglass batts"

    Are the existing batts on the crawlspace floor, walls, or ceiling?

    >"3. re opening up the crawl space vents"

    Bad idea, in any of the "-A" climate zones (zones 1A, 2A... etc.). Nashville is zone 4A. With a ground vapor barrier properly sealed to the foundation, air leakage from the outdoors becomes the primary humidity source in crawlspaces in those climate zones. Only in the "-B" and "-C" climate zones are summertime outdoor dew point averages usually below both the subsoil temperature and subfloor temperature of an air conditioned house.

    In Nashville the outdoor dew point averages are over 65F for nearly 3/4 of the summer. That's above the temperature of the subsoil, and will be pretty close to the subfloor temperture much of the season:

    If the batts are between the floor joists, the joist edges are below that temperature much of the time too, and will load up with moisture.

    >4. adding a new vapor barrier to the walls
    >5. taping up all seams in the vapor barrier.

    Overlapping the seams of the vapor barrier by a foot and sealed with a bead of caulk between them at the over lap and taped (or better yet mastic-sealed) at the edge of the overlap is the right way to go.

  3. SamuelH | | #4

    Yes, the batts are under the subfloor, between the joists. What makes my vented attic so different that I don't get sweating ducts and mold there? If I'm able to stop ground moisture in the crawl space won't the conditions be similar to the attic?

    1. Expert Member
      Dana Dorsett | | #8

      >"What makes my vented attic so different that I don't get sweating ducts and mold there? "

      Just about everything is different. The attic gets substantial solar heating, and is often hotter than the outdoors when there is an air conditioning load. That causes attic air to rise and leave pulling in some outdoor in through any roof venting, but also usually pulls in some much drier indoor air from the leaks in the attic floor to the conditioned space.

      With air leaving upward the attic and conditioned space, the first floor becomes depressurized relative to the outdoors, drawing air into the crawlspace as air leaves the crawlspace headed up to the first floor.

      The crawlspace ceiling /first floor subfloor is the coolest surface in the entire house except for air conditioning supply ducts, and is in contact with completely unconditioned outdoor air, undiluted by drier conditioned space air.

      It's certainly possible for supply ducts in the attic to have condensation issues during the cooling season (it's a very common problem) but the other surfaces are generally warmer than the outdoor air and outdoor dew points.

      Unlike attics, there is no solar heating to raise the temperatures in the crawlspace to drive moisture out of the wood, and the stack effect drives on the house aren't generally pushing conditioned space air down through air leaks in the floor, so moisture tends to collect during the summer unless air sealed from the outdoors, and ventilated with a modest amount of conditioned space air.

      Code requires either a 70 pint dehumidifier or 1 cfm of ventilation per 50 square feet of crawlspace floor area (eg: a 1000' crawlspace needs 1000/50= 20 cfm of continuous ventilation) for sealed crawl spaces, which isn't very much. There are 3 watt ventilation fans that could cover that.

      1. SamuelH | | #10

        Thank you for the detailed explanation.

  4. SamuelH | | #5

    I think I understand. The subsoil temperature cools off the wood and ductwork right up to the dew point. In the attic all surfaces are warmer, and don't condensate.

    1. Jon_R | | #6

      That's correct - except that the subsoil temperature is not the temperature of anything in the crawlspace (warm summer air heats it up). How much is impossible to predict, which leads back to a simple test.

      1. SamuelH | | #12

        Thanks Jon!

    2. Expert Member
      Dana Dorsett | | #9

      >"In the attic all surfaces are warmer, and don't condensate."

      The verb you are looking for it "condense", but even that wouldn't exactly be correct in that sentence structure.

      Surfaces don't condense, rather gases (like water vapor) condense on to surfaces, forming condensation.

      The word "condensate" is a noun, referring to the accumulated condensation.

      While the subsoil temperatures are usually cooler than the crawlspace temperatures, it is still has a moderating effect keeping the crawlspace temperatures much cooler than the attic in summer, warmer in winter.

      With cold poorly insulated (or uninsulated) ducts in a crawlspace there are existence proofs of crawlspaces that can drop below the subsoil temperatures for hours at a time during the cooling season, though those cases are outliers.

      1. SamuelH | | #11

        Thanks Dana. Yes condensate was an incorrect choice of words there.

  5. SamuelH | | #13

    >"Overlapping the seams of the vapor barrier by a foot and sealed with a bead of caulk between them at the over lap and taped (or better yet mastic-sealed) at the edge of the overlap is the right way to go."

    Can you recommend a type of mastic that will bond with a polyethylene vapor barrier?

    1. Expert Member
      Dana Dorsett | | #14

      A 1/8" coat of any vendor's fiber reinforced duct mastic applied with a cheap paint brush sticks to almost anything (including PVC ducts, hair, clothing :-) ), and stays pretty flexible forever.

      There are some HVAC tapes with a pressure sensitive mastic (eg HardCast Foil-Grip 1402 & Foil-Grip 1404) but they can be more difficult to apply when there isn't a firm stable backing like a hard-piped duct (or a concrete slab). Tape mastic sealants are cleaner to work with, but more expensive than the stuff that comes in plastic tubs (eg: DP 1030 or GAF WBA50).

      With a bead of acoustic sealant type caulk between the overlapped sheets with the edge of the overlapp sealed down with duct mastic it's pretty air tight for the long haul.

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