Rh Puzzle in Encapsulated Crawl Space

Discussion in 'HVAC Heating & Cooling' started by mckinney3, Feb 21, 2013.

  1. mckinney3

    mckinney3 New Member

    Feb 21, 2013
    South Carolina
    Background: I had high Rh in crawl space with associated mold/mildew/etc. damage in crawl space as well as 1st floor of home.

    Action: Installed new gutters and extensions. Made sure condensate lines for air handler (in crawl space) vent to outside. Made sure that sump and "spyder drains" operating properly and vented to outside. Had crawl space professionally encapsulated (looks like very thorough job) New insulated wall to isolate crawl space from wrap - around porches, etc. Had a high quality dehumidifier installed that vents to outside. Installed two transmitting humidistats in crawl space that transmit to data logger that also monitors indoors and outdoors temp and Rh.

    Results: Crawl space temp runs a constant -5 degree f below 1st floor of house. Crawl space run a constant 50-55% Rh. House runs constant 65 degrees temp. House runs a fairly constant 35-40% Rh. Outside has ranged from 25 -65 degrees F and 30-85% Rh.

    Puzzle: Obviously dehumdifier in crawl space brings Rh down to the set point of 50-55%. Temperature of crawl space behaves as expected vs. house. BUT when inside is 30% Rh and outside is 30% Rh for several days, the crawlspace is always 55%. The dehumidifier brings Rh down to the set point but WHAT is raising the Rh above 55% if inside house and outside is 30%.

    I suspect that some type of inflitration of water is still occuring to raise the humidity in the crawl space but my contractor says that the system is performing as I should expect.

    I would love to hear some discussion on this topic and why crawl space runs 55%.
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Sep 2, 2004
    Retired Systems engineer for defense industry.
    New England
    You said the dehumidifier 'vents' to the outside. If this isn't just moisture, then it's decreasing the pressure in there, pulling in outside (or interior) air.

    I'm assuming that the ground has a cover on it and it's got a vapor barrier either in it or under it.

    Relative humidity is not the same as actual humidity per volume...colder air can't hold as much moisture as warmer air, so the crawlspace being cooler could have exactly the same actual moisture in it as the house. If you put a couple foot square sheet of plastic on the floor of the crawlspace and seal the edges as best you can, does it have any trapped moisture underneath it? If so, you need to seal things up better.
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  4. mckinney3

    mckinney3 New Member

    Feb 21, 2013
    South Carolina
    Thank you , Jim!

    The dehumidifier discharges condensate to the outside but it is closed loop for air circulation. Yes, the ground has that heat sealed "thick" and reinforced vapor barrier, sidewalls plastic as well,foam around where concrete columns intersect with vapor barrier, etc. , sealed vents, etc.

    I can't imagine any ground vapor getting thru but I like the sheet on top of the plastic test. I'll do that.

    I realize that dewpoint and actual humidity are all swirling around a part of the puzzle. But the dehumidifier is running when all of the available air (house and outside) is low RH so that is why I am puzzled.
  5. Dana

    Dana In the trades

    Jan 14, 2009
    "Crawl space temp runs a constant -5 degree f below 1st floor of house. Crawl space run a constant 50-55% Rh. House runs constant 65 degrees temp."

    Crawl space temp = 60F, yes?

    " House runs a fairly constant 35-40% Rh."

    Take a look at a psychrometic chart, or use an online psychrometric calculator. The dew point 65F/40%RH air is 40F, which corresponds to an RH of 48% @ 60F, so ventilating the crawlspace with conditioned space air would bring it below the 55% setpoint of the dehumidifier. The dew point of 60F/55% RH air is 44F, which isn't going to be mold-inducing, even when the crawlspace warms up to 68F or higher in the summer.

    Standalone dehumidifier dehumidistats are not precision instruments, and very often read high when the sensing element is inside the case, due to re-evaporation of the condensed water still hanging on the coil at the end of a cycle. Monitoring the humidity in the crawl with cheap battery powered AccuRite or similar with an identical unit somewhere on the first floor space would give you a pretty good picture of what's really going on.

    Dew point is one measure of the absolute humidity in the air. The outdoor RH without knowing temperature is a meaningless number, since you don't have the temperature that it's relative to. The 30% number on it's is completely useless: If it's 90F out with a 30% RH the dew point of that air is 54F, and if you pulled that air into even a 68F crawlspace the RH you would have a fairly mold-prone 68% RH in the crawl. But if it's 40F outside with a 30% RH, it has a dew point of +13F, and venting the currently ~60F crawlspace with outdoor air would yield a bone-dry 14% RH.

    Given that it's winter and 65F in the house I'm gonna go out on a limb and guess that it's not 90F outside. :)

    But if it's a sealed insulated unvented crawlspace that's running higher dew points than the conditioned first floor it means there IS a moisture source. I could be relatively benign ground moisture getting by the ground vapor retarder , but it's probably not air-leakage from the outdoors at this time of year. If it spikes in summer outdoor air could be a factor though. Minor plumbing leaks or drain leaks, a shower pan that's dripping can all be contributing to a higher absolute humidity in the crawlspace than the upstairs. But monitor both with identical monitors, and use a psychrometric chart or online calculator to see how well the dew points track. It could be that they're dead-on, or fairly close (within the measurement error of a cheap monitor). If the crawlspace dewpoint averages >5F over the conditioned space dew point, you probably have more investigating/mitigating to do.

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