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  • 1.  Crawl Space Building Science

    Posted 06-14-2022 07:35
    One of the biggest advances in building science over the past decade is the research that proves definitively that vented crawl spaces are bad.

    If you've ever been in one, you know it's true. They're damp. They're dark. Critters have easy access to them. The batt insulation falls down. The air smells musty & moldy. And the crawl space communicates with the house, so that bad air (and the occasional critter) gets into the living space above.

    In 2002, Advanced Energy studied a group of 12 houses in North Carolina. What they found is that the 8 that had encapsulated crawl spaces had relative humidity that stayed less than 60% all summer, whereas the 4 vented crawl spaces had humidity levels that tracked the outdoor humidity.

    The graph above shows the results of the research. The red line shows the relative humidity in the encapsulated crawl spaces. (The terms closed crawl space, sealed crawl space, and conditioned crawl space are sometimes used to describe crawl space encapsulation as well.)

    The reason the vented crawl spaces had higher humidity is actually pretty simple. Warmer air can hold more moisture, so in the summertime, the outdoor air can bring a lot of extra water vapor with it when it comes into a crawl space through the vents. When that warm, humid air comes into the crawl space and cools off, the relative humidity can go even higher. You can see that in the graph above, as the blue line (vented crawl space relative humidity) is often higher than the outdoor relative humidity.

    If you want to learn more about the details of this research, you can find that and more at Advanced Energy's crawlspaces.org website.

    So, what exactly is crawl space encapsulation, and how does one make an encapsulated crawl space? Read on.

    Allison Bailes
    Energy Vanguard, LLC - BPI Test Center
    Decatur GA

  • 2.  RE: Crawl Space Building Science

    Posted 06-16-2022 09:58
    The graph only covers the summer months in NC, roughly June through September.  I live in a house in NC with an encapsulated and conditioned (supply only) crawl space and we were having massive humidity problems in the shoulder seasons.  I had to add a whole house dehumidifier to bring humidity below 70%.   David Butler argues with me that if the crawl space is properly done then dehumidifiers are a band-aid solution.  And he is probably right about that.  Do you have any data in that report in the shoulder seasons?

    Kent Kjellgren

  • 3.  RE: Crawl Space Building Science

    Posted 06-17-2022 17:19
    It depends on the pressurization of the crawl space with respect to the outside. If the crawl space is under negative pressure, outside air will be "sucked" in. In July and August in Nashville, this moist air can raise the Equilibrium Moisture Content of the wood and amplify mold growth. I suspect the conditions in North Carolina are similar. 

    Barry Westbrook, CIH, PE (KY)
    DocAir Building

    Barry Westbrook
    Franklin TN

  • 4.  RE: Crawl Space Building Science

    Posted 06-17-2022 17:20
    Hi Kent, my advice, not specific to your project, was slightly different: if a dehumidifier is necessary to keep RH under control in a sealed crawl space, then it's a band aid for poorly done sealing. The fact that RH rises in the shoulder seasons is because the AC isn't running when outside dew point is still relatively high. If the crawl is properly sealed, then the amount of supply air can likely be reduced.

    Some contractors sell dehumidifiers as standard fare for closed crawls. I would ask them if that's because they lack confidence in their workmanship, or if they're just trying to make an easy sale?!