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Community and Q&A

Sealing off unencapsulated crawlspace?

kneewall | Posted in General Questions on

I live in Zone 5A, in a 1.5 story house built in 1960. I have a basement which is half finished. The finished portion has a screen door that leads to the unfinished part (it was there when I moved in). The unfinished portion has an access hatch to the unvented crawlspace, which has a dirt floor, no vapor barrier, and almost no insulation (some of the rim joists have fiberglass batts stuffed in them). The crawl has uninsulated ducts (supply and return) that run through it. Above the crawl is my kitchen, 1/2 bathroom, and dining room. I’ve attached a diagram that roughly shows the situation. This is a separate issue, but there is a trench that runs the length of the crawlspace, and I’m not sure why, or what effect that may have.

Through reading this site, I’ve become concerned about crawlspace air making its way throughout my house. During heavy rains, I can see where water seeps in through the concrete block walls, and there is what appears to be mold on the affected walls. I’ve done my best to seal the crawlspace ducts with mastic, but one thing I’m curious about is the space above the wall that separates the crawl from the unfinished part of the basement. There’s a pretty substantial gap between the floor joists where air seems like it can freely flow. I’ve drawn a blue arrow showing my area of concern. There is a slightly musty smell apparent in the basement, and it increases as you get closer to the crawlspace.

So my questions:

1. Would sealing the area above the block wall, between the floor joists, provide any practical benefits (like reducing the smell)?

2. If I just seal off the crawlspace without fully encapsulating it, will that cause any problems for other parts of the house?

3. What other issues should I be worried about with this setup?

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  1. Expert Member
    Peter Yost | | #1

    Hi Kneewall (be great for the GBA Q&A Community to have a real name for you) -
    With mechanical equipment in your crawlspace you should really treat your crawl as "inside" your home:

    1. ground cover sealed to eliminate bare soil (could be dozens of pints of water a day making its way from the soil up into your home).
    2. air seal at the perimeter of the crawlspace
    3. insulate the perimeter walls of the crawlspace

    If you have not already, start here on this topic:


    PS - thanks for the drawing---always good to have visual to go with text.

    1. kneewall | | #3

      Thanks Peter. My name is Mat. I should have used a real name when I chose my username.

      As far as bringing the crawlspace "inside," is there a feasible way to do this in stages? Also, what would I even do about the trench?

      My other question is, how can I make sure I'm able to find the right contractor for the job? I've gotten estimates from contractors for insulation in my attic and getting a vapor barrier in the crawl, but I never felt especially confident in their building science knowledge. I don't suppose you happen to know anyone in Indianapolis?

  2. GBA Editor
    Martin Holladay | | #2

    If you create a sealed, conditioned crawl space, there shouldn't be any reason (from a building science perspective) to extend the partition up to the subfloor above. But if you are worried about smells, extending the partition might be a good idea.

    You might also want to read this article: "Building an Unvented Crawl Space."

  3. keithhoffman22 | | #4

    Hi Mat,

    DIYer answer. Your best bet for a sealed crawl space floor contractor in many areas is a radon mitigation contractor. Have you tested for radon?

    A radon system will protect you from radon and its daughter progeny as well as eject soil moisture from your building. Systems cost, in my area, roughly $1/SF of crawlspace plus $800 for the fan system. It can be DIYd. Consider adding a sump pit at the same time (main suction point should be above the 2.5' deep x 2.5' wide plastic sump pit). Such a system is very effective at removing odors, radon, and moisture.

    Given your moisture issues, I'd consider a system even if your radon is only 2.0-4.0 pci/L. It's not clear that levels from 2.4-4.0 are especially safe.

    If your crawlspace has high radon, treat the dust and dirt as hazardous material (i.e. respirator and gloves). Radon decays and attaches to dust which you don't want to inhale.

    However, I suggest trying to do some of your crawl space work as part of the same contract or before the radon system install so the radon membrane isnt damaged by further work.

    Things to think about doing are:
    - air sealing with canned foam (get a long nozzled pro style gun) or building tapes (dow weathermate, proclima tescon vana, siga wigluv, etc)
    - preventing rodent intrusion if evidenced: plywood, steel wool injected with spray foam, 1/4" hardware cloth
    - insulating joist ends: I like roxul comfortbatt for shaping to fit around wires, pipes, etc (cuts with a bread knife) but other options work too.
    - insulating water lines with rubber noodle insulation (reduces condensation on cold lines, heat loss on hot lines, sometimes quiets the pipes)
    - I wouldn't insulate the concrete block until you find and address the source of moisture. Consider roxul comfortbatt or EPS foam adhered with PMFs (plastic masonry fasteners) once you've addressed the moisture.

    Super fun working in crawlspace but when you take it from nasty, smelly, and toxic to nice and clean, you'll be pleased.

    And read Martin's linked articles. Amazing knowledge base here on GBA.

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