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Product for Sealing Joist Bottoms in a Ventilated Crawlspace

DCContrarian | Posted in General Questions on

Let me start off by saying I don’t like ventilated crawlspaces, but sometimes you have to do what you have to do. I’m working on a house in Washington, DC with a crawlspace. My go-to guide is this article by Joe Lstiburek:

Note that the “icky crawlspace” in his article is also in Washington, DC. So this is the climate I’m dealing with — hot and very humid in the summer.  We are rebuilding this crawlspace, and the old one pretty much looked like that picture — insulation between the joists with no vapor barrier below, moldy and rotted joists. It was also rodent-infested so we have to protect it better.

Dr. Joe’s recommendation is to put foil-faced insulation below the joists, to act as both a vapor barrier and insulation. The assumption is that the air under the house will be near 100% relative humidity, the crawlspace will be cooler than the interior of the house year-round, and most of the time and there will be condensation on the underside of the house. He also notes that “rigid insulation needs to be airtight so that requires tiny people with good workmanship,” which is the problem I’m running into. The clearance under the joists is 18″ and I don’t think we can get sheet goods in.

The joists are I-joists, so the contractor has suggested dropping rips of plywood onto the bottom flanges of the joists from above. They would then be sealed with caulk and the joist bays insulated. There would need to be a vapor barrier at the bottom, which could be foil-faced insulation, poly, or even a vapor barrier paint on the plywood. I’m confident we can make that assembly rodent-tight as well.

But this leaves the bottom of the plywood and the bottom flanges of the joists exposed to high humidity and condensation. As Dr. Joe notes in his article, joists can rot from the bottom up in this situation.

The question is whether there is something we can paint on the bottom of the joists and plywood to give them protection. Would regular exterior housepaint work? Or would it not stand up to continuous moisture. What about something like RedGard?



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  1. Expert Member
    Zephyr7 | | #1

    You may find 1/2" polyiso is flexible enough to get into that tight crawlspace. It might be worth experimenting with a single sheet to be sure. You can actually do much of this work from above if you're careful, but it'll be tedious. Probably more comfortable than working in an 18" high crawlspace though.

    Another option would be something like Reflectix. While it seems odd to recommend that particular product that everyone loves to hate, in this particular application it might be a good fit -- it does provide a little bit of R value, so it would insulate the bottom of the joists a bit, but it IS a good vapor barrier and it's flexible on a roll so you could get it into a tight space and stretch it across the underside of the joists while working from above. Might be something to consider.

    Your DC climate comment reminds me of when my wife was coming to meet me for the first time and flew into DCA. She flew from Denver, and it was June. I remember telling her to wear something light, like cotton, because it would be hot and humid. Denver had been cool and typically dry, so she had a parka on. When she stepped off the plane into 101F and the usual summertime humidity in DC... I still remember when I picked her up and said "it's actually not too bad for this time of year" :-)


  2. DCContrarian | | #2

    Thanks Bill. I agree that Reflectix would work here. With insulation on the bottom though I'd still have to figure out how to keep the mice out.

    1. Expert Member
      Zephyr7 | | #3

      You'll want to put your rodent proofing into the walls as much as possible and not the crawlspace "ceiling". My recommendation would be to use hardware cloth, 1/4" mesh, and embed it in canned foam as needed. The hardware cloth provides a barrier to chewing.


  3. Expert Member
    Kohta Ueno | | #4

    Just to kick around more ideas--what size sheet of polyisocyanurate can you slide into the crawl space? If you could thread pieces of polyiso in, you can "tack" them in from the topside by holding them in place temporarily, and using spray foam/froth pack to attach the corners of the joist bays. Pics below are from a friend and colleague, Henri Fennell (a spray foam consultant, former installer), showing this type of a technique used on a roof assembly.

    Henri's pics show narrower strips of foam, but it would always be better to have larger sheets and fewer joints and penetrations.

    1. Expert Member
      Zephyr7 | | #5

      I have had some luck scoring only one side of a sheet of polyiso, then folding it using the other side's foil facer as a sort of hinge. You have to be careful that "hinge" facer doesn't peel back, but if you're careful, you can unfold the sheet with the facer ensuring it goes back together pretty cleanly.


  4. Expert Member
    Josh Salinger | | #6

    Have you explored potentially insulating the stemwall and rim and going for a conditioned crawl? I'm not sure if you have footings or obstructions in the way and admittedly, this idea doesn't help with the 18" height problem. You mentioned that the contractor 'has suggested dropping rips of plywood onto the bottom flanges of the joists from above.' Does this mean you are removing the flooring and subfloor? If that's the case it wouldn't be difficult to get sheet goods down there for the perimeter of the stem wall and you could address the rim at the same time.

    1. DCContrarian | | #7

      I would much rather do a conditioned crawlspace. The crawlspace is on piers and I can't change that. DC has a strict tree preservation law, you can't disturb the soil in the root zone of a mature tree. The crawlspace overlaps the root zone of a large tree, I can't even do anything that would block the flow of water to the roots. I have to treat the bottom of the joists as the limit of the building envelope.

      1. Expert Member
        Josh Salinger | | #8

        That makes complete sense. There is usually a good reason...

  5. DCContrarian | | #9

    So I guess what I'm really asking is for thoughts on applying insulation, vapor barrier and rodent protection while working mostly from above. The appeal of dropping strips of plywood onto the flanges of the i-joists is that there's no lying on your back in a dark, damp, cramped spot while trying to hold something heavy over your head and fasten it in place, doing a careful job so you don't leave any gaps while wishing you had five hands. But if it has to be done that way it has to be done that way.

    I guess no one likes my idea of waterproofing the bottoms of the joists.

    I'm thinking maybe quarter inch hardibacker might make a good rodent-proofing layer, with half inch foil-faced polyiso above it. I could get it into the crawlspace. What would make the job a lot easier would be a way to hold the panels in place, something like a drywall lift. But it can't get in the way in the cramped quarters. Maybe temporary cleats fastened to the bottom of the joists that I can rest the panels on until they are positioned.

    The other question is whether to stagger the joints in the foam and backer so they don't lie on top of each other. While I'd like to stagger for better air-tightness, it creates a lot of work. If they aren't staggered I can stand on the floor of the crawlspace and reach underneath and probably do half of the screws without crawling on my back.

    1. DCContrarian | | #10

      This FHB piece shows what I'm thinking for supporting the panels while I work:

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