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Unvented, sealed crawlspace under pier foundation

Joe Coulterson | Posted in GBA Pro Help on

We’re in western Montana — cold, dry, very little humidity and rain, decent amount of snow. Bought a small house (< 700 sq. ft.) on a pier foundation. The crawlspace is raised about 40 inches above grade. The floor joists were insulated with fiberglass batts and covered with thin plywood. Unfortunately the plywood didn’t cover things completely, because rodents were living in the insulation between the joists. They had all kinds of tunnels, nests, etc. I’m in the process of cleaning all that out and tearing out the old batts. I’m wondering if I can make the crawlspace sealed and unvented and insulate with rigid foam on the outside skirting walls, following the recommendation of GBA’s unvented crawlspace article? The only reason I’m questioning this is because that article and other sources of the technique all seem to refer to traditional crawlspaces, rather than pier foundations that sit above grade so much. I would much prefer to insulate the walls, rather than the floor joists, because it’ll cost me less (less wall coverage than floor), it’ll be much easier, and I’ll be able to get the air sealing details done right. Feel free to point me to any reference articles if they exists. Thanks all.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Joe,
    Here is a link to a Q&A thread that discusses your situation: Insulating a crawlspace.

    After you read that thread, and the linked articles suggested in the answers on that page, you may have a few follow-up questions. If you do, feel free to post those questions here.

  2. Joe Coulterson | | #2

    Well, shoot. Yes, that is a very informative How to Insulate a Cold Floor article. I was very much hoping I wouldn't have to do that sort of thing -- I'm anticipating lots of tedious work getting Roxul batts in between the joists, then covering the floor with rigid foam and taping.

    And that Q&A also talked about insulating the skirting... I thought it was one or the other, either the floor or skirting? I guess not. And perhaps in our cold climate it makes sense to do both. Thanks for the link to the thread. I'm sure I'll have more questions as I digest this more.

  3. GBA Editor
    Martin Holladay | | #3

    Joe,
    The earlier Q&A thread may have been unclear. If you find a way to build a durable skirt, and you insulate the skirt well, you don't need to insulate the floor joists above the crawl space.

    There are several problems with this approach. If you want a sealed, insulated crawl space, the best way to proceed (but not the cheapest) is to build a new foundation -- one with either poured concrete walls on a proper footing below frost level, or one with concrete block walls on a proper footing below frost level. Then you can insulate these walls according to the usual advice (Building an Unvented Crawl Space).

    But that's expensive. If you want to try to keep your crawl space warm without paying for a proper foundation, you'll need a halfway measure like a quality skirt. But it's hard to do this right -- to end up with a skirt that is really airtight, well insulated, and rodent-proof.

  4. Joe Coulterson | | #4

    Thank you Martin. I wanted to send along an update. After more inspecting of the very shoddy and unusual floor framing, I'm not confident of the approach of Roxul batts between the joists and rigid foam on the underside. First reason is that air sealing the floor will be very difficult. I would hate to spend the $1,600 or so that I priced out for R30 Roxul and rigid foam, and then air leaks negate the performance on the insulation. The second skepticism is whether I'd effectively be able to keep critters from getting past the rigid foam layer and nesting in the joists with Roxul. We live quite remote in the mountains and there are plenty of rodents and raccoons around. An insulation contractor told me that rodents won't necessarily eat the Roxul, but when it's -20 out they'll sure like to nest up there.

    I was quoted at $2,880 ($0.90/inch/square foot) for 5 inches of spray foam on the underside of the floor. I’ve also been referencing this BSC article on crawlspaces: http://buildingscience.com/documents/insights/bsi-009-new-light-in-crawlspaces . It seems the recommendation is to spray both between floor joists and fully cover the exposed bottom edge of the floor joists. The article does mention not encasing the whole joist – Figure 9 – but then the crawlspace needs to be open to the outside. Unfortunately with plumbing under the floor and our cold weather, I don’t think leaving the crawlspace open is an option. That means covering the whole joist with foam. But, two local insulation contractors I’ve talked to say that is unnecessary and foam between the joists is all that is needed. Any other opinions on this?

  5. GBA Editor
    Martin Holladay | | #5

    Joe,
    I agree with Joe Lstiburek -- it's always a good idea to have some insulation under the floor joists. That's why my article, How to Insulate a Cold Floor, recommends a continuous layer of rigid foam under the floor joists, followed by a protective layer of OSB or plywood.

    If you are worried about freezing pipes, you may prefer to leave the floor joists uninsulated, and instead to install an airtight skirt (at the perimeter of your crawlspace -- in other words, crawl space walls) built of durable materials (pressure-treated plywood, pressure-treated 2x4s, and rigid foam, installed with attention to airtightness).

  6. Expert Member
    Dana Dorsett | | #6

    Rodents will chew through 2lb foam too, (though they probably won't nest in it.)

    Adding 2" edge strips of rigid foam board on the bottoms of the joists and filling the full depth with fiber insulation, with OSB holding it all up (with 3.5-4" screws or ring-shank nailed through to the bottom joists) is probably still cheaper than your 5" of foam, and reduces the cold-striping of the joists on the floor when it's -20F outside about as well as a full-layer of 2" foam board. If you feel the need to air seal with foam, 3" of open cell foam is enough. Most subflooring + finish flooring has a combined vapor permeance low enough to be a sufficient vapor retarder for protecting the moisture susceptible OSB.

    If you staple half-inch mesh galvanized hardware cloth to the OSB it will keep the raccoons out, probably even the badgers (but maybe not the bears :-) ).

    It may be less material and less work to take the air-tight insulated skirting approach though, depending on the particulars.

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