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Insulating crawlspace floor

jgold723 | Posted in General Questions on

I have a 22′ x 22′ addition build on posts, 18″ above the ground. Full skirt around the base. The builder used fiberglass batts under the floor which 15 years later are now sagging.

I plan to open the skirt, crawl in and remove the batts, but then what?

I’ve gotten quotes to sprayfoam the floor and rim joist (around $2,500)

But it occurs to me that it might be better (i.e. cheaper) to insulate the inside of the skirt walls with rigid foam insulation and then just spray the rim joist.

Thoughts? There is some plumbing in the crawlspace which concerns me, although in 5 years, it has never frozen.

And if this has been answered hundreds of times already (probably has), don’t hesitate to send me a link. I don’t want to waste anyone’s time.

The house is in southern Maine.

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  1. Expert Member
    Dana Dorsett | | #1

    How deep are the floor joists?

    Describe the plumbing exposure issue a bit more. Are you willing to re-plumb it to where it's under the subfloor, where it would be protected by floor insulation?

    It can be quite difficult to adequately air seal the skirting on a pier & beam crawlspace, so doing a better job at insulating the floor is probably a better bet.

    Current IRC code minimum for floors in zone 6 (all of southern ME) is R30 between the joists, or R19 minimum if the joists aren't deep enough. To avoid temperature-striping issues on the floor due to the thermal bridging of the joists it's sometimes better leave a 2" air gap between the top of the insulation and the bottom of the subfloor &/or install a continuous layer of rigid insulation under the joists.

    R30 rock wool batts are 7.25" thick, and would leave a 2" gap on 2x10 joists, but they're expensive. R30C " cathedral ceiling" fiberglass is 8.25" deep.

    A full fill of a 2x8 joist bay with cellulose would run ~R27, and a layer of 2" EPS or 1.5" polyisocyanurate would be sufficient to pretty much eliminate temperature striping. Filling a 2x10 joist bay with cellulose would be about R34, 2x12s would run ~R42. Whether or not a full layer of bottom side rigid insulation is "worth it" is questionable.

    Air sealing the band joists is a critical first step no matter which approach is taken and if there's going be an air gap above the insulation of the band joist at that air gap needs to be at least R15 (R20 is better). Cut'n' cobbled rigid foam is fine for that air-gap region insulation.

    The bottom side needs a half-inch plywood or OSB to support the cavity insulation, installed ahead of time for blown insulation, then drill & fill. It's usually possible to hit 2.5-2.8lbs or higher density in a joist bay using a box store rental blower, which is plenty for a floor dense-pack (it would be really marginal in walls though.) The technique matters, but I won't go into it unless you're seriously considering a DIY dense-pack with a rental blower.

  2. jgold723 | | #2

    The floor joists are 2 x 10. I'd rather not replumb if possible... thus my thought that if I adequately insulated the perimeter walls and put poly down on the ground I'd create a conditioned space under the floor that would keep the floor warmer and protect the plumbing. Is is really that hard to seal the skirting? I guess I'll know more today once I get under there and see what's up.

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