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Insulating floor above vented crawlspace with cellulose

user-175166 | Posted in PassivHaus on

I am building a Passive House outside of Seattle in climate zone 4C. The building site has a high water table with water only 24 inches below grade in the winter after a good rain. I’d like to have at least R-50 for my floor above a vented crawlspace. The sub-floor will be the air barrier. The HRV, all duct work and the water heater will be above the floor in the conditioned space. I would like to use dense packed cellulose between the 18 inch deep TJI. I have concerns regarding the high water table. I plan on covering the dirt floor with 10 mil poly, taping the seams and running the poly up the sides of the foundation walls. Should I be concerned about such a high water table? Is there anything else I can do to keep the crawlspace dry?

Is netting going to be strong enough to support 18 in of cellulose for 100 years? Should I support the netting with furring strips or cover it with OSB? How do I ensure that the cellulose installer is filling the entire cavity when gravity is pulling the cellulose down away from the sub floor and I can’t see above the cellulose?

Which are more economical for an 18 in deep floor, TJI or floor trusses?

Any thoughts or comments would be appreciated.

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  1. GBA Editor
    Martin Holladay | | #1

    The type of crawl space you envision will be less likely to have moisture problems if it is truly open to the outdoors. The crawl space under a house on piers (with the wind blowing under it) won't be as damp as an enclosed crawl space with two small vents.

    I'm not a fan of cellulose insulation in a crawl space, because of the potential for moisture problems. In general, I prefer to see an enclosed crawl space with insulated walls rather than a ventilated crawl space with an insulated ceiling. However, many builders in Washington state have had no problems with their ventilated crawl spaces.

    However you proceed, you have to be sure you can keep your plumbing pipes from freezing.

    If you want to insulate your floor joists with cellulose, I strongly suggest that you use OSB, plywood, or polyisocyanurate (held in place with 1x4 furring) on the underside of your floor joists -- something more robust than netting.

    I know of one Passivhaus in Washington state (the Freas house in Olympia) with a crawl space foundation. The Freas crawl space is ventilated, and it has an insulated ceiling (blown-in fiberglass). I am attaching a photo of the floor joists in the Freas crawl space, before the insulation was installed.

  2. davidmeiland | | #2

    Gerald, I've had my insulator do a couple of floors with dense-packed cellulose. We recently did one on open piers, where we installed 3/8" CDX ply under the joists before drilling holes, installing the cellulose, and then covering the holes with plugs. Under my house, where there isn't a way to get 4x8 sheets in, he used a thick fabric he calls "Canvex" under the joists. It is stapled in place around the perimeter and then reinforced with lath strips. It is a LOT heavier than the netting used to blow open walls.

  3. davidmeiland | | #3

    Oh, and as far as I can tell, there is no real way to verify that cellulose is filling every single space in areas that are blown while closed. You hire an experienced installer who knows his work, and you go by bag count. Not sure what else you can do, aside from maybe an IR inspection, which could work but would be painstaking.

  4. Expert Member
    Dana Dorsett | | #4

    The summertime dew points in Seattle are much lower than any right-coast locations: Mean mid-summer dew points in King County are still about 5F below the deep subsoil temps, which makes vented crawlspaces net moisture purgers (unlike locations in New England, where mean midsummer outdoor dew points are 10F+ above the deep subsoil temps.) That said, vented crawlspaces are not always the best approach in Seattle.

    Earth-coupling the crawlspace by sealing & insulating crawlspace walls to even R8 (say 2" of EPS foam, all the way down to the footing) would eliminate frozen plumbing issues in any King County location under 1000' in elevation. But going higher-R on the foundation insulation (say, with a bare-bones R16 insulated concrete form) may be a sufficient peak-load reducer to be able to use half the amount of cellulose, or maybe even ~3" polyiso on the joist edges. I'm not sure how well PassiveHouse tools model this configuration- the thermal conductivity of soils will vary by quite a bit, but with an insulated sealed crawlspace the crawlspace temperature extremes are much better moderated, and the peak heat flux through the floor is much lower, at any floor-R.

    Without running a formal model, note that the deep subsoil temps in your area are in the low to mid-50s F, and the mean wintertime outdoor temperature is ~42F. But the 99% design temps are in the mid-20s. With an open pier foundation you'd be looking at peak delta-Ts on the floor insulation north of 40F, average wintertime deltas of 25F+. With an earth coupled crawlspace approach the peak delta on the floor-R would be about 25F, and the average winter delta would be less than 20F.

    At those deltas, in comparison to a floor-only insulation approach you'd see a lower peak, and about the same average heat flux through the floor at about half the original R-value. (Whole-assembly R, not center-joist R.) There are some error-bars to apply to that, but it's the right ball park.


    99% design temps-

    Outdoor temperatures & dew points:!dashboard;a=USA/WA/Seattle

    Deep subsoil temperatures:

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