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Crawlspace dilemma and recycling fiberglass batts

user-3813901 | Posted in Energy Efficiency and Durability on

Hey Fellow Builders and Consultants,

I’ve got a semi-unique weatherization project that I’m about to start and need some advice.

The building in question is a 24′ x 24′, single level, four-season cottage on piers, ranging from 3′-4′ tall from grade, with a loose-fitted board skirting on 3 sides, and open on 1 side; very high and dry and well ventilated “crawlspace”……sort of.

The floor deck is 12″ I-Joists with 8″ fiberglass batts and 1′ rigid foam rips, loosely stuffed into the joist bays. (Very leaky!)

The ceiling inside, is tongue and groove pine, nailed directly to bottom chords of roof trusses, with no air barrier, (very leaky!) and 8″ fiberglass batts over it, with gable end vents, (very well ventilated attic).

My attic plan, is to pull aside the fiberglass batts, apply a 1″ layer of closed cell spray foam against pine boards from above (R-7), re-install the batts (R-24), and recycle the good-conditioned fiberglass batts from the floor deck, installing them cross-hatched over first layer (R-24), for something like R-50 total.

My floor deck plan, is to apply a 3″ layer of closed cell spray foam in I-Joist bays, against the subfloor from below (R-21), then cover bottoms of I-joists with 1-1/2″ Poly Iso, to combat thermal bridging through I-Joists (R-9.5) and tape the seams, for something like R-30 total.

My questions are:

1. If I’m careful to air-seal the layer of rigid foam on the joist bottoms, is there any type of insulation that belongs in the remainder of the I-Joist bays, other than spray foam, that isn’t too risky? (there will be 9″ unfilled space left, after 3″ spray foam) (I’ve read Martin’s article “Building an Unvented Crawl Space”, but unsure this scenario fits various prescriptions therein.)

2. Should I consider another type of rigid insulation board to apply to the joist bottoms, due to poly-iso’s poorer cold-weather performance?

3. Should I not worry about thermal bridging through the I-Joists, which is minimal compared to dimensional lumber joists, and just add more closed cell spray foam?

4. If I omit the layer of rigid foam on I-joist bottoms, and spray thicker closed cell in the joist bays, do I need an ignition or thermal barrier? ( I’ve read Martin’s article “Thermal Barriers and Ignition Barriers for the Spray Polyurethane Foam Industry,” and I’m a little unclear how to handle this particular scenario, given that there isn’t really a crawlspace…..sort of.)

5. Does my attic plan makes sense; should I “flash and batt”, re-using the fiberglass (both layers), over the closed cell foam, and call it good?

Appreciate any thoughts you all have!


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  1. GBA Editor
    Martin Holladay | | #1

    You may want to read this article: How to Insulate a Cold Floor.

    If I were insulating the floor assembly, I would proceed as you have planned, except that I would fill the spaces in the joist bays with blown-in cellulose or fiberglass batts. I would install the polyiso in an airtight manner, and I wouldn't worry about cold-weather performance of the polyiso. I would also protect the polyiso with a protective layer of OSB on the exterior side of the polyiso.

    Alternatively, you could just skip the spray foam in the floor assembly, and use dense-packed cellulose in the joist bays, coupled with a continuous layer of polyiso on the underside of the floor joists.

    Your attic plan is OK -- but it would be better if you topped everything with about 4 or 6 inches of cellulose, to reduce convection currents and improve the thermal performance of the fiberglass layers.

  2. user-3813901 | | #2


    Good point about topping off the attic with cellulose, to improve the fiberglass' performance. And I would lots rather use more cellulose and less foam, overall, as you describe.

    In that case, what if I spray only 1" under the subfloor to air seal (R-7), re-install the batts (R-24), then cover joist bottoms with taped 1" polyiso (R-6) for an R-37-ish total? Then finish the attic as you suggest, only install 12" loose fill cellulose over 1" spray foam and existing batts.
    I do need to be sure the joist cavity is filled or the bottom layer of polyiso on joists is useless, correct?

  3. Expert Member
    Dana Dorsett | | #3

    The polyiso is still doing something even with empty joist bays, as long as the ends of the bays are air tight, and not mere wind-tunnels. But it's clearly not enough on it's own.

    An inch of closed cell foam is a g'dawful expensive way to air seal a subfloor. For the same money you could install 3-4" of open cell foam and achieve even better air sealing, (and 2x the R or better). But caulked & taped seams & joists is a whole lot cheaper (and greener.) Spray foam may be an easier/better choice for air sealing the band joists, but it's pretty wasteful to use it for sealing a subfloor.

    You may be able to leave the batts mostly in place, pulling them back where you need to air seal, the blow fiber on top of them to fill the void. This is a judgement call, but with 4" of void to play with it's pretty easy to snake a dense-packing tube to do the filling. It'll compress the batts a bit, but that will improve their R/inch, and you'll end up using about half the material as a full-fill of blown-only.

  4. user-3813901 | | #4

    Point taken. I usually use spray foams very sparingly, in general, and thought I was being clever by using less. In this case, less may not be more. : )
    Initially, I was concerned about stuffing something like cellulose in between two vapor-closed layers, but I'm confident that I can make the bottom layer air tight, and I don't see any real vapor drive here, as long as the skirting remains well ventilated and open.
    Happily, the plumbing is 95% inside the cottage; I only need to deal with the water well line by making a tiny "hot-box" connected to the house via a pair of small inlets in the floor, to keep it from freezing. Drains should be fine, although I'll likely wrap them in thick pipe insulation.

  5. user-2310254 | | #5

    Should Dylan finish his floor assembly with a layer of exterior grade plywood? Since the cottage is on piers and the crawlspace is not sealed, does he need an ignition barrier along with something to discourage critters from setting up house in the I-joists?

  6. Dana1 | | #6

    "...I was concerned about stuffing something like cellulose in between two vapor-closed layers..."

    OK the foil facer is one of those vapor tight layers- what is the other layer? The initial 3" of foam?

    The moisture drive in zones 5 & higher would be primarily from the indoors, and the condensing plane would be the foil facer on the polyiso. But a plywood subfloor is a Class-II vapor retarder, and there won't be much wintertime moisture accumulation, even if the R-ratio isn't sufficient for dew point control in your climate.

    In zones 3 & lower the primary moisture drive is likely to be in the summertime, and coming from the exterior, and the vapor retardency of the foam & facers is enough to keep the moisture adsorption at the subfloor well bounded no matter how much you air condition the place.

    In zone 4 the drive is more seasonally balanced.

    But for the record, what is your climate zone?

  7. user-3813901 | | #7

    Dana, Correct. I was counting the 3" closed cell as the second vapor closed layer. And agreed, I don't think this is an issue with the foil facer placed on joist bottoms. I'm in climate zone 6, Northeastern Vermont.

    Steve, I think I'm okay as is, in terms of ignition barriers. Next season, I may cover the polyiso with some type of plywood, or OSB, as Martin suggested.

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