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Thickness of Closed Cell Insulation in 2×4 Walls

thetuningspoon | Posted in Energy Efficiency and Durability on

I’ve built a 200 sq ft structure (tiny house on wheels, not inspected) with 2×4 walls and 2×6 rafters. I’m ready to insulate and have been planning on using closed cell spray foam for the whole thing to avoid moisture issues in the unvented cathedral ceiling. (I would love to use Roxul instead if it were an option). I don’t have any can lights or skylights in the ceiling and only one penetration for the range hood.

My climate is 5A.

I’ve received several quotes for the closed cell. The first company suggested I only do 2″ in the walls since it wasn’t being inspected, which surprised me. He said that each additional inch of foam after that only gains you a fraction of what the first 2″ gets you(?)

I do like the idea of having a little bit of space to run a wire or move an electrical box if I have to do that at some point, but I don’t want to spend thousands of dollars to end up with the same R value as a fiberglass batt wall. My walls are already very tightly constructed (All exterior plywood panel edges land on blocking and are glued and screwed, so there is really no place for air to penetrate)

This company also suggested 4.5″ in the ceiling. The other companies suggested 3″ in the walls and 5″ in the ceiling.

I am inclined to do 2.5″ in the walls and 5″ in the ceiling, but please let me know if there are any other factors I’m not considering.

This is the installer I’m looking at right now (not the one who suggested 2″). Would someone with a better knowledge of spray foam installation quality mind taking a look at their portfolio and telling me whether their closed cell installation jobs look good?

Thanks very much!

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  1. Expert Member
    Dana Dorsett | | #1

    The installer is correct that the additional performance of the additional inch isn't going to be "worth it", on a performance basis, given the high cost per R.

    Of the three closed cell products listed on their w only the Lapolla is blown with the far less damaging HFO1234ze blowing agent. The Gaco and BASF products are blown with HFCs (usually HFC245fa) that are on the order of 1000x CO2 global warming potential @ 100 years, the opposite of "green building".

    An inch of closed cell foam in the walls + compressed "contractor roll"R13s would be higher performance than 2" of any closed cell foam, due to the longer higher-R thermal bridging path on the framing fraction, and would have adequate dew point control to keep the fiberglass dry even in climate zone 6.

    On the ceiling, 2" of Lapolla Foam-Lok 2000 4G (~R14) plus R15 batts would be higher performance than 4.5" of R7/inch foam, at your (likely higher than average for a full sized house) framing fraction. That would be a 45% + foam/total fraction, more than adequate dew point control for zone 5 (40% wouild be the minimum) and even the warm edge of zone 6. Even 2" of any R6/inch foam + R15 batts would have sufficient dew point margin for zone 5, but it would be just barely squeaking by with 2" of an R5/inch foam product (like water-blown Icynene.)

    For the napkin math on thermal performance of closed cell foam thermally bridged by framing see:

  2. thetuningspoon | | #2

    Thanks Dana. I hadn't thought of a 'flash and batt' technique being practical in this situation, but it sounds like I may have been wrong.

    Would you still suggest foaming the walls too, or should I do batts + interior vapor barrier?

    Also, could I use Roxul for the batt material? I'm not sure how the price compares, but I like what I hear about the material otherwise.

    Gaco does have a "low Global Warming Potential" version of OnePass as well.

  3. Expert Member
    ARMANDO COBO | | #3

    I would never recommend using CC foam insulation only on the interior of a house walls, having said that, I imagine CC foam insulation would give rigidity and structural strength to a house on wheels, specially if you travel or move it quite a bit. I've also seen a handful of tiny houses that used outsulation rigid foam to help with the thermal bridging as well. Personally I like the 3" and 5", but I have no experience nor data to substantiate my WAG comment.
    From BASF: "Spray-applied closed-cell polyurethane foam is the only insulation material that adds structural integrity throughout the wall system. Testing conducted by the National Association of Home Builders (NAHB) Research Center and the Canadian Construction Materials Centre shows spray-applied polyurethane foam insulation between wood- and steel-stud wall panels increased rack and shear strength two to three times compared with standard stick-built components with glass fiber insulation when sprayed onto gypsum wallboard and vinyl siding, and increased racking strength when sprayed onto oriented strandboard (OSB)."
    From Pentoir: "SPF also can add structural strength to buildings. NAHB Research demonstrated SPF filled walls could add from 75% to 200% racking strength to walls of OSB, plywood, light gauge metal, vinyl siding or gypsum board."
    You can search more about this topic on Google.
    PS: I should make a point that if you use outsulation (rigid foam), use open cell foam, Roxul, etc on the inside of the wall.

  4. Expert Member
    Dana Dorsett | | #4

    At 3" most closed cell foam between studs is structural enough that the exterior sheathing can usually be eliminated. But at $3-4 per square foot that's fairly pricey structure + insulation.

    In zone 5A if the siding is back-ventilated (either built out rainscreen, or vinyl siding) with a plywood or OSB sheathed wall there is no need for an interior side vapor retarder tighter than standard latex on wallboard, meeting the "Vented cladding over wood structural panels." exception:

    Installing 3" of polyiso above the roof deck and R23 rock wool in the 2x6 rafters delivers sufficient dew point control for zone 5 roof decks, but 4" would be better. At 4" of exterior polyiso would make code min (or close to it). If using reclaimed roofing polyiso that can even pretty cheap.

    Unlabeled fiber faced roofing polyiso can be presumed to be at least R5.5/inch, so at 3" you'd have at least R16.5 above the roof deck, and with R23s in the cavities you'd have R39.5 total R16.5/R29.5 = ~42%. The IRC prescriptive minimum for zone 5 comes in at 41%. If that's of concern you could back off to R20 fiberglass for R36.5 total, and R16.5/R36.5= ~45%. But bumping the exterior polyiso to 4" puts at least R22 up top, R23 in the cavities, a 49% fraction above the roof deck, which is plenty of margin.

  5. thetuningspoon | | #5

    The roofing is already complete (grace ice & water + metal roofing), so exterior foam is no longer an option. Perhaps I could have use sistered 2x4 rafters instead of 2x6s, to allow for the extra 3" that would be needed to do exterior foam (I have to keep the structure + trailer under 13' 6" to be road-ready, so there is a limit to how thick the roof assembly can be).

    The siding is 1/2" beveled cedar. No rainscreen, but I am using Benjamin Obdyke's Hydrogap underneath it.

    The wallboard will be 1/4" pine plywood to keep the weight down. I haven't decided whether I want to paint it or not.

    I would say moisture control is my big concern at this point. Total R value is of lesser concern.

  6. Expert Member
    Dana Dorsett | | #6

    Detailing the interior plywood as an air barrier and painting it with "vapor barrier latex" would be cheap insurance for protecting the wall for an all-fiber solution, but a flash'n'batt would be better.

    Compressing an R13 into 2.5" yields R10, and with an inch of HFO blown foam the total would be R17, with a foam being 41% of the total R. For walls it only needs to be about 27% in zone 5 for dew point control at the foam/fiber boundary (and you'd be at a ratio good enough for the roof!). Even air leakage into that wall from the interior via holes in the 1/4" plywood wouldn't be a wetting problem with 41% of the cavity R being foam.

    So for moisture control it's back to 1" flash'n'batt on the walls, 2" for the roof, using anybody's HFO-blown closed cell foam.

    Given that weight is a concern here, note that even compressed batts are between 1/2 - 2/3 the density of closed cell foam. Less foam = less weight. Using the minimum needed for dew point control shaves off some pounds.

  7. thetuningspoon | | #7

    All great points, thanks.

    I should also mention that cost is obviously a factor here. So if flash and batt is going to cost less, I'm all for that. I assume that a significant part of the expense of the spray foam is just getting the crew over here to do the job, so I'm not sure with such a small space if the cost calculation will be the same as it would be for a larger job where the material cost would factor more heavily.

    Another thing in the back of my mind is the (probably irrational) thought that less foam might lead to healthier air quality. But I know the stuff isn't supposed to be bad for you if it's cured and mixed correctly, and if it IS bad for you it probably isn't going to make any difference whether there's 2" or 5" of the stuff.

  8. Expert Member
    Dana Dorsett | | #8

    How many board feet (1" x 12" x 12") would it be for the 1" + 2" flash'n'batt, compared to 3" walls + 5" roof?

    At about 3000 board feet of HFO blown foam, a recent quote came in at about $1.40/board-foot for a basement finishing project an associate of mine is working on:

    If it were only 2" it would have been more expensive per board foot, but a bit cheaper overall. For very small jobs the difference in cost would be negligible.

    At 5" for your roof is most likely coming in under 1000 board feet and at 3" your walls (less doors, windows & framing area) are also probably going to be under 1000 board feet. So you're looking at maybe 1500 board-feet for an all HFO foam solution, probably at at $2500-3000. (Just a WAG- get quotes.)

    With flash'n' batt solution it's probably about 750-800 board feet total (you do the math) and perhaps $1800-2000. (That's $2.50 per board-foot- if it costs more than that maybe I'll get into the business! :-) )

    Contractor rolls of fiberglass R13s to fill the remaining 2.5" and 3.5" depths are dirt cheap about 40-50 cents per square foot. Even at box stores R15 rock wool is less than 75 cents a square foot, so it's probably less than $500 in additional material cost either way you do it. But it takes time to install them.

  9. thetuningspoon | | #9

    You're right on the money. The quotes I've gotten so far have been in the $2,500-$3,000 range.

    Did you have any thoughts on the quality of the installs in the portfolio I linked to?

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