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Closed Cell In My Unvented Gable Roof 5.5 in or 4 in To Save Money???

leavytinyroof | Posted in General Questions on

Hi there,

My name is Hazen and I’m going through the motions on a new construction build of a mini home on trailer. The home will be in Washington state climate zone 5. I’ve 99% decided to have a local spray foam contractor put in closed cell in the 2×6 unvented ceiling rafters after pondering flash and batt. Walls are rockwool. Floor is rockwool and foamular.

4in quoted at $1,350 R28
5.5in quoted at $2,050 R38

Is the money savings worth going down to 4in? R38 is code, but then again, it’s rare that a tiny house ever has R38 in the roof, whether below or above the sheathing, because of height and weight restrictions.

I’ll hang 1/4in drywall after to seal out inside air and then finish with some wood t & g of some kind.

On the other side of my roof is OSB, titanium PSU 30 ice and water shield, standing seam metal.

4in of closed cell will do the trick to prevent condensation yeah?

Thanks for feedback.

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  1. Expert Member
    PETER G ENGLE PE | | #1

    Just about any amount of impermeable insulation will prevent condensation, so long as you don't have an appreciable amount of permeable insulation inside of it. It is the ratio of permeable to impermeable that you need to watch. in your climate zone, 50-50 is great, 70-30 would probably still be OK, though tiny houses have lots more moisture general per unit volume than bogger houses due to the more people per unit volume. In most places, tiny houses on trailer frames don't have to comply with residential energy codes. And, by nature, tiny houses use far less energy and materials than larger ones so you start way ahead of the game in terms of lifetime carbon costs. All that said, considering the thermal shorts of the rafters themselves, going to 5" provides little extra real energy performance.

  2. Expert Member
    Dana Dorsett | | #2

    Peter has it right.

    Dropping the closed cell foam layer to 2" of cc foam (R12-ish & ~0.6 perms) and stuffing the remaining 3.5"" with R15 batts (R15-ish),at ~45% foam would be moisture safe for the roof deck (for climate zone 5), cheaper, and would thermally outperform 4" of cc foam (with no fiber).

    1. leavytinyroof | | #4

      I sure am loving the sound of this; putting in rockwool over the 2in of spray foam. I didn’t know 2in was sufficient. Outperform for less money and less offgassing. How could I not go for that recipe? The insulation contractor must have a slightly higher view on the R value compared to your R6 per inch since I got the R28 and R38 out of their estimate. Thanks again.

      1. user-2310254 | | #6

        If budget is a concern, high-density fiberglass batts will be cheaper than rockwool.

        1. leavytinyroof | | #7

          Hi there, budget isn’t much of a concern. The insulation contractor actually recently told me the cost will be the same for 2in vs 4in since they have a $ minimum for bringing out their trucks to the job. Cheers.

          1. Expert Member
            Dana Dorsett | | #8

            With 4" foam (= R28, for some HFO-blown foams R24-R25-ish for most cc foam) it becomes harder to fill the remaining 1.5" of space decently with fiber. With 2" foam there is 3.5" of remaining space, which is enough space to allow compliance/compression of the fiber despite the now uneven surface of the foam layer.

            Center-cavity R isn't as important as whole-assembly R. In a tiny house the framing fraction of the roof is likely to be ~25% or higher, and the heat transfer through the rafters becomes quite significant. At 4" the foam might be R28, but the 4" thermal bridge of the more thermally conductive rafters is only ~R5, severely undercutting the whole-assembly performance. With a full cavity fill the 5.5" of rafter-R becomes ~R6.6, a ~24% reduction of framing losses.

            At 2" foam (R14) plus 3.5" of HD fiberglass or rock wool (R15) the center cavity R is R29, or about the same as 4" of foam, but the framing fraction losses are much improved.

  3. walta100 | | #3

    Seem to me you could cut your costs by eliminating the spray foam by replacing it with sheet foam over the roof deck. If you can find reclaimed / used foam even better.


    1. leavytinyroof | | #5

      Hi Walter, the roof deck is already done; 1 layer of OSB, ice and water shield, standing seam metal. Thanks for your input and idea.

  4. jollygreenshortguy | | #9

    I just read through all the comments and as usual learned things today. My thanks to you all.
    A few days back I became aware of the Bonfiglio wall here on GBA. Would this be an opportunity for something similar, but for the roof, space permitting? Peter Engle mentions the thermal bridging at the rafters. Perhaps build out the rafters with 1" of EPS? Then take advantage of that add'l inch to do a bit better with the flash&batt?

    Rafter + 1" = 6.5"
    3" flash = R18
    3.5" batt = R15
    R18/R33 ratio is more than adequate

    1.5" flash = R9
    5.5" batt = R18 (R19 batt compressed 1/2")
    R9/R27 = .33, just meets the ratio requirement for zone 5

  5. DennisWood | | #10

    Spray foam for this roof at 4" or 5" makes the most sense structurally. This is a tiny home on a trailer which means it will be towed (I'm assuming) at some point. The increase in racking strength on the ceiling assembly with closed cell makes it the best choice in my opinion. Old post I know, but no one had mentioned the structural difference.

    Check this video at 1:43 which has racking test results from a few assemblies:

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