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Closed-cell spray foam insulation: between rafters or joists?

Jon_L | Posted in General Questions on

Hi,

I live in an old (~1900) house in climate zone 5 (southern CT).  We are having our attic re-insulated, and have gotten quotes for both rafter (un-vented attic) and joist (vented attic) insulation with closed cell foam.  The quote includes 5″ of closed cell foam in both applications.  The cost difference is not significant (~$300), and I have read a lot of conflicting advice between the two options.  Currently, the attic is vented with fiberglass insulation between the joists. There is no HVAC equipment in the attic, however, we may add central air in the next few years. The contractor recommended insulating in between the rafters and conditioning the space.   Any thoughts on the best option based on these conditions?

Best,

Jon

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Replies

  1. Expert Member
    Dana Dorsett | | #1

    5" of closed cell foam doesn't meet current code min in CT on it's own. Is that in combination with fiber insulation?

    If you're planning to install HVAC equipment in the attic it's better to condition the attic. But it's also important to install more than R30-R35 in the attic in your climate.

    How deep are the rafters?

  2. GBA Editor
    Martin Holladay | | #2

    Jon,
    Dana hit the important points. Five inches of spray foam isn't enough to meet minimum code requirements.

    As Dana said, if you have HVAC equipment in your attic, the insulation should follow the sloped roofline. For more information, see "Creating a Conditioned Attic."

    If you decide (for whatever reason) to insulate the attic floor, you don't need spray foam. All you need to do is to perform air sealing work first, and then install at least R-49 of cellulose insulation.

  3. Jon_L | | #3

    Thanks for the replies. The contractor did mention that it would be better to have more than 5 inches, but the rafters are 2x6’s. This contractor rated the 5 inches at r-38, but I suppose that is a bit exaggerated. We had another contractor come and suggest 3 inches on the floor...which is obviously not enough. We had a home energy audit qnd they rated our current level at R-9. Would having only 5 inches create other issues?

  4. Expert Member
    Dana Dorsett | | #4

    At 5" even HFO blown closed cell foam is only R35-ish. But at 4" would be sufficient dew point control to install R23 rock wool (5.5") to the interior side, and you'd be more like code-minimum. That would mean increasing the rafter depth to total of 9.5". If they are full dimension 2x6 that's just 3.5, if milled 2x6 (rare in 1900) it would be 4".

    Pre-building some 6" x 9.5" deep plywood or OSB gussets and side-nailing & gluing them onto a 2x2 about 24" on center they can then be side-nailed to the rafters prior to he closed cell foam. cutting some 1.5" or 2" foil faced polyiso to fit between the 2x2 & rafter would more than double the R-value of the framing fraction, and close them up from being potential thermal bypass air channels for the batts.

    With 4" of HFO blown foam you'd be at about R27 center-cavity, adding R23 brings that to R50. With cheaper but more environmentally damaging HFC blown foam you'd be at about R24-R25 on the foam, and R47-R48 at center cavity. Stapling PERFORATED radiant barrier onto the to the exposed attic side of the 2x2s to keep the batts in place would add another R1 of performance without creating a moisture trap or fire hazard. R19-R21 fiberglass could be used too, but it's less fire resistant, and may need a timed thermal barrier such as half-inch drywall for the inside air barrier. (Paper facers on batts should never be left exposed.)

    If you prefer to install the foam first, and DIY the batts later at your leisure that can still work, but leave yourself at least 1.5-2" of rafter to side-nail onto. If that means going less than 4" (unlikely), whatever the R-value of that foam layer is, in US climate zone 5 the fiber R should be no more than 1.5x that of the foam-R (eg: If it needs to be 3" of R6/inch foam at R18, keep the fiber insulation to no more than 1.5 x R18 = R27 to avoid potential moisture issues. So in that example R25 fiberglass or R23 rock wool would be fine, R30 rock wool or fiberglass would be pushing your luck. But if it's 3" of HFO blown foam or R6.5/inch HFC blown foam you'd be at about R20-R21, enough for R30 fiber insulation.)

    If it's 5" of HFO blown foam, R15 batts would get you there, and you'd probably be able to achieve the 3.5" necessary depth with simpler Bonfiglioli strips:

    https://www.finehomebuilding.com/membership/pdf/9750/021250059.pdf

    1. Jon_L | | #6

      Hi Dana,

      Thanks so much for the thorough response. It makes me wonder if I would be better of having the insulation installed between the joists instead and adding batts on top to achieve the desired R-value? I would avoid the thermal bridging issue, correct? I am concerned that going from a vented attic to unvented may create ice dam issues...but with all the conflicting info I have read, I am not even sure that is true. Would the 4-5 inches of closed cell foam in between the rafters be sufficient to prevent ice dams while I get around to adding the rock wool? For what it's worth, the insulation I was quoted on is Demilac...not sure if that makes any difference.

      Thanks again!

      Jon

      1. Expert Member
        Dana Dorsett | | #7

        >"Would the 4-5 inches of closed cell foam in between the rafters be sufficient to prevent ice dams while I get around to adding the rock wool?"

        Prevent ice dams? Absolutely not!

        Reduce the ice damming potential? Absolutely YES!

        With 4-5" of insulation between the joists (even at R10/inch), the 4-5" path through the rafter is only R5-ish, which is an ice-dam starter. Deepening the total insulation thickness to 9.5 would be R10-ish, even if all wood- half the heat loss, half the melt-out potential. Deeping it using some amount of rigid foam makes it even less prone to ice damming.

        At my home in central MA (a snowier, somewhat cooler climate than southern CT) putting R20 of open cell foam between full-dimension 2x6 rafters in the tiny attic spaces behind kneewalls in the built-out 1.5 story made a huge difference in the amount of ice damming, but didn't prevent it completely. When re-roofing in short years I'll be doing a foam-over with R25-ish polyiso to better protect the roof sheathing, which should also pretty much eliminate ice damming except around heat leaks such as skylights (which mi esposa demands we keep.)

        Demilec HFO HighLift is an HFO blown foam that they claim runs R7.5/inch, and can be blown in lifts up to 6.5" in a single pass . Their HeatLok XT is blown with climate damaging HFCs, run about R6.5/inch, and would need to be installed in lifts of 2" per pass with a cooling/curing interval time between passes.

        They're claiming that HeatLok Soy 200 is R7.4 @ 1", and can be installed in 3" lifts (also HFC blown.)

        So 3" of HFO HighLift (R22.5) would be adequate dew point control for R30 rock wool batts (7.25" deep), for a total depth of 10.25".

        And 4" of HFO HighLift (R30) would have even more dew point margin, and would get you to code min with R20-R23 batts (5.5" deep) at 9.5" total depth.

  5. David Williams | | #5

    Because I wanted a whole house attic fan in our new cabin and it looked like the fan (plus the windy location) might end up disturbing blown-in insulation, we opted to go with closed-cell foam. The insulators built up a "false ceiling" between the trusses and the roof deck to allow air movement from the soffit to the ridge vents, then built up the foam over multiple passes to around 9". Definitely more complicated than the original plan, but hopefully if will get the job done.

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