GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Unvented roof need r38 in 7 inch rafters

Johnny83 | Posted in General Questions on

Hello! Need advice, I have 7 inch total rafters in a unvented garage roof which is under conversion to bedroom, best way safest way to hit r38 in the space, climate zone 5, thinking a mix of spray foam and batts, do I need a vapor barrier?, was thinking of covering whole ceiling with 1/2  inch xps as well, bad idea to sandwich spray/batt/xps? Grooved pine will be ceiling instead of drywall.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.

Replies

  1. Brad Murphy | | #1

    Hi Johnny,

    Having had a similar challenge, what I did after getting solid advice here at GBA from the building science experts was as follows.

    1 - Please note that I am in zone 4A, so your best scenario might look slightly different

    2 - To deepen my 7.25" rafter cavity I furred out the bottom of the rafters with 1" of EPS Foam, and then 5/8" of baltic plywood... I did this for two reasons: 1st, it bumped me to almost 9" of depth. 2nd it reduced thermal bridging which is CONSIDERABLE in such a shallow depth cathedral ceiling.

    3 - I then sprayed 2" of CLOSED Cell foam on the underside of the deck with allows some VERY MODEST drying to the inside, while also acting as a complete air seal.. which is critical to reducing risk of rot from vapor in the air.

    4- I then sprayed another 6" of OPEN Cell foam on top of the 2" closed cell foam.

    5 - With this combo I achieved around R35 in the cavities .

    6 - I finished the underside with wood, T&G nailed to the rafters... In hindsight I wish I had installed an Intello membrane to help better manage the wet/dry cycle from humidity... but I did use a slightly "open" roof underlayment (20 perms) so that the roof deck can dry to the outside from moisture driven down from the top... and I think my 2" Closed Cell / 6" Open Cell gives me decent drying to the inside capability.

    Dana and Martin are the two most active and knowledgeable experts here on the forum and they each helped me arrive at the above design. Because you are in Zone 5, you might need more drying to the interior potential than I have... but I'll let those guys be the experts, not me.

  2. Expert Member
    Dana Dorsett | | #2

    A vapor barrier is neither necessary nor desirable here. As long as more than 40% of the total R is closed cell spray foam on the roof deck latex paint is good enough as vapor retarder.

    Using an HFO blown closed cell foam you get about R7/inch, so you could get the full R38 in 5.5", not that I'm recommending it.

    I'll assume that you have 2x8 rafters, which are 7.25", not 7". With 3" of HFO blown foam you'd be at R21 leaving 4.25" or the remaining R17. An R21 fiberglass batt compressed to 4.25" would hit pretty much right on the money at R17. See:

    https://s3.amazonaws.com/greenbuildingadvisor.s3.tauntoncloud.com/app/uploads/2018/08/08062722/Compressing%20fiberglass_2-700x310.jpg

    With R21/R38= 55% of the total R as closed cell foam there is huge dew point margin at the fiberglass/foam boundary.

    If an inspector argue the points with a compressed R21, go with R23 rock wool instead. The R/inch of R23 rock wool is R4.2/inch @ 5.5 " and increases with compression to a higher density. At 4.25" x R4.2/inch even with no uprating for higher density it would be at R18, and the center-cavity R would be R39.

    Unfaced half-inch XPS is about as vapor retardent as a couple coats of standard latex paint- it's not a significant impediment to drying, but it's also not buying you anything.

    If you wanted to get more performance out of the batts, make edge strips for the framing out of half-inch or 3/4" foil faced polyiso, giving the R21s or R23s more loft, bringing it up to R19-R20, and put an R3-R5 thermal break on the rafters. Even if the edge strips were 1.25" thick, enough to get the full R-value out of the batts there still ample dew point margin at the foam/fiber boundary.

  3. Brad Murphy | | #3

    Dana,

    Is 40% on the roof deck requirement specific to Zone 5 only?

    1. Expert Member
      Dana Dorsett | | #4

      Yes.

      If you take the IRC's condensation control prescriptives in Table R806.5 combined with the total R prescribed in Table R402.1.2 it's actually 41% for zone 5. (R20 out of a total R49).

  4. Hugh Weisman | | #5

    Somewhat similar situation bun new construction...Thinking about 2x10 rafters @24" O.C. with 1x3 strapping on the interior. Insulation would be 3" closed cell foam (R20) under sheathing followed by 7"" (R26) of open cell foam. Then a GypBd ceiling.. Strapping is pretty standard on Martha's Vineyard for ceilings and provides an extra 3/4" for insulation, but more importantly reduced thermal bridging significantly with bridging only where rafters and strapping cross. Total R value of insulation is only R36 with R49 required for Zone 5, but Ekotrope's R value calculator for roofs indicates that the OVERALL R value of the assembly is about 40.5. I believe this meets the U-value alternative for an assembly with a U-factor equal or less than Table R402.1.4 of the 2018 ICC code, 0.026 for Zone 5.... An overall R40.5 for the assembly results in a U-factor of .0247....Is there a less expensive alternative for the 7' of open cell foam that still provides a total of R26?

    1. Expert Member
      Dana Dorsett | | #6

      >"Thinking about 2x10 rafters @24" O.C. with 1x3 strapping on the interior. Insulation would be 3" closed cell foam (R20) under sheathing followed by 7"" (R26) of open cell foam. Then a GypBd ceiling.

      Total R value of insulation is only R36 with R49 required for Zone 5... "

      Call me old fashioned, but when I add R20 to R26 I come up with R46, not R36. Works the same when I count out 20 pebbles in one bucket, 26 into a second bucket, then dump them both into a third bucket and count the rocks in the third bucket. ;-)

      >"...but Ekotrope's R value calculator for roofs indicates that the OVERALL R value of the assembly is about 40.5. I believe this meets the U-value alternative for an assembly with a U-factor equal or less than Table R402.1.4 of the 2018 ICC code, 0.026 for Zone 5 ...."

      That math works, assuming Ekotrope's calculator isn't fudging anything.

  5. Hugh Weisman | | #7

    Hmm 2+2=3? Me bad...'C' in 3rd grade in math but 800 on the Math SAT'S...It seems to me that the calculator must be pretty accurate...2's at 24" have a .0625 actual framing ratio,....the calculator is pretty conservative to account for doubled rafters, etc and uses .11 as the 24" o.c. framing factor...and I tried using the calculator for a prescription standard roof with r49 insulation and framing at 16". That resulted in an assembly R value of R37.6, marginally less efficient but close, and a U factor of about 0.0266.
    https://ekotrope.com/r-value-calculator-roof/

    My gut tells me that R46 of insulation plus another R1 or so for sheathing, shingles, and gypbd, shouldn't be reduced by too significant an amount by the .11 framing factor (with it'a own insulating value of about R10). It looks like the calculator reduces the total R value of R47+ by 12 to 14% for the whole assembly.

Log in or create an account to post an answer.

Community

Recent Questions and Replies

  • |
  • |
  • |
  • |