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Conditioned attic conundrum

Steve Lawson | Posted in Green Building Techniques on

Conditioned Attic Conundrum

I am building a house in Climate Zone 4A (in a storm-prone area) with a conditioned crawl insulated with Roxul Drainboard and 2×6 walls of 1″ Zip-R that I plan to flash with closed cell foam and back with blown fiberglass. Unfortunately, the roof insulation system has me scratching my head and second guessing myself at every turn. I have read all of the articles on this site and elsewhere, and have designed the system accordingly – but am seeking a bit of additional advice.

What I have decided for sure is that it will be a conditioned/unvented attic with a 9/12 pitch standing seam metal roof over ¾” plywood. I have chosen to make the attic conditioned since I will have HVAC equipment and ductwork up there that I cannot relocate. There will be no combustion appliances in the attic.

My initial thought was to spray closed cell foam on the underside of the roof deck, furr the 2×10 rafters down with 2×2’s, and then fill that with blown fiberglass to meet the required R-38. I have, however, always been concerned about the possibility spray foam hiding roof leaks until a catastrophic failure and have also read a couple of articles about failures of roof decks with open cell foam due to vapor migration and condensation. While closed cell foam seems to be the better choice, I still have concerns that roof leaks will be hidden (I am in a storm prone where wind-driven rain often finds its way into every nook and cranny). In my search for another alternative, I have been reading in this forum about using foam board on top of the roof sheathing and have approached my roofing contractor with that idea. He likes the idea and has done exactly that on countless commercial jobs, but he would prefer to put another layer of plywood over the foam to have a good substrate for the metal panels. Assuming that does not create any moisture problems, I would still need to air seal the attic to complete the insulation system. To that end I have investigated the Knauf EcoSeal spray caulk and think it could provide the air seal I need for this assembly. My plan would be to install the unvented metal standing seam roof over ½” plywood over 2½” of polyiso foam board on top of ¾” plywood (2½” of polyiso is the minimum required on a roof in my climate zone per the excellent article on this site). I would then install blocking of some sort between the rafters at the top plate so that I could seal it by spraying EcoSeal caulk in the corners and then in all corners and gaps on the underside of the rest of the roof sheathing to create the required air sealing. Finally I would install blown fiberglass over netting between the rafters. So the assembly would look like this:

– Standing seam metal roof with no venting
– High temperature ice and water shield roof underlayment (roofer has spec’d Tamko TW Metal and Tile Underlayment, which is rated at 0.05 perms)
– ½” plywood
– 2½” polyiso foam board
– ¾” plywood
– Knauf EcoSeal air sealing spray caulk
– 9¼” blown fiberglass

I calculate this system to have an R-Value of 16 for 2½” polyiso foam + 32.375 for the 9¼” of blown fiberglass = 48.375, which is well in excess of code, not to mention I have short-circuited the thermal bridging of the rafters. If I do experience a roof leak, then the water should make its way through the blown fiberglass to a place where you would see it, find the source, and fix it.

If anyone would be so kind as to weigh in, my specific questions are:

1. Does the Knauf EcoSeal actually seal well enough for this purpose, or is spray foam the only way to get a truly tight seal? I will be certifying the house to EarthCraft standards, which requires an envelope leakage ratio < 0.50 or 7 ACH50, whichever is more stringent. 2. Am I going to trap moisture and/or create a condensing surface in the plywood and foam sandwich assembly? My concern is the ice and water shield under the metal roof will be a water and vapor barrier, and that the Knauf EcoSeal may create a second vapor barrier that could trap moisture. Would the ¾” plywood be permeable enough, however, to allow drying to the interior? Many thanks, Steve

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Replies

  1. D Dorsett | | #1

    There's no point to flash-foaming a 2x6 ZIP-R wall with a shot of closed cell foam. The ZIP-R already has more than adequate R-value outside of the fiber insulation for dew point control at the foam/fiber interface, and ZIP-R is designed to be dead-easy to air seal from the exterior. The thermal improvement of the 1" of closed cell foam over an R21 fiberglass or R23 rock wool solution is negligible, less than R0.25 from a whole-wall performance perspective. Sealing the framing to the ZIP-R with caulk in every stud bay would be more than adequate, and far cheaper (and greener) than an inch of closed cell foam.

    Your roof stackup makes it for your climate, and pretty common in my area (though with 4" of polyiso here in zone 5A, due to the cooler climate and higher total R requirement.) The IRC spells out R15 out of R49 for zone 4A, which or 30% of the total R to be on the exterior for dew point control at the structural roof deck. Derating the polyiso to R5.5/inch for the climate & location within the stackup means you're only at about R14, and and a total R of . If you're putting R32 in the rafter bays, with a derated R14 on the exterior that's R46 total, so the derated R14 JUST makes it to 30%. But 3" polyiso would be better since it gives you a bit of dew point margin, and would bring you up to the IRC code-min.

    http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_11_sec002.htm

    http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_8_sec006.htm

  2. GBA Editor
    Martin Holladay | | #2

    Steve,
    Q. "Does the Knauf EcoSeal actually seal well enough for this purpose, or is spray foam the only way to get a truly tight seal?"

    A. It's not the product that results in a tight house; it's the attention to detail of the installer. Knauf EcoSeal can result in a tight envelope, and so can spray foam, but (a) using these products does not guarantee that all leaks have been located and sealed, and (b) plenty of tight homes don't use Knauf EcoSeal or spray foam.

    Your installer needs to anticipate air leaks at each point where dissimilar materials have seams, and needs to have a plan to seal those leaks. If your installer isn't thinking this way, it doesn't matter what products you use. In many case, a high quality tape and good caulk are all you need.

    Q. "Am I going to trap moisture and/or create a condensing surface in the plywood and foam sandwich assembly?"

    A. No.

    Q. "My concern is the ice and water shield under the metal roof will be a water and vapor barrier."

    A. It will.

    Q. "... and that the Knauf EcoSeal may create a second vapor barrier that could trap moisture."

    A. I couldn't determine whether Knauf EcoSeal is vapor-permeable or vapor-impermeable, but the product's permeance hardly matters. It is used in small beads like caulk, so it covers a very small percentage of the area you are concerned about.

  3. Steve Lawson | | #3

    D,

    Thanks for the advice against the CC flash layer, the polyiso derating insight, and the code references - all very helpful!

    Martin,

    Thanks for the concise answers! And I could not agree more that the installation is a huge driver of performance. One option under the metal roof is a 40 mil peel and stick underlayment, which should be a very effective air barrier if installed correctly - but does anyone have any experience with this application? I would still need to create an air barrier at the soffit, but assuming I do that, would peel and stick on top of roof take care of the rest? Or would I still need to use the Knauf EcoSeal?

    Thanks,
    Steve

  4. GBA Editor
    Martin Holladay | | #4

    Steve,
    Either the plywood roof sheathing or the rigid foam layer (or both) can be detailed as an air barrier if you want. I would probably just tape the plywood seams if I were you. For more information on this approach, see Airtight Wall and Roof Sheathing.

    For more information on selecting the right air-sealing tape, see Return to the Backyard Tape Test.

  5. Steve Lawson | | #5

    Thanks Martin, This is extremely helpful!
    -Steve

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