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Community and Q&A

Does this roof assembly work?

skyfarm | Posted in Energy Efficiency and Durability on

I am building a house in upstate NY. I am about to finish framing and have begun to doubt my unvented roof design. Specifically whether I need to add a path for drying the layer of plywood that will be beneath my metal roofing. This is the sequence from the bottom up: 2 x12 rafters filled with cellulose, layer of 5/8″ ply, 3″ of polyiso foam board, 5/8″ ply completely covered with ice and water membrane, metal roofing. It is a pitch of 1″ per foot.
Should there be an air space between the foam and the ply above? My air barrier was to be the ice and water membrane!

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  1. Expert Member
    Dana Dorsett | | #1

    The more commonly used & reliable stackup puts the ice & water membrane between the structura roof deck plywood at the rafters & cellulose, not the nailer above the foam. Ice & water shield is not an appropriate underlayment for the metal roofing, and it's putting a vapor barrier at the exterior, which is poor placement in any NY climate. Metal roofing needs an appropriate slip-surface underlayment to manage both moisture and expansion/contraction. The underlayment should be semi-permeable, and wicking to provide at least some drying, or better yet, mount the metal roofing on 2x purlins, gapped to allow cross ventilation.

    At your total-R you are a bit shy on foam-R for dew point control at the roof deck at most upstate locations. At R50 total center-cavity R you need at least R20 (40%) as exterior foam for climate zone 5 locations, and R24 (48%) as foam for R for zone 6 locations. But you're at around R60 (R41-42 cellulose + R18-19 of iso), with only 30% of the R as exterior foam. That's fine in zone-5 (parts of Westchester county), but most of upstate NY is either zone 5 or 6. To bump it up to the proper ratio you'd need 5" of iso for a climate zone 5 location 6.5" for a zone 6 location.

    If that's too much exterior foam to manage, you can either use a smart vapor retarder like Certainteed MemBrain between the gypsum & cellulose to limit wintertime moisture diffusion, or shoot a flash-inch of closed cell foam on the underside of the roof deck prior to insulating with cellulose. That places a semi-permeable non-wicking condensing surface between the wood and foam, and brings the ratio between foam & fiber into full compliance for zone 5, but still wouldn't be a problem in zone 6 (despite stepping slightly outside the letter of the IRC prescriptive ratios). The reason it works well despite IRC non-compliance is that it's nearly a class-II vapor retarder, and the cellulose wicks & redistributes any moisture that forms during those brief condensing temp periods without damage or even temporary loss of function. At about a buck a square foot the flash-inch of closed cell is usually cheaper than adding 2" of exterior foam, and it goes a long way toward air-sealing the roof deck, but quote it both ways.

  2. GBA Editor
    Martin Holladay | | #2

    Here is an article that you may want to read: How to Build an Insulated Cathedral Ceiling.

    In your climate zone (Zone 6), the minimum R-value of the rigid foam above the roof sheathing is R-25, so 3 inches of polyiso doesn't quite make it. You'll need at least 4 inches.

    I would skip the Ice & Water Shield. Instead, just install #15 or #30 asphalt felt as an underlayment. That way, your upper layer of roof sheathing will be able to dry to the exterior somewhat (especially if you install the metal roofing on 1x4 or 2x4 purlins over the underlayment). If you want a tight air barrier, you can either tape one of your polyiso layers, or you can tape the plywood roof sheathing.

  3. homedesign | | #3

    One Inch in 12 inch slope.... hmmm

  4. Expert Member
    Dana Dorsett | | #4

    Martin- only half of upstate NY is zone 6- most of the population (Westchester-Hudson Valley, including Albany,& the Rochester/Syracuse/Buffalo) lives in zone 5.

    The prescriptive R24 min for zone 6 or R20 for zone 5 in the IRC is based on a presumed total center cavity value of R49- it needs at least a hair more than R4.5" to cut it at his total-R even for zone 5. With the lower-when cold derating of iso that needs to be rounded up to 5" to have any margin at all.

    But an inch of ccSPF on the underside of the deck cures all.

  5. GBA Editor
    Martin Holladay | | #5

    I wasn't paying attention to the pitch -- 1 in 12 isn't much. I guess that explains the Ice & Water Shield.

  6. homedesign | | #6

    hmmm.. I may be wrong... my understanding is that WR Grace Co and the Building Code will not allow anything less than 2 in 12???

  7. skyfarm | | #7

    so....first- ice and water on top of roof deck plywood, then 4" of polyiso, then 1/2" ply, 30# felt, 1x3 purlins,metal roofing? Does the roofing get enough support with a purlin every foot? One could walk on that? What about creating an airspace between the polyiso and the upper ply with 1x4 then plywood,felt, roofing?

  8. homedesign | | #8

    My question
    Is a metal roof a code approved option with less than a 2 in 12 pitch.....??
    My understanding is "No"

  9. GBA Editor
    Martin Holladay | | #9
  10. homedesign | | #10

    thanks Martin , I was not sure
    I may have "learned" wrong from a local contractor

  11. skyfarm | | #11

    ICC says minimum slope for standing seam metal roof is 1/4 in 12.

  12. skyfarm | | #12

    Grace does require 2 in 12 for their ice and water shield. Thanks for all the responses, it is appreciated! Any further thoughts on placing the airspace below final plywood layer?

  13. Expert Member
    Dana Dorsett | | #13

    I still contend that 4" of exterior iso is not enough exterior R when you have ~R40 cavity fill, since that's R64 for total R, not R49 as presumed by the code. The dew point isn't magically lowered by having R64 rather than R49- it's the ratio of foam-R above to the fiber-R below that determines the average temp at the roof deck.

    R24 would meet code in zone 6 at R49-total, but would fail to protect in zone 5 at R64 total.

    Got a ZIP code, so we can tell which climate zone you're really in?

  14. skyfarm | | #14

    Thanks Dana. Zip here is 12546. Millerton. High side of Hudson valley near Mass / Conn line. I have a hard time knowing what r-value to give for the polyiso. It is factory seconds with paper backing.

  15. Expert Member
    Dana Dorsett | | #15

    In Millerton NY the January mean binned hourly temp runs about 25F- it's a zone 5 location, but the warmer edge, definitely not zone 4 (but you're not far away), and definitely not zone 6. See:!dashboard;a=NY/Millerton

    That means the average mid-winter temp through the outer third or so of the R (the foam) is going to be in the low 40sF in your app. Derating 2lb roofing iso for 40F on it's temperature curve would put it around R5.7-5.8/inch, aged value for most goods out there, and not lower than about R5.5/inch for any density/manufacturer/age.

    So, figure 4" at R22-R23.

    The magic ratio of foam to fiber you're looking for to meet IRC expectations anywhere in zone 5 is R20/(R49-R20), or about R2 foam for every R3 of fiber. At 11.25" (3.5lb cellulose will run about R40-R42. That means ideally you'd want the foam to be 2/3 x R40, to 2/3 x R42 which is R27-R28.

    Bumping it to 5" and assuming the worst case R5.5/inch would put you at R27.5 so it makes it, even fully derated. If you double-layer the 3" goods it's a slam-dunk, with huge margin.

    At 4" even if you assumed against all reason that it was R6.5/inch forever you'd still be a bit shy of what's needed, though the moisture buffering of the cellulose might actually make a derated R23 work if dense-packed to 3.5lbs or higher, but there would still some small long-term risk to the north facing roof deck.

    At 3" yer on yer own- wouldn't meet code any way you slice it, but even there with plywood rather than OSB as the structural deck, and a darker shade of roofing for higher solar gain & higher average deck temps, and 4lb cellulose for a buffer, it might be just fine. Or not...

  16. skyfarm | | #16

    5" it is. Or will be! Excellent advice.Thanks Dana! Kudos to all responsible for this site, it is very helpful!

  17. [email protected] | | #17

    There are many types of standing seam roofing. Each has their own slope requirements.

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