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Climate Zone 6: Low-slope warm roof materials / assembly

MeriMeri | Posted in Energy Efficiency and Durability on

I am designing a large mono-pitch shed roof in Climate Z6. The slope is 1.5 in 12 or about 7 degrees. A ventilation space seems impossible due to the scale of this roof plane. 60 feet sloping up and about 25 feet wide. I would like the finished roofing to be standing seam metal as the roof is visible from various vantage points on the property.

It my understanding that we will need to install a high-temp waterproof membrane like Grace Ultra over the entire roof sheathing to prevents leaks. This means the roof assembly is only able to dry to the inside.

As I see it I have three options.

1. Specify a warm roof with closed cell spray foam to limit the vapor drive to the backside of the sheathing. For instance 10 inches of SPF @ R6/in is an good roof. But this technique is risky if there is a leak as condensation that occurs on the back side of the plywood would not be able to dry out. I’m also not trilled with the idea of using so much SPF. The idea of SIPS have been discussed by the structural engineer.

2. Another option is to use a smart vapor retarder with dens packed cellulose between the rafters. The dens packed cellulose would limit vapor drive but if and when the condensation does occur it would seem that the cellulose acts as a storage medium for the liquid and that eventually it would dry back to the inside, especially when the summer sun heats up the dark colored metal roof. Possibly add rigid insulation above the deck as well?

3. Expose all the rafters as part of the interior aesthetic and then place rigid board above the sheathing. I’m least familiar with this concept and it will result in a really thick assembly.

Does anyone know of a waterproof membrane that is vapor open and approved for use on low slope roofs with metal roofing?

Any advice would be greatly appreciated.

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  1. Expert Member
    Dana Dorsett | | #1

    Go with a variation on #2, with these details in mind:

    In climate zone 6 as long as 50% of the total R is above the roof deck , the roof deck will not have moisture accumulation issues with just standard latex paint as the interior side vapor retarder.

    IRC 2012 code min for zone 6 would be R49, which means if you put R25 above the roof deck you could use R24-ish sprayed cellulose or batts between rafters, as per the IRC prescriptive levels:

    An exterior-foam R25 that works, and actually performs at R25 in a zone-6 winter that isn't super thick would be 2" of either 1.5lb or 2lb polyiso directly above the roof deck IR12-R13) with 3" layer of Type-II (1.5lb) EPS above that (R12.6). In mid winter the 3" of EPS would exceed it's labeled R, but the 2" of polyiso would under-perform by a bit. If the polyiso were 4-5" thick the outer 2" would be cold enough to underperform by a LOT, which is why a dual-foam type is preferable.

    If you are going higher total R you need to keep the ratio of exterior foam-R to interior fiber-R at 50%. For the exterior foam layers, keep the EPS to polyiso labeled-R roughly equal as well, always stacked with the EPS on the top side, directly under your roofing membrane (or whatever slip-surface underlayment it uses.) So at any total R value the basic stackup would be:

    Standing seam


    25% EPS-R

    25% polyiso-R

    Roof deck

    50% cellulose-R

    Semi-permeable air-barrier & vapor retarder ( 1-8 perms)

    Since R25 cellulose is only about 6.5-7" deep, you could use an exposed edge rafter look on the interior. You'd need to set it up to put laminated wood paneling or sheet rock as an interior air barrier for the cellulose, but that's not too tough. If you nailed in some 2x2 ledgers for attaching air barrier ahead of time and staple the blowing mesh to the ledgers you can then use a roller to flatten it out the pillowed mesh before installing the air barrier. Most laminated wood would already have sufficiently low permeance to be protective, but if you use sheet rock it needs standard latex paint to get it down to ~5 perms or so, which is fine.

    To do #3 would involve 10" of exterior foam, which gets to be a bit unwieldy.

    Both EPS and polyiso are environmentally benign compared to closed cell polyurethane or XPS, since they are blown with pentane, which is a fairly low-impact blowing agent.

  2. GBA Editor
    Martin Holladay | | #2

    Q. "Does anyone know of a waterproof membrane that is vapor open and approved for use on low slope roofs with metal roofing?"

    A. A vapor-open membrane (roofing underlayment) won't help, because your standing-seam metal roofing prevents any drying to the exterior in any case.

    Dana's advice is good. For more information on insulating this type of roof, see Insulating Low-Slope Residential Roofs.

  3. MeriMeri | | #3

    Dana, thank you for these insights regarding the dual type rigid board ratio and the underperformance of poly-iso. I'll be sure to keep the 50/50 rule in mind.

    Martin - of course you're right about the metal roof! and I've been looking at this article a lot lately it's very helpful

    In order to fasten the standing seam roof I think I'll need another plywood layer above the top layer of EPS with a product like Grace Ice and Water Shield HT. Is there any concern about installing EPS in a high-temp roof assembly.

    The Contractor is interested in using SIPS panels. I'm concerned about condensation affecting the top layer of OSB. since this roof assembly cannot dry to the outside. On the flip side it would be easy to air/vapor seal from the interior if there are no peentrations.

  4. Expert Member
    Dana Dorsett | | #4

    They use EPS under torch-down or hot-mopped tarred roofs all the time- don't sweat that one!

    The service temperature for long-term exposure for most EPS is about 165-170F, but even limited excursions to 180F are fine, if limited in duration & frequency. If you use a jet-black finish to the metal roof you might occasionally hit those temps, but a medium brown or even a dark green would probably still stay within spec.

    If you make it bitumen-black you'd almost certainly hit the 180F mark occasionally.

    The lightest color that doesn't look crappy, or a darker shade of "cool roof" finish that has low absorption in the infra-red portion of the solar spectrum (which is about half the incident solar radiation ) at a solar reflective index (SRI) north of 25 would be a good idea for a roof that low-angle, even in US climate zone 6. Berridge Hartford Green or Dark Bronze or Charcoal Grey all look pretty dark, but all have an SRI of 30:

  5. MeriMeri | | #5

    Follow-up question regarding the 50/50 ratio of air-impermeable rigid board above the deck to air-permeable fiber insulation on underside of the deck. (foam-R to fiber-R)

    I read about this way to calculate how to keep the roof sheathing above 45deg when the interior is at 68degrees.

    In my town, the average min. outdoor temperature for the three coldest months (Dec, Jan, Feb) is 22°F. The required exterior insulation is then 22°F/68°F = .323 or roughly 32% of the total installed R-value.

    Total Required R is 49. So at 32% R16-Foam and R33-fiber. But this doesn't meet the code min. of R25-Foam.

    So here's the question. I want a higher R-value Roof than R49. Let's say R60. If I keep the code min. of R25-foam this is 41% exterior and 58% R-fiber. Does this raise any issues? Can the fiber insulation be dens-packed cellulose or is another density of cellulose preferred?

  6. GBA Editor
    Martin Holladay | | #6

    Here is a link to an article that explains the calculation method used to answer your question: Are Dew-Point Calculations Really Necessary?

  7. MeriMeri | | #7

    This article is perfect to follow - thank you

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