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Designing an Unvented Roof

planesandteeth | Posted in General Questions on

My “unvented” head has too much info to configure this roof!

I’m going down the rabbit hole and/or getting too far into the weeds about the unvented roof I’m trying to plan.  There’s so much information in the Q&A section; some old, some new, some science, some “best practices” and I think I’ve gotten too much information swimming in my head I can’t sort.  Can someone please point me in a better direction because my architect needs some better guidance than me giving him 9 different “working” examples I’ve found.  I think I need help with some details after a hundred hours of reading…

My scenario:

·      Climate 4a (Middle TN).  Desiring high performance roof.

·      Single ridge, 6:12 roof with standing seam metal roofing.  Think large simple design like a rectangular ranch/bardominium style house.  Simple design and engineering, but large roof sq footage.

·      Roughly 2/3rd of the house (rooms split equally between either ends of the rectangle) is cathedral ceiling with the remaining (center section) 1/3 a large, conditioned attic that’ll contain the HVAC and ERV.

My latest idea:

·      Entire roof unvented.  Overhangs to meet some* passive standards.  R- 54 total.  From the conditioned side going outwards:

o   Flash and batt (2 inches of closed cell spray foam under decking) and as much fiberglass as the rafter bay will allow.  I’m thinking R-19 batt, R-14 spray foam (R-33 total interior)

o   Traditional plywood (or OSB if Hunter will allow it their panel to be screwed to it) deck nailed to the top of the trusses and/or rafters.

o   4” of exterior polyiso insulation on top of the decking in the form of a Hunter panel (H-Shield NB). R-21 is what they report.  Taped seams.  Their H-Shield NB panel is OSB topped.

o   Synthetic underlayment

o   Standing seam metal roof attached directly on top of the synthetic underlayment screwed into the OSB Hunter panel.

My questions I’m turned around trying to solve:

·      Is the first layer of decking (the one directly on the trusses) going to have issues being sandwiched between closed cell foams (spray and polyiso)?  Should I be concerned with which way the roof dries, which admittedly is confusing?  Doesn’t seem like the truss sheathing will dry any direction.  Should I abandon the flash and batt here?  Is this what ICC R806.5 is talking about with the class 1 vapor retarder?

·      Will the polyiso have so much thermal drift I’ll lose the roughly 31% delta recommended exterior R value that Martin Holladay wrote in his articles?  Also, ICC R-15 min for CZ4.  Thermal drift has been a vague research topic, and aggressively negative if you manufacture EPS…

·      Do I need underlayment on the first layer of sheathing, or just the layer directly under the metal roof?

I’m thinking of the roof this way to try to somewhat limit complexity for the labor, who seem to be throwing up cheap houses to take advantage of the hot real estate market.  Faster and cheaper seems to be the builder mantra in Mid TN and my confidence is waning in finding a builder with A+ crews.  I think most crews could accomplish this idea, especially a decent roofing crew who would finish everything after the first layer of sheathing.  I thought about doing the entire roof with SIPs as there is a manufacturer locally, then I saw the cost…  I can essentially fabricate the SIP on site using conventional tools, lumber, and labor with Hunter H-Shield NB panels for about half the cost (not as much R-value, but still).  Please correct me if I’m wandering off the rails here.  Thanks in advance.

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  1. Expert Member
    ARMANDO COBO | | #1

    Since you are all over the place, I propose a good and cost effective design for your unvented roof assembly, especially since you want metal roof, and applying 2021 IECC (why go to an older code). FYI, 2021 IRC/IECC insulation in CZ4A is R60 min.

    Roof framing (trusses/lumber/I-joist), taped plywood/OSB sheathing, 3” R15 min rigid foam (see R806.5) to match 2-2x4 perimeter nailers, underlayment, 1x4 battens, and metal roof. If you use trusses, I would use 13” R45 min ocSPF. If you use 2x12s R40 or 14” I-joist (better), then you can use dense-packed cellulose R49 (better).

    Hunter roof panels with rigid foam need a structural sheathing/substrate attached to the framing members. Why spend money on two sheathings.

    1. brendanalbano | | #3

      Reading your comment, I realized that the previous version of the IRC and the new version appear to have slightly different guidance regarding the ratios of continuous insulation to cavity insulation.

      Table R806.5 Insulation for Condensation Control in both IRC 2018 and IRC 2021 recommend R-15 for CZ4A.

      But table N1102.1.3 2018 requires R-49 while 2021 requires R-60.

      IRC 2018: 15/49 = 0.31
      IRC 2021: 15/60 = 0.25

      I'm not sure if this difference matters in practice, but it is interesting to note!

  2. brendanalbano | | #2

    I think you are over complicating things with using both exterior continuous insulation (the hunter panels) and closed-cell sprayfoam.

    Using R-60 as an example, and assuming the ratio between CI/ccSPF and total insulation you want is 30%, these two approaches to insulation in an unvented roof seem more straightforward to me:

    ccSPF approach:

    R-18 Closed-Cell Spray Foam below the roof sheathing.
    R-42 Additional insulation (cellulose, fiberglass, mineral wool, or open-cell spray foam)

    Continuous insulation approach (Armando's reply has a lot of great details for this approach):

    R-18 Rigid insulation above the roof sheathing.
    R-42 Additional insulation below the roof deck (cellulose, fiberglass, mineral wool, or open-cell spray foam)

    There are some good reasons to avoid using closed-cell spray foam, although the availability of the new HFO-blown foams does make it a little more reasonable from an environmental standpoint, even if it's not the best. There are horror stories about lingering smells that are worth reading before deciding to use it. The big advantage of closed-cell spray foam is that if your contractor is not comfortable with installing a thick layer of rigid insulation above your roof sheathing, they are much more likely to be comfortable with a closed-cell spray foam approach.

  3. planesandteeth | | #4

    Thanks Armando. Yeah, looking at old post I was using the old info. Im glad I posted then! I'll go to the 2021 IECC and take a look. I need far more R value, clearly. So with the 1x4 battens you are suggesting a vent between the rigid foam and metal roof? I was trying to stick with "stupid simple" for the labor and thats why I figured they could instal a hunter panel because it just acts like a thick sheet of sheathing. I think almost anybody could do a decent job doing that instal. Maybe more expensive but more difficult to screw up. Rigid foam with battens over the top though? Not so sure. Have you been to TN and seen the build quality? ha. I will explore further none the less. Thanks

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