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Cool roof design for hot climate

DennisGray | Posted in General Questions on

Working on design for new build in hot and humid CZ-3 (a few miles from being CZ-2). Considering the following roof (outside to inside layers): Composite shingles, roofing underlayment, LP Tech Shield foil coated OSB (foil side down), a 3/4″ vented air gap between 1×3 furring strips (running up roof pitch), Zip roofing panels, and R-40 open cell spray foam between rafters or trusses above conditioned attic. Will have venting from under eave soffits through the air gap up to ridge vents on a 9:12 pitch roof.

My thinking is the foil radiant barrier will greatly reduce radiant heat to the Zip panel roof sheathing (by over 90%) and the heat conduction off of the upper layers will largely be vented out by convection in the air gap. As attics in this area can get up to 150F in summer I am thinking this would be worthwhile approach to reducing summer heat gain. Also, reducing the roof deck temperature should result in longer life of roofing material (some roofing manufacturer warranties require under roof venting).

Any experience with this approach? Is 3/4″ enough air gap? Problems I might have? Too expensive? Appreciate any comments.

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Replies

  1. Expert Member
    Armando Cobo | | #1

    I think you have a great opportunity to build a conditioned attic with a Vapor Diffusion Port. There are good articles here on the GBA and Building Science Corporation.

    1. DennisGray | | #2

      Not sure that approach gets me anywhere. As I understand that would apply if I have a structure with thick air permeable insulation. If the roof is supported by a truss some additional elements would have to be added to support the insulation up to the underside of the roof deck. With spray foam no such structure is needed. Seems to me the vapor diffusion port is for vaulted ceiling structures with deep rafters and best if ceiling is at the bottom of rafters. If foam no air movement and no need for diffusion port. Or am I missing something here? If using trusses but still wanting conditioned attic space I don't that see that working.

  2. Expert Member
    Armando Cobo | | #3

    1. You mentioned R40 ocSPF against the roof decking between rafters OR trusses… that’s against code. You need to install at least 2” ccSPF against the roof decking and then 8” ocSPF, plus installing TechShield AND Zip sheathing can get expensive, but it is a good system, especially if using trusses. I do think TechShield needs 1.5" space for radiant barrier to be effective. Check their website.
    2. A VDP is allow by code with permeable insulation like cellulose, fiberglass or Rockwool. Depending on the span, you can use 2x12s or 11 7/8 TJI. My detail uses 14 TJI because we used the 2021 Code. This is much easier to build and less costs. A big plus, you can install your HVAC in the attic. 2 for 1, and if you ad storage, is 3 for 1.
    You can see in my detail that this is a full attic, not a vaulted ceiling, but that would be easy to do as well. My intent was to give you options.

    1. DennisGray | | #4

      Thanks much for your thoughtful reply.
      I could not find code reference regarding the R40 ocSPF. I am under 2018 IRC. Where should I be looking? (maybe in 2021 IRC?) The described mix of layers does start to sound price alarms.
      The only thing I could see in TechShield literature is 3/4" air gap, but that was above insulation, not a hard surface.
      My span for vaulted area is 33' (first floor great room/kitchen). I was making assumption that scissor truss was the way to go, but maybe TJIs would make sense. I could lower pitch from 9:12 if TJIs and still end up with more ceiling height than with trusses, but the roof beyond is 9:12 and not sure the pitch mixing would be aesthetically pleasing. You think I would see cost savings with scissor trusses vs TJIs?
      For sure wanting a conditioned attic - for HVAC and storage. House is on slab so that stuff has to find a place inside and upstairs.

  3. Expert Member
    Armando Cobo | | #5

    That code was at least in use since the 2009 IECC R806.4. Using scissor trusses still an option, but you would need to install 1" R5 rigid foam on top of the roof decking and 10" R35 ocSPF under the roof decking minimum.
    In this assembly, I would typically use 1.5" R10 iso rigid foam on top of the roof decking and 8" R28 ocSPF under the roof decking, so I can use a 2x4 nailer around the roof deck perimeter to attach the second roof sheathing if using shingles or battens if using metal or tile roof.
    You can still run ducts in there, but you couldn't install the furnace nor use it for storage. Another option is attic trusses.
    Just options and trade-offs.

  4. Expert Member
    Armando Cobo | | #6

    My brain couldn't stop thinking about a compromise between a scissor truss and making room for the HVAC system. So, if you could talk to your truss designer to design you a truss similar to this... Now you'll have a higher ceiling in the first floor great room/kitchen, but also have room for the HVAC system. Just a fun challenge 🤪

  5. DennisGray | | #7

    Looking at 2018 IRC R806.5 it sounds as if you are going on the assumption that ocSPF is an air-permeable insulation requiring the above deck insulation board (5.1.2) or ccSPF below deck (5.1.3). Is that right?
    Oh wow, you have revealed something new to me regarding open cell spray foam. I have heard for years that ocSPF, while vapor permeable is air-impermeable and an effective air barrier. Now I read some places that ocSPF allows sufficient air flow to exclude it from being considered an air barrier. Code says to be air-impermeable must be </= 0.02 (L/s)/m @ 75 Pa. tested to ASTM E2178 or E283, while Johns Manville (example) publishes that they meet <0.02 but then say that value is by testing to ASTM E90. Although I have seen claims on both sides, I still have the new question; is ocSPF considered air permeable or air impermeable by IRC?
    Just a note, as there exists an entire industry that manufactures open cell foam air filters for all sorts of applications, I have to question claim of the material being an air barrier. Always just figured they were talking about different materials.
    I like your sketch of the scissor/attic truss - looks cool. To clarify, on our house we have two different roof areas; a vaulted truss ceiling in great room, and then a 1 -1/2 floor area for remainder of house with normal rafters and ceilings giving ample conditioned attic areas outside of the occupied spaces. We don't need any usable space above the vaulted ceiling (except for ductwork).

  6. DennisGray | | #8

    Well, now I find another spec sheet from Johns Manville stating for the ocSPF that it tests at <0.02 (L/s)/m @ 75 Pa. tested to ASTM E283. The one stating E90 testing was for "Corbond" ocSPF. If this is case, only ocSPF would be required under the deck by 2018 IRC R806.5-5.1.1 as it would be considered an air-impermeable insulation. No ccSPF or above deck insulation board would be required. Or am I missing something?

  7. Kyle Bentley | | #9

    I'll just throw this out there -

    a 2x3 has the same amount of wood in it as a 1x4. They're about the same price, but the 2x3 will give you a larger vent area, be stronger in the direction of loading, and create a smaller bridge footprint between the two surfaces. Bonus is that you can nail into it with standard 2 +3/8 nails without worry.

    1. DennisGray | | #10

      Good point on the nailing. The air flow would be much improved with twice the vent area (1.5" versus 3/4" gap). A bit concerned on roof structure as within wind zone 2 (up to 150 mph) - don't want the roof deck flying away.

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