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Can this assembly work for a flat unvented wood roof in Atlanta, GA (Zone 3)?

apate54 | Posted in Energy Efficiency and Durability on

Hi all, I am building a flat un-vented wood roof in Atlanta, Ga (Zone 3) and I want to make sure I am doing it correctly. The assembly that I have so far is:

-TPO roofing
– 1/4″ exterior grade drywall cover board
– 2.5″ Tapered insulation
– 4″ Rigid tapered Insulation
– 2″ Rigid Insulation
– Waterproof Coating on osb / or peel and stick waterproof membrane
– 5/8″ osb
– 2×10″ beams (all at the same height)
– 2″ closed cell spray on foam insulation
– 1 layer 5/8″ type x Drywall
– Resilient channels
– 1 more layer 5/8″ type x drywall

I have read alot of papers on what to do in this situation but I have had to combine what I have found to fit our needs. I am very worried about water getting into the osb and causing roof rot. I know that the XPS insulation once taped acts as a vapor barrier but it is still possible that water could leak in through the membrane and never find a way out because the XPS is so impermeable. At the same time – I know that I need to protect the inside face of the osb from infiltrating vapor and I should spray closed cell foam on the inside. So, this assembly has a water and air barrier on both sides of the osb in an effort to protect it – I’m not sure if this is actually protecting it, though.

Also, with this assembly, the inside of the osb has an air barrier made by the spray on – so is it possible then to used recessed cans here? If I can’t use recessed cans here – then I will still have to attach some kind of lighting to the ceiling – which will still puncture the drywall.

What do you all think! Thank you all so very much for the help!

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  1. apate54 | | #1

    Sorry - I marked park of the assembly wrong!

    - 2.5" Tapered insulation
    - 4" Rigid tapered Insulation

    - 2.5" Tapered insulation
    - 4" Rigid tapered Insulation
    - 2" Rigid Insulation


  2. Expert Member
    Dana Dorsett | | #2

    Any unvented assembly would have to dry toward the interior. Moisture trapped between the membrane and foam is not a problem since they are both moisture tolerant. The wooden roof deck is and below is your biggest concern.

    The performance of XPS drops to that of EPS of equal density as the HFC blowing agents leak out over a couple of decades, doing it's climate damage. EPS is also usually cheaper per labeled-R, and blown with comparatively far more benign pentane, not HFCs.

    Per Table 806.5, in zone 3 you only need R5 above the roof deck to be able to use standard latex paint on the ceiling below as the only vapor retarder, with up to R30 of insulation below that. No matter what foam you are using above the roof deck, you have HUGE dew point margin for your proposed ~R12 under the roof deck.

    The 2" of closed cell foam is a waste of good foam when between rafters, and less resilient, since it is far more vapor retardent at 2" than it needs to be. Like XPS most closed cell foam is blown with a high global warming potential HFC. You'll get far more bang per buck installing 5-6" of open cell foam (R20 ish, give or take), which would still be well over 5 perms- half pound foam would be typcially 10-12 perms @ 5.5"/R20. That's more R, and at a lower installed cost than closed cell foam, with a vapor permeance capable of rapid drying should the roof ever leak down to the wood.

  3. apate54 | | #3

    Hi Dana, I cannot thank you enough for the quick response!!!! I just want to clarify a few last things. So, I modified my assembly, based on my understanding of your advice, to:

    - .6 mil TPO
    - 0"-2.5" Tapered EPS Insulation
    - 3" of EPS Insulation (about R-10)
    - Waterproofing Coating on OSB or Peel and Stick
    - 5/8" OSB
    - 5" Open celled spray foam insulation between joists
    - 2x10" Joist
    - 1 Layer 5/8" type X Drywall
    - Resilient channels
    - 1 more layer 5/8" type X drywall

    1. Does this assembly match your advice to prevent the roof rot?

    2. The open celled spray just needs to go between the joists right?

    3.. Can I use the recessed cans now that we have air-sealed the inside of the osb?

    Thank you again for all of your help!!!!!

  4. GBA Editor
    Martin Holladay | | #4

    A. Patel,
    It's true that in Climate Zone 3, all you need is R-5 of rigid foam on the exterior side of the roof sheathing for this type of unvented roof assembly. But you still need to meet the minimum code requirement for the total R-value of your roof -- in your case, R-38.

    Your proposed assembly adds up to only R-28.5 or R-29. So you need to figure out a way to add another R-9 to your roof assembly -- either above the roof sheathing (by beefing up your EPS from 3 inches to 5 inches) or below the roof sheathing (by beefing up your open-cell spray foam from 5 inches to 8 inches).

    For a complete discussion of the type of roof assembly you're talking about, see this article: Insulating Low-Slope Residential Roofs.

  5. Dana1 | | #5

    Depending on the rafter spacing it will stilll just about make it on a U-factor basis, (per Table N1102.1.4 of the IRC) which only needs to be under U0.030 (R33-ish whole-assembly, including all of the air films, the R-value of the roof deck, the gypsum board, etc all added in. Another inch of EPS or open cell foam would almost ensure it, but we'd have to look at the framing details in greater detail to be sure.

  6. JC72 | | #6

    Just an FYI, Georgia follows 2009 IECC which indicates min ceiling R-value of R30.

  7. apate54 | | #7

    Wow, I'd like to first say how much I appreciate all of the help! So,Thank you all!

    Hi Martin, I did read the "Insulating Low-Slope Residential Roofs" , "How to building and Insulated Cathedral Ceiling", and a few others - which is why I am so concerned about the air-barrier on the inside. This is actually my main reason for asking these questions. From my understanding - you should make sure that the drywall is air-tight unless you have used spray foam insulation as an air-barrier. - But then at the same time in other places I see that you are supposed to allow the assembly to dry out to the inside - which is why I additionally added the membrane under the exterior EPS to ensure there would be minimize the damage from any water infiltration. I really apologize for all of the questions, but I really want to make sure I am doing this right. I have added the 5" of open cell spray foam on the inside between the rafters to create an air barrier and provide the additional R-Value. By doing this, can I add the recessed cans? I need to add alot of lighting to the ceiling here and one way or another there will be air punctures in the drywall and I don't want this to cause roof rot - but at the same time I should be letting the assembly dry out to the inside, right? What do you think is the best solution for the interior insulation knowing that there will be air penetrations in the drywall?

    I would really appreciate anyone's opinion on that!

    Also, I have added 1" to my EPS insulation above the osb to make sure I meet code. Thank you all for making sure I met that!

  8. user-2310254 | | #8

    A. Patel. Surface-mounted and low-profile recessed LEDs offer an alternative to traditional recessed cans. If you are wedded to using recessed fixtures, you could also create a utility space below the air barrier layer.

  9. Expert Member
    Dana Dorsett | | #9

    If the recessed cans are completely inside the thermal & pressure boundary (rather than penetrating the open cell foam) it's fine.

    At a 7% framing fraction (typical for 24" o.c. milled 2x rafters) with 5.0" (R18.5) of o.c. foam one layer of OSB and one layer of gypsum, with one upper and one lower air film you're at about R18.0 "whole assembly", or U0.056. Three inches of continuousType-II EPS (1.5lbs density, the minimum "walkable" roof density) adds R12.6 brings that up to R30.6 whole-assembly. The additional layer or 5/8" gypsum plus the additional air films in that cavity adds another ~R1, bringing it up to R31.6 whole-assembly, or U0.032.

    Adding a mere half- inch of continuous EPS it would add another 2.1, which would make it R33.7, which squeaks under the U0.30 code-max.

    If you kept the EPS as-is and added 1/2" to the open cell foam bringing it to 5.5" it would add R1.6 to that R31.6, bringing it to R33.2 whole-assembly which just barely makes it to U0.30 when you include the R0.1-R0.2 or whatever for the roofing & membrane.

    If you add a full inch to either the EPS or the open cell foam it's fully compliant everywhere, with margin.

    Some open cell foam installers shoot literally miles of 5.5" foam between 2x6 studs & rafters every year, and the true pros are good at eyeballing it. But to be sure you can snap a chalk-line or otherwise mark the 5.5" deptin on both sides of each rafter as a both guide for the installer and to facilitate inspecting it after the fact. Most open cell foam can be safely installed at 5.5-6" in a single pass without quality issues.

  10. apate54 | | #10

    Thank you all for all of that information! I really appreciate the time you took to write responses to me. They had exactly what I needed to know!

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