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Unvented cathedral ceiling / dew point inquiry

user-2945479 | Posted in Energy Efficiency and Durability on

We have a project under construction (Zone 5) and are considering a late modification.

The plan has always been to have an unvented cathedral ceiling… the exterior allows us to have a continuous air barrier (Blueskin) from the foundation up to the ridge.  I am a firm believer in solving for air infiltration/exfiltration as a priority.

Originally, we were going to apply 4″ of XPS (R-20) above the structural roof sheathing and fill the rafter bays with mineral wool (R-30).
But after running a thermal gradient (Ext 14deg, Int 68deg, 40%RH), we net out at 36deg on the underside of the sheathing… that’s below the dew point of 43.

The alt would be to ‘flash-and-bath’ with 4″ of ccSPF (R-6.7/in = 26.8) and R-23 mineral wool below.   With this assembly the inside face of the foam is right at the dew point…. not super, but significantly better than the earlier option.  And while condensation isn’t good, at least it’d be on the foam and not the structural sheathing.

Both systems net out a value of a bit over R-51… obviously the latter incurs the penalties of thermal bridging.

Thoughts?  Much appreciated.

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  1. Expert Member
    Dana Dorsett | | #1

    >"Originally, we were going to apply 4″ of XPS (R-20) above the structural roof..."

    Read the fine print. At 4" the manufacturers will only warranty R18. (90% of labeled R). But even that may not cut it over the long term. At full depeletion of it's climate damaging HFC blowing agents (that are responsible for the higher thermal performance) it's performance is about R17.

    Going with 4" of 2lb roofing polyiso would be labeled R23, and would still deliver the code prescriptive R20 even when derated for climate, but 4.5-5" would give more margin.

    What is the significance of the +14F outdoor temperature?

    Unless your binned hourly mean wintertime temperature for the 10 coldest weeks of winter is as cool as 14F (not likely, in US climate zone 5) the fact that the sheathing drops below the indoor dew point under that condition is irrelevant. Yes, it will begin taking on moisture as adsorb (not condensate) at that temperature, but it releases it when the temperatures aren't as cold. It's the total wintertime accumulation that matters, not whether or not a few hours or days of accumulation occur when it's cold out, followed by more temperate days when it's realeasing moisture.

    In my own zone 5A location even the January mean temperature is over 25F, and the 10 coldest weeks of winter even higher, even though there are often days during mid winter when the binned hourly mean temp for the DAY might not be more than 10-15F:

    1. user-2945479 | | #4

      14deg came from an old text that listed it as the winter design temp... with the thermal gradient calc'd in an excel chart, I have inputted several interior and exterior temps. Anything above 25deg has no issue.
      Excellent point on the adsorption.

      1. Expert Member
        Dana Dorsett | | #6

        If 14F is your 99% outside design temp, it means that 99% of the hours in a year are warmer than that. That's an important number from a heating system design point of view, but not relevant to wintertime moisture accumulation issues.

        The IRC prescribes a minimum of a bit more than 40% of the total R (R20 out of a total R49) on the exterior to provide adequate dew point control in a zone 5 climate.

  2. GBA Editor
    Martin Holladay | | #2

    The information you need is in the following two articles:

    "Combining Exterior Rigid Foam With Fluffy Insulation"

    "Flash-and-Batt Insulation"

  3. user-2945479 | | #3


  4. Expert Member
    Akos | | #5


    You unitial plan ends up with a whole assembly R value of 47.

    With the spray foam/batt approach, that drops to around R34.

    With the 4" polyiso Dana recommends you get R55 roof. This is much higher performance and probably cheaper than either of the other two options.

    If you are looking for a budget roof, going with 3" of polyiso and R25 fiberglass batts would get you an R45 roof even with a sub standard batt install. The energy consumption difference between an R55 and R45 roof is minimal in Zone 5.

    1. Expert Member
      Dana Dorsett | | #7

      When it's continuous foam over the roof the center-cavity R becomes less relevant, and compliance could/should be done on a U-factor basis.

      For zone 5 the IRC calls out U0.026 max (= R38.5 "whole assembly"), and that COULD be achieved with 3" of continuous foil-faced polyiso (R18) held down with a 5/8" plywood nailer deck, with R25 batts between the rafters, depending on the rafter spacing and the R-values of the sheathing, ceiling gypsum, roofing material, interior & exterior air films, etc.

      1. user-2945479 | | #9

        Points well taken, Dana.
        Per my comments to Akos. I'm going to stick with 4" of polyiso. That way I can use an R-30 mineral wool batt.

    2. user-2945479 | | #8

      Thanks for your insight, Akos. FWIW when I calced the 'center of cavity' on the 2 initial assemblies they netted out nearly equal. Obviously you have pointed out the thermal bridging pitfall with the ccSPF option, which is much appreciated.
      Also, just as a point of fact, the costs of the 2 proposed options are pretty similar. While the ccSPF costs more, I have added labor from the framers and roofers to install the exterior insulation and secondary deck and/or battens. I don't think there will be much of a savings btwn 3" of polyiso and 4.5" of XPS (unless you have a better source for polyiso that I'm not aware of), and for the batts, I am really not a fan of fiberglass. I have 9.5" of rafter depth (LSL timberstrand rafters)... I'd hate to 'waste' that cavity space with R-23 mineral wool.

      1. Expert Member
        Akos | | #10

        For poyiso, get pricing from a roofing supplier. Here for the same R value, roofing polyiso is usually cheaper.

        If you don't have a complicated roof, exterior foam is pretty straight forward to install. One method that I've used is to install purlins on edge with 2' insulation boards between them, this puts the purlins at slightly more than 24OC which is fine for some metal roofs. You get slightly less R value, but much quicker to install as you are not trying to drive 6" screws blindly into rafters. I guess you can even putt batts between the purlins provided the roof panel doesn't need full support.

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