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Is former ventilation space a problem with exterior rigid foam.

Ddreed | Posted in Energy Efficiency and Durability on

Hello, thanks for all the info here on GBA!
I have a cape cod and I keep on coming back to the idea of exterior rigid foam to insulate my roof with shallow rafters. I was talking with a contractor and he said rigid foam above roof sheathing with the ventilation gap beneath wouldn’t be good for moisture. Is this considered true? 

I thought it didn’t matter because the new roof assembly would either be not vented because of a sealed roof envelope and the option to vent the new roof decking if need be. 
Thanks for the help, 
Dustin

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Replies

  1. Expert Member
    Zephyr7 | | #1

    A vent gap between layers of insulation defeats much of the purpose of the extra insulation. I don't know if I'd call it a moisture risk (assuming it's still allowing for airflow, i.e. it's still "ventilating"), but it isn't a good thing for thermal reasons either. If you want to put rigid foam on the roof, you need to change the interior to get rid of the vent gap (either by removing the interior insulation, or moving it up against the sheathing). I suppose you could put ALL of your insulation on the exterior using really thick rigid foam, then just block off the vent gap, but that probably isn't what you want to do.

    Bill

    1. Ddreed | | #3

      Bill, thanks for your input. I guess my specific question is if I could strip my current roof down to the decking without touching the cavity insulation, which currently has an air gap. The current cavity insulation isn't enough due to the limited space of a 2x6. I was wondering if the process of exterior rigid foam on a roof, by making the old decking and eaves part of the new envelope, would that small gap between the cavity insulation and the old decking be a problem, even though the temperature would never reach dew point in that air gap space.

  2. Tyler Keniston | | #2

    You can blow in from above possibly

  3. Tyler Keniston | | #4

    I'm curious what others here say. I have seen it said many times before that insulation should not be 'split' like that, but I've never heard all the reasoning or whether one of the reasons is moisture related (other than this argument made by Allision Bailes in regards to attics, which is a bit different I think: https://www.energyvanguard.com/blog/3-reasons-to-remove-attic-floor-insulation-in-a-spray-foam-attic/ )

    But I can see how such a gap could reduce thermal performance of the assembly a bit. Consider that your existing interior finishes are probably not air-tight. There could be a pathway for air to bypass the lower, interior insulation and enter that open cavity, at which point you have only the R-value of your new exterior layer as thermal protection.

    Basically, the convective currents in that space can contribute to a sort of short circuit effect—how pronounced this effect is would depend on the air-tightness of your interior finishes and on the type and quality of install of your existing insulation. Also pitch of roof, delta T, and probably other factors.

    Whether excess moisture would accumulate there (it would be a cooler space than before the ext. insulation, but may allow more interior moisture laden air to reach it than if it were filled) I don't know.

    1. Ddreed | | #7

      Tyler, are you saying that the air space affects r value negatively? In my mind it's the same because it's still on the other side of the finished interior. And overall there's a fixed amount of insulation it just has an air space between the two. But maybe I don't get it.

      1. Tyler Keniston | | #8

        Dustin,
        Practically, I'm not sure. It's been said before by Martin and Dana and I feel I've read it elsewhere. But I won't claim to be able to defend the position. My understanding is that the issue is all air movement related. As I've described above. Loose batt insulation would be most susceptible to this. And if your interior finishes are truly airtight, it may not matter at all. Sorry, not too definitive. Hopefully others chime in.

  4. Jon R | | #5

    Discussion of non-vented mid-wall air gaps here.

    > the temperature would never reach dew point in that air gap space

    I suspect this makes a difference. As well as how good the air sealing is.

    IRC R806 requires "directly below". But it also allows R ratios where some condensation could occur.

    1. Ddreed | | #6

      Thank you, that article was good.

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