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Rigid foam over roof questions

user-6423311 | Posted in Green Building Techniques on

Our house is a two story brick with the usual fiberglass insulation in walls and 1/2” of pink foam sheathing between studs and brick facade. The attic space is finished with a fairly low ceiling, r19 in rafters, drywall, and knee walls. I suspect the ceiling of the second floor (which is the floor of the finished attic) is also r19 but there is finished floor down so I haven’t confirmed. Roof has the styrofoam trays for venting and eave and ridge vents. Upstairs ac unit is in the unconditioned space behind the knee wall and sadly is bare metal duct with flexible ducts to the upstairs rooms. Predictably the ac runs a lot. 

I found the articles here about foam over the roof and it looks like we are climate zone 7 so I believe that calls for 6” of XP’s or iso foam. That would mean two layers of 3” foam, nail base, and lots of screws. 

First, is this the right approach? It would prevent ripping out good drywall, would condition the knee wall area to improve hvac efficiency, and would get us to r50 or so for the assembly. 

Second, do the screws for the final layer of sheathing have to hit rafters or is there a schedule that allows screwing to the first layer of sheathing? What screws would you recommend?

I don’t want to get into a situation with a contractor where they are frustrated with the project due to difficulty installing etc. 

Thanks for your insights!

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  1. Expert Member
    Dana Dorsett | | #1

    With 6" of polyiso above the roof deck would have the equivalent performance of R50 between joists or rafters even without anything rafter-cavity fill, and would meet IRC 2018 compliance on a U-factor basis:

    U0.026 is equivalent to R38.5 "whole assembly" which includes the R-values of all layers, such as the roof deck, nailer deck, roofing, and even the interior & exterior air films.

    In zone 7 as long as at least 60% of the total R is on the exterior of the roof deck the deck stays warm enough to mitigate against excessive wintertime moisture accumulation: (the presumptive total R is R49 in the Table R806.5 prescriptives.)

    It may be easier and cheaper to use a U-factor approach, with some of the insulation on the interior tight to the underside of the roof deck, along with some amount of rigid foam above. For instance, R15 rock wool or 3-4" of open cell foam under the roof deck and 4.5" of polyiso above the roof deck would hit the U-factor performance, and still have a bit of dew point margin at the roof deck. To figure out the easiest/cheapest/safest way to go, how deep are the rafters? What is the rafter spacing? Were you planning on keeping the R19s?

    With nailbase panels most vendors advise NOT screwing them into the rafters, but instead screwing it to the structural roof deck using prescribed patterns and fasteners that often vary with roof pitch and other particulars. eg:

    1. user-6423311 | | #2

      Thanks! The rafters are 2x6 16” oc and have drywall on the inside that would be nice to keep but not mandatory depending on what really makes sense to do. Do you think we could keep the r19 and drywall and add the polyiso? I guess air sealing would be the issue with that. I wonder if we removed enough drywall from the knee walls for access and spray foamed that area roof deck and waves to seal but left the ceiling intact with drywall and r19 batts if that would make sense. It would solve the air sealing I believe.

      1. Expert Member
        Dana Dorsett | | #3

        If it's possible to gain sufficient access to insert a dense packing hose between the foamy vent channels and roof deck you could just fill it with cellulose or blown fiberglass, which would deliver ~R20 performance. In your climate ~R20 in the cavity would demand at least 5" of polyiso (6" better) up top for dew point control at the roof deck.

        1. user-6423311 | | #4

          Okay thank you it sounds like we should be thinking 6” of polyiso then. What do you think about sealing the soffit and knee wall area with spray foam?

          The harder part of this is finding a contractor who will work with us in NE TN. I looked and found a company who sells themselves as a green roofing company but when I contacted them they said oh we just do roofing.

  2. thrifttrust | | #5

    I'm confused. Where is NE TN? If it is north east Tennessee you're in climate zone 4, not 7. CZ4 calls for 30% external insulation rather than the 60% required in CZ7.

    1. user-6423311 | | #6

      Oh gee I think I got plant zones and climate mixed up. Thanks very much for pointing that out!

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