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Community and Q&A

Rigid insulation under mineral wood in rafter bays? (Zone 6 Vermont)

user-1142226794 | Posted in General Questions on

  My wife and I are adding an addition with a vaulted vented ceiling with no dormers, skylights, or electrical. It is 18’X10′ added to our existing bedroom. We ran a 2X12 ridge beam with 2X10 rafters 24″ oc , Advantech roof sheathing, Ice and Water shield then asphalt shingles. I am now scratching my head trying to get to R-49 in the ceiling. I thought of furring out the rafters to fit more mineral wool, or doing a layer or two of rigid insulation under the rafters, with strapping, and finishing with sheet rock. There are a couple articles  and Q&A’s I found that mention this strategy, mainly:

However, most situation seem more complicated (dormers + skylights). I was wondering which strategy makes the most sense (furring out / ridged), and if anyone has more info on adding ridged insulation to the underside of rafters.

Cheers and thanks for any suggestion/information.


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  1. Expert Member
    BILL WICHERS | | #1

    If you go with the code-required overall U factor for the roof, instead of the prescriptive R factor, you can get by with less internal rigid foam than you might think. Assuming you have a 1" (code minimum) vent channel in that 2x10 framed roof, you can fit about R36 worth of mineral wool in there. Code requires a ceiling U factor (based on the 2018 ICC for your climate zone) of 0.026, which is equivalent to about R38.5. Your framed assembly with that ~R36 worth of mineral wool is about R30.2, allowing for wood at about R1 per inch, and using the real U value calcullation and not a simple average. Since continuous insulation like rigid foam under the rafters is not affected by thermal bridging, you need only about R8.3 worth of rigid foam under the rafters here. 1.5" polyiso is R9, so that's the best fit, and would give you a whole-assembly R value of about R39.2, which us U0.0255, and meets code.

    I would do the interior side rigid foam here since I think it would be easier to build, and you could detail it easily to act as a vapor barrier (assuming foil faced polyiso with taped seams), which would make your vented cathedral ceiling a little more robust.


    1. user-1142226794 | | #2

      Hi Bill,

      Thanks for the reply. I was looking for a way to get my air barrier from the external (taped plywood with Vana tape) to the interior. So this method seems to fit the bill. My buddy pointed out that if I run 2X4 strapping on the underside of the poly iso I will have a good channel for electrical if needed/wanted. Thanks again and have a good one.

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