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Community and Q&A

Insulation for dormer with thinner studs

Andrew David | Posted in Energy Efficiency and Durability on

I am in the process of building a small cabin in Climate zone 7-8. All the walls are 2×6 and I am planning on using Roxul to fill the stud bays. Exterior has no foam just 1/2 plywood and Tyvek. I will be installing a rainscreen and using some sort of cement board siding.

What I’m wondering about is my dormers, there are two in the loft. The contractor who finished the roof for me including the dormers, went down to 2×4 studs for the walls of the dormers. I’m wondering is it okay to drop down to R-15 Roxul for these or should I try to maintain the R-23 the rest of the walls will have by using the cut and cobble with EPS and Polyiso layers inside the studs? Might I just be over thinking it? I certainly wouldn’t like the dormers to be any colder than the remainder of the building.

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  1. GBA Editor
    Martin Holladay | | #1

    It's your cabin, so you can build it any way you want (assuming that your local building inspector agrees with your plans).

    One thing is for sure: 2x4 walls insulated with Roxul will have a lower R-value than 2x6 walls insulated with Roxul. If you want similar performance, you can add a continuous layer of rigid foam on the interior or the exterior of the 2x4 walls.

  2. Expert Member
    Dana Dorsett | | #2

    That's right. If you can give up 1" of interior space, R6 foil face polyiso covering OVER the stud edges (not cut'n'cobbled between the studs) will bring the wall performance up to something comparable to R23 in 2x6 framing.

  3. Andrew David | | #3

    Brilliant! I don't know why that never occurred to me, but I think that's what I'm going to do. So much faster!

    Thank you both.

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