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Information on insulation

PMNY | Posted in General Questions on


I’m fixing up a small 1930’s cabin in upstate NY (zone 5) and wondering if I can get some advise on insulation, specifically for a Cathedral ceiling.

The building is made up of three parts, the original cabin 14’ x 28’ with a bump out extension (shed roof) that comes out 6’ and is about 14’ long. There is also a garage attached on the gable end that runs 24’ with the roof line matching the original cabin. Existing rafters are 2×6 on 24’ throughout.

It wil most likely be unconditioned both summer and winter, with either the windows open to allow cross breezes in the summer and a small stove to heat in the winter. It is used mainly at weekends at the moment, but that could change in the future. I realize I will have to address the heating cooling at some stage with something like a mini split system or something along those lines.

Because of the bump out extension, and the 2×6 rafters, I can’t put the venting on the inside, so I will create the venting above the exterior insulation. I’ve looked at some of the solutions on this site and here is what I have concluded as the best approach for my situation, listed from outside inwards:

Standing seam metal roof
Structural OSB, or 1×4 purlins attached to 2×4’s on the flat on top of rigid foam insulation, secured to the rafters below.
Two layers of 2″rigid foam (offset and taped) outside layer of XPS over layer of Poyiso
Snow/ice membrane around eaves area and ashfelt paper on the remainder of OSB sheeting.
6’ rafters with Roxul Rock wool R23 24” spacing
1/2” drywall taped as an air barrier.
Painted horizontal lap boarding on walls and ceiling.

Is there any issues with moisture in the wall cavities I’m missing, or any issue with not having it heated during the winter/cooled in summer? Should I use Membrain air barrier and vapor retarder? If I do use it, do I need the drywall since I will be using painted shiplap on the interiors?

Any advice is much appreciated.


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  1. GBA Editor
    Martin Holladay | | #1

    It sounds like you are on the right track. In case you didn't see it, you may want to read this article: How to Install Rigid Foam On Top of Roof Sheathing.

    Q. "Are there any issues with moisture in the wall cavities I’m missing, or any issue with not having it heated during the winter/cooled in summer?"

    A. You didn't tell us much about your walls -- only your roof. If your building is unheated, you'll need to drain your plumbing pipes and put antifreeze in your toilet bowl.

    Q. "Should I use MemBrain air barrier and vapor retarder?"

    A. Your roof assembly doesn't require you to do that. However, you need some kind of air barrier (most people use drywall, as you are planning to use) on the interior side of your mineral wool.

    Q. "If I do use MemBrain, do I need the drywall since I will be using painted shiplap on the interiors?"

    A. It's possible to use MemBrain as an air barrier, but I don't know whether it holds up as well to all the fastener penetrations as drywall does. I would use drywall.

  2. PMNY | | #2

    Thank you Martin for your response.

    The walls will be set up like this from inside out: Painted Shiplap, air barrier (Membrain and/or drywall); Roxul Rock wool R23 spacing between 2x6 studs 24" on center; OSB; 1" foam, housewrap, Cor-a-vent, SV# and Sturdi-batten for rainscreen gap, followed by painted board and batten siding.

    Is the 1" foam enough with the Roxul on the inside, is there any issues with drying to the inside with this set up?

    Again, is there an issues with having a non-conditioned environment winter or summer?


  3. GBA Editor
    Martin Holladay | | #3

    Your plan won't work. In your climate zone (Zone 5), if you want to install exterior rigid foam on a 2x6 wall, the rigid foam layer must have a minimum R-value of R-7.5. So you'll need 2 inches of EPS, or 1.5 inches of polyiso. One inch of foam isn't thick enough.

    For more information, see Calculating the Minimum Thickness of Rigid Foam Sheathing.

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