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Zip Sheathing and Interior Rigid Foam

tyltur | Posted in Green Building Techniques on

I’m in the process of insulating, resheathing, residing, and adding heating and cooling to a detached garage to turn it into a working shop space. Basically everything but the framing and roof is being replaced, so it’s the perfect time to do it right.

I’m in climate zone 4A, and I plan to add a 12k BTU mini split to the structure, so moisture will come from both the interior and the exterior seasonally. The plan is to use Zip Sheathing as the air barrier and James Hardie over a 1×3 rainscreen, and there are no windows in this structure – only a garage door. This means detailing on the outside will be simple, but there isn’t enough roof overhang to add exterior rigid foam. Given that the roof was just replaced last year, I don’t intend to insulate on the outside of the sheathing. 

The walls are 2×4, so to increase the R value and decrease thermal bridging, I’ve been considering adding an inch of interior rigid foamboard. 1″ of EPS has a permanence of between 2 and 6, so even though it would operate as a second air barrier, this would still allow the wall to dry to the interior. In my climate kraft faced fiberglass is common, and it appears the kraft facing has a variable perm rating between 0.4 and 4.2 (https://www2.owenscorning.com/literature/pdfs/10017860%20Moisture%20Control%20Perform%20Kraft-Asphalt%20Facing%20Tech%20Bulletin.pdf), so the EPS should allow drying even better than most structures in my area. Combined with the simple exterior detailing, it seems that I should have a pretty safe wall assembly. Am I missing anything?

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Replies

  1. DennisGray | | #1

    Just something to consider: The majority of infiltration and conductive heat loss/gain is going to be through garage door. Even an insulated garage door has terrible effective R value. The normal R value rating quoted by garage door makers is "calculated door section R value" which looks at the insulated panels only, not the entire metal door structure. Make sure your heat loss calculations are realistic with this in mind.

    1. tyltur | | #2

      Yes, I'm assuming some pretty horrible air leakage through the garage door. Not much I can do about that, however, short of building a custom carriage door. I would be open to any suggestions on tightest garage doors, though I assume most of that is up to the installation.

      My goal here is to build the rest of the structure tight enough and with a high enough r-value that I can heat/cool the structure despite that. The calculator at https://www.loadcalc.net/ seems to suggest a reasonable heating/cooling load for my structure even with a "poor" air leakage rate.

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