Downside to exterior foam only?
Heated outbuilding in Climate Zone 5
30x40x12 nominal size w/ 2×6 walls on trench footings (16″ OC)
Attic trusses upstairs with open stairway
Radiant slab for primary heat
Two 10×11 garage doors
Since this is going to be a garage, my preference is to not fill the stud cavities with insulation and have to cover them with drywall. That just makes future wiring tweaks a total PITA.
Wall code here is R19 or R13+5
Roof is R39
Building to R19, with the 2×[email protected] is really only R15 or so net (assuming R1 per inch of stud)
Seems like a minimum of 3″ reclaimed polyiso would beat the R19 wall most of the time, and 3.5-4″ would soundly beat it even in cold weather.
Is there a downside to this plan:
Siding over furring strips
over 3+” polyiso with taped seams
over 1/2″ OSB with taped seams
over 2×6 stud walls
The only issue I see is having the polyiso in contact with the ground, as I do want to insulate all the way down, so I’m thinking of putting EPS or XPS around the perimeter, just 1-2′ high and do poly up from there. Maybe that’s not even an issue if I flash it properly?
Truss designs will likely drive me to some foam over the sheathing, with only a 2×6 top chord in some places but that’s TBD at this point as I should be able to fur build inward.
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