How best to insulate newly built garage with flat roof

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How best to insulate newly built garage with flat roof


I live in Chicago (city of), Zone 5, and I have a new detached 2 car garage 20x20x11. The construction is wood frame with metal siding, drywall finished on the inside. I'd like to heat this garage to 50F during the winters in a cost effective manner (both insulation cost and energy cost), but I also want to do this right and not cut corners. I plan to heat this garage with a small ceiling mounted gas heater, like a Reznor unit.

The roof construction is 2x12 TJI @ 12" O.C with only 1/2" rigid insulation and single ply modified bitumen membrane. No real insulation on the walls or ceiling. This is an unventilated roof. There is no soffit, only a parapet that extends an additional 3.5 feet to accommodate a wood roof deck on top.

My biggest concern is moisture. I want to insulate the walls and ceiling such that I do not have to worry about moisture getting trapped anywhere. I've been recommended a few configurations, and the one that sounds most plausible to me is as follows:

Tear out the ceiling, spray closed cell foam to 3" (R20), fill in with fiberglass 9" batt (R 30). Replace drywall. Total (R50) for roof.
Fill wall cavities with open cell foam to 3.5" depth (R12)
Insulate the garage door with some rigid board R4.5

Does this sound like a good cost effective plan that will reduce my energy costs, prevent moisture problems, and allow me to easily heat my garage to 50F in Chicago winters?

Asked by Brian CL
Posted Tue, 01/22/2013 - 00:30