What’s the R value and heat needed to heat a detached garage that is somewhat insulated – GreenBuildingAdvisor

Hope everyone is well wanted to ask a very specific energy calculation. If I insulated a 20×25 detached garage with r-13 batts in the wall, 1/2 polyiso on exterior walls, with 1 inch of polyiso on 2 8×7 wood garage doors, but the ceiling and rafters was left open what would be the r value of the garage? If the ceiling is open is it like having no insulation? I have a 2400w infrared electric heater, would it do anything in the winter if I turned it on while using the garage?  I understand heat rises, so is ceiling insulation more important than walls? What about an insulated ceiling only directly above the area I want to heat. Or does hot air move immediately to the open ceiling area?

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