I recently raised a small carriage barn frame, about 20 foot square, that will serve as a small garage/shop. Originally I had planned on not insulating, and to date have boarded and shingled the roof and sheathed the sides with 1" pine shiplap, and was planning to add windows and finishing with clapboards come spring.

I'm located in NH and now that the weather's cooling off, I'm rethinking the insulation thing: specifically, adding enough insulation to help retain some heat while I'm out there, just for the first floor and not in the attic/storage.

My overhangs are only 4" on the gable ends, so wrapping and strapping with foam panels, etc., probably won't work too well, plus I'm on a bit of a budget. I'm considering using 2" foam panels on the interior cut to infill the spaces between posts & braces (foil to the inside) and covered with 1/2" sheet rock, and likely the same foam panels on top of the attic floor.

My main concern with this approach is a vapor barrier or lack thereof, and do I really even need one? The building won't be kept heated year round, so basically I'm looking at a buffer to temperature swings.

Should I be OK with this basic approach? I'm familiar with the "proper" way to wrap and strap a frame that is to be fully insulated and heated year round, but need some advice when it comes to changing things up.

Thanks,
Audi