I know I’ve seen this discussed before but can’t find specifics.  I have a stand alone garage about 20′ by 22′.  It had a concrete foundation that rises above floor/ground level about 2 feet, on top of which is 2×4 walls covered on the exterior with ship lap siding (no separate sheathing).  The ceiling is exposed joists with plank boards across for attic floor.  I’d like to maintain temperature  in the 40s for a workshop, with temps raised a little higher when I’m working out there. 

I’ve got a lot of surplus rigid polyiso foam insulation, so planning to put spacers in the stud cavities and fill with the polyiso, leaving a gap between insulation and siding, then cover walls with continuous taped layer of polyiso and finally screw plywood through to studs for interior wall finish.

I need to leave the ceiling joists exposed in the garage (there’s just enough room to rotate a full sheet of plywood), so was planning to lay polyiso on top of the ceiling in attic.  Wondering if I should put down a sheet of poly under the insulation and tape to wall top plate (which extends a few inches up into the attic) to air seal garage below.  I was not planning to air seal and insulate the attic itself, just use the poly sheeting and insulation on floor to keep warm air in the space below.  Is this a bad idea?

Thanks for any feedback.

p.s. I built swinging barn doors that are insulated and air sealed, so not worried about losing a lot of heat that way.