Pay me now, or pay someone else (POCO) later. Spray foam, IMO, is the best insulation you can hope to get in your situation. I had a client who spray foamed an addition in the back of her garage. I went over to check something out, and my truck thermometer read 100 degrees F. I unlocked the room, and with no air movement at all, it was a comfortable 80 degrees on her wall thermostat/garage door opener.. I turned on the window unit and within about 15 minutes, I heard the compressor kick off and it was 70 degrees. Amazing stuff.