Air movement is what you want to look at. If you can prevent air infiltration of the building and heat a large quantity of air at a time, you will be warm. I live in WI and have a 40x80 pole shed that I use as a shop. No insulation whatsoever. I can get it fairly warm with a wood stove. We are not talking about a balmy 70-80°, but a lot better than the arctic temps outside. Anything above zero is good if it is -40 outside and near freezing is better than zero. It is around 100 near the stove and gets cooler as you move away. A blower will help put warm air on you. A draft will make it feel 10 times colder. Simply having an airtight space will more than double your heat retention. Losing some heat through a wall is better than losing your warm air altogether and having to heat up even colder air. Start at the top and work your way down toward the floor. Foam, tape, plastic, whatever, just use it. The same with any insulation, start at the top. I am going to add bubble foil insulation to my shop starting next year. The roof will be first, then the walls. The floor is mostly dirt and will get foam under the concrete to help with heat sink issues. I have 2 11x16 doors, one rolling and one panel style lift door. They allow good circulation in summer, but stay closed when at all possible in winter. A draft wall can help with temps too. A 4-5 foot wall a few feet inside the main wall that cold air must go over to get to the main floor area. Forcing this cold air up makes it mix with the warmer air and will help preheat it. Perceived heat is what we are after. If your feet are cold, you are cold, if your feet are warm, you are warm. Get it?