I’m in the process of converting an old detached garage to a wood working shop and am wondering about air sealing and insulating. The walls are 2×4 with 1×8 tongue and groove sheathing and 1×8 shiplap siding with aluminum siding over that. I’ll be heating and cooling the space with a portable dual hose AC/Heater unit run through the wall. Climate zone is 4C marine. 

My question is what would be the best way to air seal the stud bays? The gaps in the sheathing boards are very large and there appears to be some old tar paper under the siding. If I apply any kind of vapor closed sealing such as spray foam the sheathing and cladding won’t be able to dry inward or outward due to the aluminum siding. However the garage will only be heated intermittently anyway so maybe not that much inward drying potential anyway. What’s the best option with an intermittently conditioned space like this?