spray foam vs ridgid foam board insulation?

[Dan Schrecongost]

I would like to spray foam but I don't think I can justify the cost for a building I won't be heating. I mainly need it relatively sealed up and want to reduce temp swings to reduce condensation on my tools.

In my experience, condensation on tools happens when there is a warm-up outdoors and moisture-laden air comes in contact with cold steel or cast iron. Reducing air infiltration pathways will go a long way to keeping moist air out of your shop and sprayed foam is very good for that. But water vapor moves through building materials also, so pay attention to the vapor permeability of the foam you choose.

Insulation slows the transfer of heat, but it does not stop a building from cooling down to ambient temperatures, so your tools will still be cold much of the time, and vulnerable to condensation.

[Wes Grass]

An air gap is an insulator to a certain point. Make the gap too wide and you get convective flow transfering heat faster than a narrower gap.

A double pane window can transfer heat faster than a single pane if the gap is too wide.

[Steve Tripp]

I have a pole shed and did closed cell foam for the roof/ceiling. For the walls I did 1 1/2 inch foam board and sealed it, then furred out the walls with 2x4s and did 3 1/2 inches of rock wool. I'm in Minnesota and have been very happy with the results. I also have a ductless mini split for cooling and heating and have been surprised at how well it works for me.