I briefly considered "attic" trusses, too, but that would have created a much taller building and would stand out a bit more than we want on the small property. The proportions of a 24x36x10 with 4/12 pitch are pleasant for where the building will be sited.
Spray foam is on the menu...I'm a believer and have been for a long time, but it will also come down to economics. Some of the additional cost of spray foam could be absorbed by not needing to put in a ceiling and that's not a small number in itself. I'll be exploring that with a family friend who's in the business once I get a little farther along.
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The most expensive tool is the one you buy "cheaply" and often...