Greetings Charliehorse,
It is also possible, probable even, that there is no vapour barrier under the concrete floor. If there is enough space and height you can build a room within a room with a slightly raised floor. You can retro fit building paper the the walls from the inside, pressing it against the exterior cladding and folding it back and stapling it to the studs. Insulation followed by lining with two or three more interior walls and an insulated ceiling and you have a decent space. This is roughly what I am doing. Our garage floor slab has a vapour barrier and I have lined and insulated the exterior walls so I don't have as far to go. Cars bring a lot of moisture into a garage (assuming yours get into the garage, ours don't) so a separate space makes a lot of sense.
Regards Grandpamac.
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