To Antifreeze, or not to antifreeze...this is the question

Cosmo

I am designing a radiant job, a beautiful post and beam frame from an 1800's barn installed on a new foundation, with foam core panels making up the outer skin of the building. The problem I have is the customer wants to radiantly heat his tractor shed; ten feet high, ten feet wide, by 30 feet long. Is is surrounded by the rest of the building, with 3 exposed walls. With 12" spacing I can bring the room up to 65degrees on a design temp for my area of 7degrees F. I have never radiantly heated a garage before and wonder how conerned I should be about freezing. Would be nice just to treat it like another zone and forget the water/antifreeze yearly maintenence- but what if the garage door is left open for too long? I know from experiance the owner is a don't fix 'till its broke type, and will likley get tired of the extra expense.
Teach me!
Thanks,
Casmo