Still, the only way to solve it will be some air movement of some type, either getting the inside air more consistent or exchanging it for outside air to expel some of the moisture outside of the building rather than having it condense on the bottom surface of the roof.  I have a similar setup for the building my shop is in, although I have a wall and door rather than a tarp separating my heated area from the colder part.  If I didn't have the moisture barrier under my roof steel, I'd have a lot more condensation problems than I do now (only problem is a few days in the early spring when it warms up a lot during the day).

Mike