Condensation in Unheated Building

In summer of 2006, I built a gambrel metal barn 32X48, unheated, just used for storage, with second story. Winter 0f 2007 discovered a lot of water on north side of building, first and second floor. Building kit provided by local lumber yard. We called building contractor about problem. He then tells us that we need to put 'celotex' underneath metal roof. We agreed. He pulled roof completely off and laid celotex under entire roof. Winter of 2008 we have the same problem, as bad as it was in 2007. Is there any solution to our problem?

Do you have any ventilation? Normally condensation is the result of a lack of ventilation.