condensation

I had a pole barn built with a concrete floor and I have installed insulation in the walls and ceiling. I use it as a cabin. My metal roof was installed over the trusses with no barrier under it. I have since learned of a product that some people have used that is place down before the roof goes down. Should they have put a barrier under the roof? I am worried about condensation dripping onto the insulation. If so what do I do about it now? I do have soffit vents and ridge vent. Thanks

No they did not use anything under the roof. The roof is attached to the purlins. The purlins are attached to scissor trusses. I placed unfaced insulation between the trusses and used poly as a vapor barrier facing the living area. My ? is - what do I do about possible condensation dripping from the roof onto the insulation. I can't very well take the roof off and put down a barrier on top of purlins and then put the roof back on. Do I have any other options at this point. I have not really had any issues with condensation that I can see. Because I have the insulation in place I can't really see how much condesation is taking place- its what I can't see that has me woorried. I don't want to drywall the ceiling only to find out later I have a problem. What about a gable end fan? Thanks for your replies!