condensation on metal roof

[Joseph Hammel]

I cant figure out what forum to put this under. I own a 16 unit apartment building in Hammond Minnesota near Rochester. It had a flat rubber roof over about 2/3 of the building. It was needing replacement and as more of a long term fix I decided to have a truss system built and put a steel roof on the building. However we didn't sheath the roof the steel is on purloins. We vented the soffit and ridge. Now we thought we had leaks but it turns out its condensation forming inside on the steel dripping on the rubber and finding a way in. We had to cut the rubber around the perimeter to install the Lam beam to set the truss system on. I looked into spray foam. The spray foam guy suggests a 1 inch layer on the back of the steel and totally closing up the system. So we would block off the soffits and foam it all in tight including the ridge vent. This would be about $5000. My other Idea was to use cellulose to put a r-30 (or more) layer on top of the old rubber. We could reseal the edges with rubber scraps to act as a vapor barrier.  R-30 would be about $1300 for material plus labor but I can hire some $20 hr guys to do that. Any other suggestions/ comments? I wish I would have done this differently. Oh the other advantage to the blow in is I can do it now where the foam would have to wait till spring.