Frost on underside of roof

I have read now several people who have installed an metal roof on there home and now have moisture issues on the underside of the roof in attic. I left one layer of shingles and placed a sheeting over shingles to protect underside of steel roof per building code. I have a vented ridge vent as well. I have never had moisture in my attic till now. The metal roof is causing moisture in my attic. Nothing different has been done other than installing metal sheeting over existing roof. Note authorized building dept. Why is it freezing in attic. I checked for leaks, but see none and besides never had this issue till now? House is at 45% humidity. We do have plants and humidifier, but had prior to installing steel sheeting with no issues prior. I am puzzled and worried about rotting sheeting as well.

Was the roof vented previously? How is it vented now?

Are you sure the ridge is not obstructed? How about the soffits?

As I mentioned before: has soffit vents same as before steel was applied nothing changed. The ridge vent is not obstructed. Just took off old shingle ridge vent and installed metal vented ridge vent. You know the stick on soft foam like ridge vent stuff. Stint screws attach to hip on steel panels across roof. I have seen similar inquires on this forum. Did nothing but add steel and now have freezing issues in attic. Absolutely have done nothing, but add steel to existing shingled roof. Never had issue prior.

There is nothing about the steel that would change the vapor permeance of the roof as that point. I shingle as well as a sheet of plywood is a class 1 vapor retarder. If the venting is the same, there is nothing about the attic that is keeping more moisture as thing point.

If you are certain you never had a similar manifestation before (keep in mind that many roofs with manifest with frost), the only theory that I could guesstimate is that the steel roof is actually making the roof surface colder by not absorbing any of the radiant heat during the day vs. a shingle holding on to some of that heat and making the inside plywood warmer.

That said...whatever temperature differential there is should be over by AM when dew point is usually happening.

See if you can put a data logger in the attic that will record relative humidity as well as verify via smoke pencil that the ridge and soffits are working.

Thank you, will follow your advice!