Metal Roofing Question

marverick

I am refinishing an old building, and it has a corrugated metal roof. Currently it is just metal roof>2x6 purlins spaced at 6 in centers. I will be adding heating a cooling to the area and I need an idea of a way to insulate this and control condensation.

I am thinking metal roof>purlins>2 in ventilation area with soffit and ridge vents>batt insulation (hopefully Owens Corning R-38c)>polyethelene sheeting(or vapor barrior of some sort>Drywall ceiling.

Is this the right approach?

I have seen some people suggest putting double bubble on top of the purlins underneath the metal roof- what is the benefit of this?

Also the raters are only 2x8's and the insulation and ventilation area would be 12.5 inches after 10.5 inches of insulation and 2 inches of ventilation area. How can I do this without replacing all the rafters?

How big of a deal is it to drop the whole ceiling, is there alot of extra support that needs to be added?

By the way I am in Southern Wisconsin

Also, by the way- this is a cathedral ceilng- no attic space.