Charliedan

We are erecting a steel portal frame 8m x 12m i have positioned the 9 inch treated purlins with small wooden blocks beneath them so I can attach white corrugated tin beneath the wooden blocks will enable the edges of the corrugated tin to slide in, and sit over the i-beams by approximately 20 mm above this layer of tin and between the 9-in purlins I wish to use rolls of fiberglass insulation of which I have a very large quantity people have mentioned Vapor barriers and air gaps beneath the black top layer of tin but I can't find an exact detail of the Construction I am proposing any advice would be gratefully received I'm very keen not to add a vapor barrier if it's not necessary