When I built my steel building, I laid short pieces of galvanized roofing between the beams, resting on the lower flanges,,
that material is, in my case, 6 inches below the actual roof,,
That gap creates an air gap, and the summer heat can not radiate down,, it stays cool in the building,,
My SIL has virtually the same building about 1/2 mile away,, but he does not have the extra layer of material,,
his shop is like an oven in the summer,,,
Since my drop in pieces were only 48" long, the local roofing company had hundreds of them, as they were drops from cutting roofing to length,,
I was able to buy the pieces for $0.50 each,, which was WAY more than the roofing company could get for the drops as scrap,,,