I am undertaking upgrading an old corrugated iron roof, that features such marvels as no insulation whatsoever and degraded building paper that passes under the purlins! Photo attached. Spectacular, huh! It has the added bonus of zero air tightness!
The ceiling below is solid T&G, so…
I’m looking to install layers of Phenolic foam/foil faced panels.
First layer laid crossways, between the purlins (50mm)
Second layer laid down the roof over the purlins, taped to provide a full continuous layer of foam/foil. (50mm)
Have a large amount of Kingspan insulated roofing to re-roof with it’s a sandwich of 5-rib with 60mm of foam and a flat steel under-layer.
My question is what, if any, underlay, paper, wrap, whatever might be required in that layup? Does the foil do the job of that layer?
is it a bad idea to have the roofing sitting down directly on that foil layer? I worry about electrolysis/corrosion at that interface. Should I have something in between?