The moment fiberglass insulation receives damp (and it will get damp, as a result of condensation) it will eventually shed r-benefit! Water trapped amongst fiberglass insulation and the outside of a creating will also induce mould, fungus growth, rust and oxidation - Conditions that can degrade the metal and shorten https://www.freecaliforniaclassifieds.com/ads/hand-crafted-metalworks/