Bought a home recently that has a detached structure which was used as a shop/art studio. The roof is metal over battons. No felt or sheathing underneath that. However, below the battons is about 8 inches of fiberglass insulation between the rafters, with a layer of clear plastic underneath that. Does it make sense to have added the fiberglass and plastic like this or would it be better to remove it? Seems like a weird method to me guaranteed to cause mold, some of which is visible through the plastic. Our use of this building will be as a shop/barn. It will be a cold building, never heated.
If I had to guess, the reason you see mold is that you have some water leaking from the metal roof. Probably a small amount but enough to encourage the mold to grow. If the roof were fixed so it didn’t leak and it’s going to be unheated anyway, not sure what purpose the insulation and plastic serve. While the plastic is probably what’s keeping the ceiling from dripping, it and the insulation is preventing any airflow which, when combined with moisture, provides a perfect environment for the mold to grow.
Sounds like good reasoning. Thanks.