mobilehomerepair.com

I'm refurbishing a '76 double wide for use as a project shop. we are currently resolving leakage issues and replacing damaged material. when taking down the ceiling we have two layers of insulation. one along the roof itself and another layer against the ceiling. There is are two plastic moisture barriers on the inside of both layers, one for the roof batts and another one for the ceiling batts. I see no attic vents of any kind. on account of the plastic barrier on the roof, the leakage runs down over the exterior wall and tends gets into the subfloor. It also appears to protect the trusses from incoming moisture. Not sure how common this was.

when I replace the insulation, does this plastic have to be there, or am I better off adding a few gable vents and using the permeable vapor barrier that comes with new insulation, or just buying batts without?

The problem with metal roofs is that you WILL have condensation. it is how it is handled is the difference. I wish I could say "all you have to do is....."
Spray foam insulation is a solution that I hear works, but gets expensive fast.
Plastic can contain the moisture, but you need ventilation to evaporate it.

Greg