Originally Posted by gbaglo
If, and I repeat if, the vinyl liner sagged, how would condensation get through it to get behind it?
I thought the issue being discussed was water from a leak getting trapped behind the vinyl and I repeat, I've not seen a post on the topic in the eight years I've been on the forum.
Just because "you" haven't seen a post on this Forum doesn't mean it hasn't happened. Many owners don't use this forum for various reasons including they are uncomfortable with the discreditation posts by folks who should be to 'lubricating' discussions, not causing 'friction'.
In simplest terms, the ‘sag’ becomes a terrarium growing mold…..
I would guess you have lived in a house with single pane windows. When it got cold outside you saw water, coming out of the air near the window, condense on the window. If you never wiped off the window frame and sill you would see mold forming and it would slowly get to be a moldy mess.
Quote from article: “This describes the situation with molds. They don’t magically appear or re-incarnate when conditions are right. They don’t seep through solid walls as is often assumed. They are always there, floating in the air or carried in moving water, in a dormant state, just looking for the right combination of moisture, heat, and food sources to begin rapid growth.”
When the headliner sags the area behind it fills with air from somewhere or there would be a vacuum and the headliner would never drop to the pull of gravity.
That air is pumped in/out to some degree from changes in barometric pressure 'refreshing' the air. And the thermal stress of heating and cooling in the area slowly expand the “sag”.
When your 1/8" (.125") of fiberglass shell cools below the dew point moisture from that air condenses on the shell. (like the single pane window) some of that water gets into the foam (like the window sill) and mold forms. The mold spores are ‘there’ (see quote above) and multiply rapidly.
After condensing add some heat from the sun beating on the fiberglass and water vapor is formed. It may find its way to the spores or returns to the air. And the cycle continues.