Floor insulation/sound deadening

[russcc]

I used some 1" HVAC duct insulation on the floor of the '40 coupe behind the seat. It's been in there a year or so, and when I took it out to check something there was some mold in the stiffening valleys. What I would like to find is a breathable membrane to go between the steel floor, and insulation. They use a thin mesh like that today between the wood roof sheathing and, in my case, red cedar shingles. Any other ideas ? Thank you Fordbaners.