I want to insulate the roof of the 39 behind the headliner. I've had issues with the butyl rubber/ foil backed products not adhering when used on upside down and vertical applications. Especially when cooking in the florida sun. I dread having issues above the headliner requiring headliner to get pulled back out. The old Poncho has/had thick tar paper from the factory and it stayed 80+ years but is degraded. So what's a good plan? Styrofoam? Open or closed cell foam? Ideas? Experiences good and bad are welcome..