I would avoid using great stuff wherever possible. I think the adhesion properties are a negative and not positive. Like others said it will not foam properly if not given a place to vent to.
Those foam panels are great, are they just across the middle section or fully from left to right?
If I had that "gift" of insulation, I think I would put it right back, then get some of the Styrofoam boards and cut into strips and use as "chinking" you know, like how a log cabin has rope as caulk between the logs. The trick is to get the size right so you don't compress the foam.
I would then follow up by taping all the joints with HVAC tape, then add thermal break strapping (wood planks) screwed into the roof ribs for something to attach stuff to later.
After THAT, I would add another layer of insulation that sits flush with the wood strapping to try and bulk your insulation up some more.
After that insulation is in, tape the seams between new foam and the strapping strips.
Don't forget to insulate your floor. If you can, do it from underneath - don't you have plywood floors? It might make a great attachment point from under the bus so you don't loose height on the inside.
Originally Posted by Robin97396
Would it be bad to spray foam, then press in the panels? Might affect the drying or curing of the foam. I'm saying that because going around the insulation panels with Great Stuff wouldn't seal the same as spray on insulation.