G'morning Tony,
Check out that foil/poly stuff I mentioned: Reflectix. In 5/16" it's seven layers with foil on the outsides. Seven layers makes this stuff quite firm, even if it has bubble material inside. Firm and flexible In an email exchange, they suggested using a variant that has white poly on one side - that side would be glued to the hull.
There is a spray adhesive advertised in the gardening catalog called
Camie 363. It's a type of contact cement, I guess. It is meant to be sprayed on in spots (on the glass & frame of yer greenhouse.) The bubble/foil is pressed on after it sets. With a magnifying glass you can see on the can in the picture that it's 'highly flamable'
These cements are very toxic, so I just try to avoid them, especially in closed spaces. I tried to garner info on what Reflectix would recommend but they wouldn't commit. I'll ask again, You don't want this stuff to let go from the hull in heat or cold.
But I would argue that you have to make sure whatever insulation you use, especially if you cover it, that it is totally glued to the hull. No spaces. And if I end up doing it with the ribs, I will make sure that there are no spaces behind the liner. Which means that the ribs and the insulation have to be the same thickness.
(I was going to add: It's not only condensation and mold in those spaces you worry about. It would make a great hotel for bugs like cockroaches.)