Hi, I’m doing that now on my Safari. We had a vent not closed tight when it was shipped to us last January. We didn’t realize the subfloor was soaked until 2 weeks ago. Anyway, we pulled down what Airstream calls insulation to let it dry.
I had the same concerns about putting the insulation against the subfloor. We made “hat” sections to space the insulation board 1/2” from the floor. I made them out of 20 ga stainless steel- because that’s what I had (we own a sheet metal shop
) and screwed them to the underside of the floor. I put some rivnuts on the flat section so I could screw the foam board to to them. I was worried that sheet metal screws would come loose over time. Screws will have a drop of Loctite on them.
Key is going to sealing around the gap between the floor and the insulation otherwise the insulation won’t be effective. My plan, although I didn’t have time to try it today, is to use expanding foam around the parameter of the panel and put it in position before it cures. It should make a form fitting gasket against the subfloor. I hope.
In the photo, you can see the headless screws that will punch through the insulation board when dry fitting it so I can locate where the screws go. That part works great.
I’m traveling all week so it will be next weekend before I get this done. I’ll let you know how it goes.
Steve