My sunroom was at one time a back porch built of solid cedar that has since been enclosed and insulated by a previous homeowner. They used paper-backed fiberglass that has long since been shredded by rodents, and they had run flexible ductwork to two floor vents that is also a dangling mess. You can see what that looks like and the access from underneath in the album below.

I might be able to get under there and replace all of that with some other insulation (mineral wool batts? XPS boards?) between the joists and do a better job of covering over it with steel mesh or plywood to keep animals out of it, but I am really dreading it and would prefer to work from the top if I can find a way.

Or, could I use something like closed cell polyisocyanurate boards directly on top of the deck boards, then cover that with subfloor? It's not a typical application I can find, but I have found example products rated for >22psi compressive strength, which seems plenty with it evenly distributed between the subfloor and decking. There is room to raise the floor a couple of inches if needed to accommodate this.

Are there other options I should look into?

The floor vents are getting removed and covered over no matter what. The space is small and has a large double door opening to the rest of the house. I plan to put in either vinyl or laminate floating floor and may include in-floor heating at some point.

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Imgur album link