Rigid foam as floor insulation

rkhpapa

I'm converting an existing screened porch (New Hampshire) into a 3 season room. The floor is 5/4 X 6 decking which I need to cover and finish. I only have 1 13/16 to play with until I come flush with the existing threshold. I was thinking about 1" thick foil faced rigid foam directly down on that decking and then covering that with 5/8 T&G plywood. That would leave me somewhere around 3/16 or 1/4" for indoor/outdoor carpeting or some other finsh. Here are my questions:
-do I need "sleepers" to keep that foam from crushing or can I just screw the plywood down through the foam?
-if I do need the sleepers, how would you space them?
The floor is 16' across the joists by 10' deep.
Thanks for your thoughts.
Rich

redline

can you install the insulation on the under side of the floor decking?

rkhpapa

That's my problem. There isn't enough room between the grade and the floor joists for me to crawl under there. That's why I'm trying to insulate it from the topside.
Thanks,
Rich

fqp25

Supposedly, you just lay down the polystyrene foam insulation, then the top flooring and screw it into the joist. I've only used it under durarock and tile floor. I don't know what kind of effect it might have with a plywood top, and a space that large.