I am putting an addition on my house this summer. It is going OVER an existing deck which faces a lake, I can’t do a foundation under it because of the lake. I live in Northern NJ and it can get cold in the winter. I have been reading alot about Radiant heat and am wondering if it is installed under the floor sheathing, what type of insulation can I use (or other methods) in the floor joist to insure it will not freeze up on those occasional super cold winter nights??? I was going to use a combo of fiberglass and rigid insulation over that.

I would appreciate any insights