Probably the easiest way would be to have it sprayed with closed cell foam and then paint it with a UV protector (assuming light can get to it). If you use fiberglass you will just be making a great nest for critters unless you put plywood or some other material over it.
Welcome to the forum! Is the deck designed for the concentrated glass load? What R-value is called out on the deck/living space conversion plans?
Gary
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