I just wonder whether a prefab type trussed attic would cause problems with the ceiling below if floored.
All Prefab trusses are best left aloneThey were never designed to store your stuffYou can add some 2x8 lumber, from bearing to bearing, deck that floor; and store your junk thereAnd as an added bonus, you still have 12 of insulation on the ceiling.
Remove the styrofoam insulation,install a batt insulation,the pink itchy stuff,cover and seal it with 6mil cgsb poly then proceed to reinstall your new drywall.The thickness of your insulation will depend on the thickness of your framing,eg 2-1/2 studs would require an r8 insulation,3-1/2 studs an r12.
