If you go with bat insulation i suggest you put some nails in the underside of the joists and lace the insulation in place with wire.
Screened porch floor insulation.
Do it yourself video on how to insulate the exposed floor over an unheated area using roxul comfortbatt r28 canada or r30 usa and roxul comfortboard is i.
Enclosed but uninsulated sitting areas are known as three season porches but if you live in a northern state cold weather can reduce that to two or even one and a half seasons.
Adding insulation batting to the walls and floor of your three season porch can extend its usefulness during cold weather.
If your floor joist are 16 inches on center you can easily install bat insulation that is assuming you can get to the underside.
If your porch is screened the first step will be either to add glass all around or to replace the screen with standard glass panels.
Roll out the fiberglass insulation sheets between the joists facing side up making sure it is snugly in place.
If it is above grade winter drafts will make the room too cold to use no matter how well the walls and ceiling are insulated.
Start with the basics thermally efficient glazed doors and windows are essential to keeping the cold out.
Use a vapor barrier to keep your floor above free from condensation caused by the new insulated area.
Make sure that you wear gloves and a mask for protection.
This can be stapled up between the floor joists.
Insulating the floor is an important step in porch conversion.