The MA building code says:

"5314.2.3 Attics and Crawlspaces. Within attics
and crawl-spaces where entry is made only for
service of utilities, foam plastics shall be
protected against ignition by 1 1/2-inch-thick (38
mm) mineral fiber insulation, 1/4-inch-thick (6.4
mm) wood structural panels, 3/8-inch (9.5 mm)
particleboard, 1/4-inch (6.4 mm) hardboard,
3/8-inch (9.5 mm) gypsum board, or corrosion-
resistant steel having a base metal thickness of
0.016 inch (0.406 mm)."

The advantage of fiberglass and cellulose (other than price) is that they can more easily be installed to fill the spaces between the attic floor joists.

Here is a link that might be useful: MA one & two family building code

Thanks Mightyanvil for the info. My hope in using the polyiso would be to obtain a much higher r value than using either cellulose or fiberglass. Would the ceiling from below and the floorboards that are now on the floor in the attic be sufficient to meet the building code requirements? Also, what qualifies as mineral fiber insulation? You are right that using cellulose would be ideal if only the r value was higher. Thanks

"Wood structural panels" means plywood or OSB sheets rather than boards but if they are t&g you might be able to get away with it.

I don't understand how you would be able to tightly fill the attic joist bays with foam boards. I would use sprayed closed-cell polyurethane foam to get maximum R value and an air barrier. Can you insulate the rafters too?