This weekend I started using my 5th box of Poly-Fill. You know that soft fluffy stuff that makes pillows and stuffed animals so warm. It makes great insulation. Especially when you don’t want the itch of fiberglass. This is going everywhere that the rigid foam board did not fill the whole space. In this case the gap between the vapor barrier and the foam. It also fits nice into every little crack and nook. Just give it a little nudge with a ruler or pencil eraser and the gap is gone.
I cannot speak to the ‘Flame’ characteristics of using this product. I made a choice based on where it would be and likelihood of flame/heat contact.