PFT installation expert Jared explains how to cleanly cut foam insulation to prevent “fly-aways”.
You want to start by laying your insulation on flat surface. If it is contoured insulation, such as Fullback for vinyl, lay the panel down so the flat back of the insulation is facing upward. Use a long-bladed utility knife with the blade extended all the way out, cutting at an aggressive angle through the insulation. This makes for a nice, clean cut.
What you want to avoid is using a short-bladed utility knife with a sawing motion. This is a common mistake that leads to an unclean cut.