Use a ruler and pencil to mark vertical lines the length of polyisocyanurate board showing where the bend will start and where it will end. Draw vertical lines every two inches between these two lines. The side of the board you are drawing on will be the outside of the bend.
Score along the lines you have drawn using a ruler to ensure accuracy and making sure you only cut through the top layer of the insulating board.
Orient the board so that the correct side is facing you. If you are pushing the board into a corner, the scored side faces away from you. If you are wrapping the board around an object, the scored side faces toward you.
Press the board gently into the bend. On corners the board can be held in place using insulating tape but if you are wrapping a pipe it is worth using metal strapping. Tighten strapping around the pipe and use the dispenser to cut and secure the strap.
Put tape and then caulk over any cracks.