Measure the wall, ceiling or other installation area to determine the width and length of extruded polystyrene sheet that you will need.
Lay the polystyrene sheet on a flat surface -- such as a table or work bench -- and mark the length and width measurements on it using a tape measure and a pencil.
Lay a straight edge along the pencil line and use a utility knife to cut 1/4 inch into the sheet all the way down the line.
Set the utility knife and the straight edge aside and slide the polystyrene sheet over the edge of the table until the cut line hangs off by 1 inch.
Place one hand flat on top of the section that remains on the table and grasp the free edge with your other hand. Snap the free edge downward using a quick motion to snap it off the sheet.
Things You Will Need
- Tape measure
- Straight edge
- Utility knife
- Extruded polystyrene can also be cut while standing upright. After cutting 1/4 inch into the front, push a knee into the backside of the sheet where the cut is, then pull the two sections back until they snap. You may need to use a utility knife to cut through the last remaining bit of plastic covering.