How to Cut Polystyrene Foam Insulation

Polystyrene foam insulation, commonly used for exterior house insulation, is manufactured in a wide variety of thicknesses and strengths. This type of insulation resists moisture and provides superior insulation from heat gain in the summer and heat loss in the winter. To cut polystyrene foam insulation, all you need are a few household tools and a supply of very sharp utility knife blades.

Foam insulation is commonly installed under the siding.
  1. Measure and mark the dimensions for your cut using the measuring tape and marker.
  2. Place the carpenter's square or yardstick on your drawn measurements, to use as a straight edge to guide the utility knife as you cut. This will ensure a straight, neat cut.
  3. Gently score the surface of the board along your mark with the utility knife. Repeat the action, applying slightly more pressure on the knife. Resist the temptation to saw the board through with the knife, or the cut will be uneven with a jagged edge.
  4. Change the utility knife blade as needed, as soon as you notice the foam snagging on the knife blade.
  5. Stroke the utility knife repeatedly into the scored surface of your cut, applying more pressure for every stroke. Work the blade through the cut until the blade goes through the board fully.

Things You Will Need

  • Measuring tape
  • Carpenter's square or yardstick
  • Marker
  • Utility knife
  • Sharp knife blades


  • To prevent jagged edges or uneven cuts, do not attempt to cut the foam board with the utility knife with one stroke, and do not use the knife as a saw to puncture through the board.


  • Use heavyduty work gloves to protect your hands from the sharp blades.
  • To prevent injury, always move the blade away from your hands and your body as you make the cut.
  • Do not use a saw to cut polystyrene foam insulation, as this will produce severely jagged ends.

About the Author

Rebecca Mecomber, a former radio broadcaster, has been a professional blogger and writer since 2006. Her articles and interviews have appeared in "The Wall Street Journal," Salon.com and several other publications, covering topics such as Federal Trade Commission policy and media regulations, blogging, home improvement and New York travel.

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