Cutting Rigid Foam Board Insulation

A large stack of thick pieces of foam board insulation.
  • 2-3 hours
  • Beginner
  • 0-60
What You'll Need
Safety gloves
Safety glasses
Tape measure
Crayon, magic marker, or dry erase marker
Long straightedge
Small rubber clamps
Utility knife with adjustable blade

Foam board insulation is a relatively inexpensive type of insulation that adds a high R-Value to areas where adding standard installation is either cost-prohibitive or not required. Foam board insulation is often used in garages, work sheds, or other spaces that may not require interior finish or wall trim.

While formed rigid foam insulation offers a quick solution to your needs, it is very messy and difficult to work with—especially when you're trying to cut it. Follow these steps for a better result and an easier cleanup.

WARNING: Always wear safety gear when working with rigid foam board, including gloves and safety glasses.

Step 1 - Measure and Mark

Measure the distance on the foam board insulation that you wish to cut, and then make a mark there. This will serve as your cutting line. Always remember and follow the adage "measure twice, cut once" so you don't get the wrong sized pieces and end up wasting material.

Step 2 - Prepare to Cut

Next, take your long straightedge and line it up with your cutting lines. Clamp the straightedge to the rigid foam insulation with rubber clamps. Make sure they are securely and firmly in place so that the pieces will not shift or slip during the cutting.

Step 3 - Cut the Foam Insulation Board

With your utility knife, score the foam insulation board by firmly running the utility knife along the outer edge of your straightedge. This will result in a small impression or etching in the board.

While cutting, make sure that your knife cuts to the outside edge, or "waste area," of the board. That way, if you make a mistake, it will not damage or destroy the entire piece.

Once you finish the scoring the board with your utility knife, remove the clamps and straightedge.

Step 4 - Complete the Cut

Lift the foam insulation board so that the sheet is sitting upright. Then, snap along the scored cut by pressing hard against the backside of the score (etching) line while securely holding the side edges of the board. If you have properly scored the foam board, it should simply pop or snap off. Remember to snap the foam board from the uncut side of the insulation board or it make not work as you want it to.

Continue this method for all the pieces you need cut for your insulation project. This procedure should produce precise, clean cuts and less waste in the end. However, keep in mind that this method may be hard to use without help for particularly large sheets of rigid foam insulation, so adjust as needed.