Methods for Cutting Rigid Insulation


Rigid foam insulation can be used in wet and underground locations. Due to its composition, this foam board can be used to insulate the outside of home foundations and the exterior and interior of basement walls. In most cases this foam is glued to masonry walls using a construction adhesive. The foam board insulation can be purchased in thicknesses that range from ½-inch thick into blocks of 2 feet thick. The material can also be glued together to form thicker blocks and sections.

Razor Knife

  • Thin sheets of the foam board can be accurately cut with a sharp razor knife. Typically thickness up to 1 inch can be cut in this manner. Lay the foam board on top of a sacrificial ¾-inch thick piece of 4-foot by 8-foot plywood. The foam board comes in this same dimensional size. Mark the foam using a tape measure and make a small score mark with the knife. Use a chalk line box to pop a line for cutting. You can use a long, straight 2-by-4 as a cutting straight edge or a 4-foot long drywall T-square as a guide for the knife. Cut the foam board to size. You may have to make a couple of passes with the razor knife for thicker material.

Circular Saw or Handsaw

  • For thicker material that is deeper than the length of the razor knife you can use a circular saw or handsaw. Lay out the cuts in the same manner using a chalk box and measuring tape. Set the depth of the circular saw so the bottom of the blade is held up from the ¾-inch thick plywood by 1/16 inch to 1/8 inch. This will keep the blade from cutting into the wood. Cut the foam with the circular saw. The foam will still have to be broken free from the thin remnants of the 1/16-inch to 1/8-inch material. Handsaw cutting is still measured in the same method as described above. Cutting the material may require the use of another person to hold the edge of the foam. Lay the foam over the edge of a set of sawhorses. The cut line will have to be free of any interference from the sawhorses.

Hot Wire Cutting

  • Ni-chrome wire can be used as a hot wire cutting tool for foam board. Typically these electrical cutters can be fashioned from a piece of electrical conduit. The conduit is shaped like a large bow. The Ni-chrome wire is stretched between the open ends. The wire must be electrically insulated from the metal conduit bow. Electrical power can be supplied from a 12-volt car battery or a home-built power supply. The wire is heated by the electrical power. This cuts the foam by melting its way through. The material is laid out and measured in the same methods described. The foam can be set on sawhorses just like the handsaw cutting method.

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