Rigid foam insulation board claims to be easy to cut. It says right on the board, usually. But when someone saw me doing that in the Home Depot parking lot, he said, “You’re a braver soul than me!” But if you know my trick, it’s not hard. Here’s how to cut rigid foam insulation board, with simple hand tools, and minimal mess. The best way to cut foam board even allows you to cut it right in the Home Depot parking lot, so the board will fit in a small SUV.
Thin foam board will cut with just a utility knife, but you can easily cut the thicker 2-inch boards, even with just simple tools.
Using a drywall saw
If worse comes to worse, you can cut rigid foam insulation board with a drywall saw. It cuts really easily. And if you have to cut a small amount, say, less than a couple of inches, this probably is the easiest way. Draw your line, stand the board up, then saw along the line as close as possible.
It’s not especially hard, but it can take a few minutes, especially if you have to cut the full length of an 8-foot sheet. And I don’t like the mess. The polystyrene shavings attract static electricity so it really likes to stick to the broom or vacuum cleaner.
And if you need to make a small cutout, this is probably the best way to do it. The 2×3 cuts to accommodate an electrical box come to mind. You might be able to minimize the mess by cutting with a utility knife first, then finishing the cut with the saw.
So I prefer another method that’s much faster, more precise, and makes less mess.
Best way to cut rigid foam insulation
The best way to cut rigid foam insulation requires very simple tools:
- A board, 6-8 feet long (I prefer a 1×4, but a 2×4 will work)
- Knee pads (optional but recommended)
- A utility knife
- A permanent marker
- A tape measure
Sometimes the knife, marker, and tape measure are optional, especially in the parking lot.
You can get the board at the store too. Be sure to check the cull lumber section. Cull lumber is my very favorite DIY trick.
Using the factory spacing to get perfect 16-inch widths
If you look closely at the foam board, at least the thicker ones, there are slight grooves cut into it at 16-inch intervals. If you placed your studs exactly 16 inches apart, you can use these to break your board into perfect 16-inch widths. If you haven’t framed your studs yet, use one of these as a spacer when you do, to make your life easier.
To break the board into perfect 16-inch widths, just place your 1×4 board right along one of the grooves. Stand on the board, or kneel on the board if you have knee pads, kneel on it. Then reach down and pull the board up toward you. It snaps along the line surprisingly easily. You can do it yourself in the parking lot of the store, just park in the back, where you’re likely to have a bit of room. I like to use the adjacent parking spot, if I can find a lonely enough section of the lot.
And hey, someone who sees you doing it might even be impressed with your bravery, or skill.
Cutting rigid foam insulation board to custom lengths or widths
Frequently you’ll need custom lengths or widths of foam board, so that’s where the tape measure, marker, and knife come in. Measure the dimensions you want, then mark them with the marker. Cut along the line with your knife. Repeat on the other side for the cleanest possible cut. You can skip this step, but you risk a jagged break.
Then place your 1×4 board along the cut, and stand or kneel on it, and pull the board up toward you. It will snap. You’ll get a clean, smooth edge with little to no mess.
Cutting thin rigid foam insulation board
Rigid foam insulation board comes in various thicknesses. The half-inch or inch-thick boards can simply cut with a utility knife, if you want. I generally find it easier to make a shallow cut and snap it like I would 2-inch board anyway, but try whichever way you find works best for you.