Last Updated on May 12, 2023 by Dave Farquhar
Foam insulation board claims to be easy to cut. It says right on the board, usually. But when someone saw me cutting thick foam boards in the Home Depot parking lot, he said, “You’re a braver soul than me!” In this blog post, I’ll show how to cut foam insulation board with simple and very inexpensive hand tools, and minimal mess. The best way to cut foam board even allows you to cut it right in the parking lot, so the foam will fit in a small SUV.
Thin foam board will cut with just a utility knife, but you can easily cut the thick 2-inch boards, even with just simple hand tools.
Using a drywall saw: the most common way
The most common way is to cut foam insulation board with a drywall saw. It cuts really easily. All you need to do is draw your line, stand the board up, then saw along the line as close as possible. This method can take a few minutes, especially if you have to cut the full length of an 8-foot sheet. It’s more time consuming than difficult. The most difficult part is keeping your blade straight, but that requires more patience than strength.
But I wouldn’t call this method of cutting thick foam insulation board clean. And the mess is harder to clean up than cutting the foam was. The polystyrene shavings attract static electricity so it really likes to stick to the broom or vacuum cleaner. So I don’t like to use this method to cut the full length or width of a thick foam board.
But if you need to make a small cutout inside the board, a drywall saw is probably the best way to do it. The 2×3 cuts to accommodate an electrical box come to mind. I have found I can cut down on the mess a bit by cutting with a utility knife first, then finishing the cut with the saw.
So I only use my drywall saw to cut openings in the foam board. To cut foam insulation the full length or width of the board to break a 4×8 board down into the dimensions I need for my project, I prefer another method that’s much faster and more precise. Not only that, it doesn’t make a mess either.
Best way to cut foam insulation board without making a mess
The best way to cut rigid foam insulation without making a mess requires very simple, cheap and common household tools, even with thick 2-inch board:
- A wooden 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, depending on the widths you need. You can break a 4×8 sheet of 2-inch foam insulation board into thirds with just a piece of lumber.
You can get the 1×4 lumber at the store too. Be sure to check the cull lumber section. Cull lumber is my very favorite DIY trick. But a 1×4 furring strip costs less than $3 if you don’t have one laying around and the selection at the cull lumber bin is slim.
Cut foam insulation to perfect 16-inch widths with only a wooden board
If you look closely at the foam board, at least the thicker ones, there are slight grooves cut into it at 16-inch intervals. You can use these to break thick foam board into perfect 16-inch widths to fit between your studs, as long as you placed them exactly 16 inches apart. If you haven’t framed your studs yet, use one of these as a spacer when you do, to make your life easier.
This means you can break the board down into perfect 16-inch widths without a knife, and with little or no mess.
To break thick foam board into perfect 16-inch widths, just place your 1×4 board right along one of the grooves. Stand on the 1×4, or kneel on the 1×4 if you have knee pads. Then reach down and pull the foam 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.
How to cut foam insulation board to custom lengths or widths
Frequently you’ll need custom lengths or widths of foam board. 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. Don’t worry about cutting all the way through, or even halfway through, because we’re going to take care of that in the next step. Repeat on the other side for the cleanest possible cut. You can skip this step, but you risk a jagged break.
Then set the foam on the ground. Place your 1×4 board on top of the foam along the cut. Stand or kneel on the wooden board, and pull the foam board up toward you. The foam board will snap along the cut. You’ll get a clean, smooth edge with little to no mess.
Cutting thin 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 foam board anyway, but try either way and go with whichever way you find works best for you.
And that’s how to cut foam insulation cleanly, in a nutshell. Cut part way with a utility knife on each side of the foam, put a length of 1×4 lumber along the line, stand on the 1×4, then pull the foam board toward you.
David Farquhar is a computer security professional, entrepreneur, and author. He started his career as a part-time computer technician in 1994, worked his way up to system administrator by 1997, and has specialized in vulnerability management since 2013. He invests in real estate on the side and his hobbies include O gauge trains, baseball cards, and retro computers and video games. A University of Missouri graduate, he holds CISSP and Security+ certifications. He lives in St. Louis with his family.