Cutting batt insulation can be a real pain, especially if it's the thicker R-30 variety; here's an easy way to accomplish your cuts without making a huge mess, wasting the material, and having to hit the bathroom first aid kit because, in your enthusiasm, you nicked a finger.
A brief review -- there is a variety of construction insulation, but homeowner-friendly material generally falls into four categories: loose fill, foam board, spray, and batts or rolls. Loose fill doesn't require cutting (unless you count cutting open the bag), spray comes in a can, board is generally found in a 1", or smaller, thickness and is readily cut, and batts. That's our project.
So we begin:
- You need -- a smooth cutting surface, a piece of straight material against which to cut (e.g. 2x4, 6' level, framing square, big carpenters square), a sharp knife (perhaps an old Ginsu knife left over from a late-night infomercial buying spree) or razor knife, and, of course, a piece of roll or batt of insulation. I have used a batt for my purposes.
- Place the batt on the cutting surface, take your measurements ("measure it twice, cut it once") and lay the straight edge along the imaginary line corresponding to your measured size. A hint -- with batt insulation, it's preferable to make your cut piece a bit wider rather than narrower; you can always squeeze the insulation to fit into the cavity, but a piece too narrow allows unwanted heat transfer (thus negating the insulating value of the material.)
- Push down hard on the straight edge to compress the thick batt and allow for easier cutting; I always cut with the insulation facing up, rather than the paper lining facing up. The more you compress the insulation, the easier it is to cut.
Cut the batt in short strokes, rather than in one single motion -- that makes it easier to keep the insulation compressed with the edge. (You can disregard this instruction if you have 9' arms and you can cut an 8' batt in one fell swoop.
A final note -- be sure that you always have the appropriate safety gear on hand and in use; I'm thinking protective mask, gloves, and a stable cutting surface.
Voila' -- you're finished; shove that baby between the studs and grab another batt.
(All photos by the author.)