Fiberglass pipe insulation sections are cylindrical in shape which makes cutting sections straight somewhat difficult.  Small pipe sizes can be cut normally without much issue. Problems arise however when we cut pipe insulation sizes that have outer diameters great than the length of the knife blade.  Cutting these larger sections incorrectly will provide two uneven sections.  When butting up even the slightest uneven edge of pipe insulation will result in a gap in the insulation system.  On a cold system, this gap will be vulnerable to condensation.  On a hot system, the gap results in a break in the thermal performance.

HOW TO CORRECTLY CUT FIBERGLASS PIPE INSULATION STRAIGHT:

Safety is always first!  Make sure to wear an N95 Dust Mask and eye protection.  We also highly recommend making all cuts outside if possible for dust prevention.  

1.  Find out exactly how long you need the piece of pipe insulation to be.  Take your tape measure and mark the measurement onto the pipe insulation piece.  You can take your knife and make a small gash in the ASJ jacket or simply mark with a pencil.  

2.  Now, turn the pipe insulation section slightly and remark the pipe insulation.  Repeat this multiple times (the larger the piece of pipe insulation the closer the marks should be made together.)  

3.  Now take your knife blade and make your cut by connection the dots.  We recommend not to cut through the center of the insulation to the other side.  You should cut one half at a time and connect back at the beginning.

*This method can be used by cutting the outside of the pipe insulation (as described above), or you can open up the pipe insulation section and mark/cut the inside of the pipe insulation.  You may prefer either method depending on the size/thickness of the pipe insulation being cut.

You can correct any uneven cuts by shaving off fiberglass with the knife at the longer points.