What type of knife for cutting batts?


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Old 11-13-18, 12:13 PM
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What type of knife for cutting batts?

I started installing the batt insulation in my garage over the weekend. Things were going pretty smoothly, but my blade began to get dull and tear the vapor barrier while cutting, which was extremely frustrating. Cutting the insulation itself is not an issue. It's cutting through the vapor barrier that is becoming a problem after a few cuts. I've been using a heavy duty utility knife with snap-off blades. Is there a better knife that I can use that won't dull so quickly?
 
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Old 11-13-18, 12:57 PM
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A utility knife is fine. Use a framing square as a cutting guide and to compress the batt to make cutting easier.
 
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Old 11-13-18, 01:41 PM
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You have to change blades often because the fiberglass dulls the blade.
 
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Old 11-13-18, 01:51 PM
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I've been limiting my use of a framing square and trying to make as many cuts as possible with the knife only as I install. This is the quickest, but is frustrating without a sharp knife. Is a longer blade better than a shorter one? In other words, the snap off kind that can be extended several inches compared to a box cutter. Maybe something a little higher quality that doesn't require constant replenishing, like shown below. Or I could cut the vapor barrier with one blade and the insulation with another. Tearing the vapor barrier is the irritating part. I see why some prefer to install unfaced batts and the vapor barrier as a separate step.
 
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Last edited by mossman; 11-13-18 at 03:08 PM.
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Old 11-13-18, 02:10 PM
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A large scissors will work also, but I usually use a simple utility knife.
 
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Old 11-13-18, 02:12 PM
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I use a 6 inch hunting knife against a board or square. The blade is honed occasionally especially at the tip to keep it sharp.
 
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Old 11-13-18, 05:08 PM
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Get yourself some 12" or 15" scissors, you will always find a use for them, like cutting insulation!

https://www.doityourself.com/forum/a...1&d=1542154114
 
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Old 11-13-18, 07:20 PM
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I use a simple utility knife but always do my cutting on a piece of lumber. This prevents dulling of the blade by hitting concrete under your cut. I cut vapor barrier down, slice through the insulation in several passes and then do one final pass through the vapor barrier.
 
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Old 11-13-18, 07:57 PM
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Just to comment, but current guidance says to skip the VB in all but the extreme north and deep south. Focus on air sealing and allow the wall assembly to dry to inside or outside.

Citations available.

Bud
 
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Old 11-13-18, 08:26 PM
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Best I've heard for cutting lots of same-sized batts is to compress the batt between a pair of 2 x 4s and then zip along with an angle grinder using a thin cut-off-wheel that you'd use for cutting fiberglass panels.
 
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Old 11-13-18, 09:15 PM
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...and not to be nitpicky but the kraft facing is a vapor RETARDER, not a vapor barrier. Kraft facing has a perm rating of around 1.0.

I never knew cutting fiberglass was so difficult. lol
 
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Old 11-14-18, 07:49 AM
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The fiberglass is the easy part. It's the dulled blade tearing the vapor retarder that is the issue. I was trying to avoid constantly measuring wall cavities and cutting on the floor because I know the more efficient way is to cut it to fit on the wall.
 
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Old 11-14-18, 09:21 AM
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Actually, measuring and cutting on a hard surface does a much neater job, and the installation performs better. A lose fit equals poor insulation. Insulation crews do it the way you want to but that is because it is faster, not better.

Bud

And I seem to keep avoiding your actual question, standard utility knife (thin style) and a large pack of blades. You can use multiple knives so when all need a blade change you do them all at once, with coffee.
 
 

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