What type of knife for cutting batts?


  #1  
Old 11-13-18, 12:13 PM
M
Member
Thread Starter
Join Date: Mar 2009
Location: Northern Virginia
Posts: 3,913
Received 27 Upvotes on 21 Posts
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?
 
  #2  
Old 11-13-18, 12:57 PM
W
Member
Join Date: Feb 2006
Location: USA
Posts: 6,394
Received 63 Upvotes on 55 Posts
A utility knife is fine. Use a framing square as a cutting guide and to compress the batt to make cutting easier.
 
  #3  
Old 11-13-18, 01:41 PM
XSleeper's Avatar
Group Moderator
Join Date: Dec 2004
Posts: 27,021
Received 1,900 Upvotes on 1,706 Posts
You have to change blades often because the fiberglass dulls the blade.
 
  #4  
Old 11-13-18, 01:51 PM
M
Member
Thread Starter
Join Date: Mar 2009
Location: Northern Virginia
Posts: 3,913
Received 27 Upvotes on 21 Posts
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.
 
Attached Images  

Last edited by mossman; 11-13-18 at 03:08 PM.
  #5  
Old 11-13-18, 02:10 PM
Norm201's Avatar
Member
Join Date: Sep 2013
Location: United States
Posts: 10,949
Received 716 Upvotes on 637 Posts
A large scissors will work also, but I usually use a simple utility knife.
 
  #6  
Old 11-13-18, 02:12 PM
2
Member
Join Date: Jan 2014
Location: USA near Boston, MA
Posts: 2,246
Received 387 Upvotes on 337 Posts
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.
 
  #7  
Old 11-13-18, 05:08 PM
Marq1's Avatar
Member
Join Date: Sep 2016
Location: USA MI
Posts: 9,745
Received 1,210 Upvotes on 1,098 Posts
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
 
Attached Images  
  #8  
Old 11-13-18, 07:20 PM
czizzi's Avatar
Member
Join Date: Jul 2007
Location: USA
Posts: 6,541
Received 15 Upvotes on 13 Posts
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.
 
  #9  
Old 11-13-18, 07:57 PM
B
Member
Join Date: Oct 2008
Location: New England
Posts: 9,460
Received 47 Upvotes on 43 Posts
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
 
  #10  
Old 11-13-18, 08:26 PM
H
Member
Join Date: Nov 2012
Location: USA
Posts: 2,291
Received 283 Upvotes on 243 Posts
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.
 
  #11  
Old 11-13-18, 09:15 PM
XSleeper's Avatar
Group Moderator
Join Date: Dec 2004
Posts: 27,021
Received 1,900 Upvotes on 1,706 Posts
...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
 
  #12  
Old 11-14-18, 07:49 AM
M
Member
Thread Starter
Join Date: Mar 2009
Location: Northern Virginia
Posts: 3,913
Received 27 Upvotes on 21 Posts
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.
 
  #13  
Old 11-14-18, 09:21 AM
B
Member
Join Date: Oct 2008
Location: New England
Posts: 9,460
Received 47 Upvotes on 43 Posts
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.
 
 

Thread Tools
Search this Thread
 
Ask a Question
Question Title:
Description:
Your question will be posted in: