# Cutting drywall without damaging vapour barrier?



## Red Squirrel (Jun 29, 2009)

I am redoing a section of outside wall as it is badly insulated and no proper vapour barrier (there is one but it was not tied in properly). What is the best way to cut the drywall cleanly without cutting vapour barrier too? I need excess so I can tape the new one to it, if I cut it flush with the stud then I wont have any to tape to. I started off with an utility knife with blade slightly shorter than thickness of drywall but that leaves a mess of drywall in the back so the new sheet wont be flush. If I make the blade longer I risk cutting the vapour barrier and it would be next to impossible to tape that so close to the existing drywall. How is this typically done?

This is what I'm working with:



The vapour barrier is not even tied in with patio door opening so I'll add more and tape it on one of those other studs. I'll also check why there are so many of them, might be able to remove some for better R value.


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## Ron6519 (Mar 28, 2007)

I would forget that particular goal. It's not feasible. Just redo the vapor barrier.


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## Red Squirrel (Jun 29, 2009)

I still need to tie it in to existing though, and if I cut it flush, I wont be able to get the tape on it properly. Though, I just finished doing the rest of the area and and it's a pretty big mess, there's parts where there's not even any vapour barrier coming out from the uncut portion so I have nothing to tie into. Guess I'll just do the best I can, but proper vapour barrier is critical. 

Like here, I'm not sure what I'm going to do:



I guess I just tie it to the wood with staples and acoustic sealent and hope for the best? Wood is not really air tight though but guess it will have to do.

I might be able to get rid of some of those 2x4's too, some of them tie in to a steel beam over the patio door but some actually are 1 inch away from the top plate. I think some work has been done here before, I can probably remove most of the ones not holding the beam.


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## ToolSeeker (Sep 19, 2012)

Remove the drywall, fix the paper barrier correctly, re-drywal.


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## Red Squirrel (Jun 29, 2009)

ToolSeeker said:


> Remove the drywall, fix the paper barrier correctly, re-drywal.


Obviously... what I'm asking is how do I properly cut the remaining drywall without cutting through the vapour barrier. If I cut it flush with the existing drywall that I don't want to touch, I can't put my tape to tie in new vapour barrier correctly. 

In my second post I also have a situation where I can't even see where the vapour barrier starts, so when I put new do I just shove it in the crack with lots of glue or what? Vapour barrier needs to be properly tied in with the rest of the house vapour barrier or I'll be back to square one. I get frost on the drywall in this area in winter which is what prompted me to check this in first place. I don't want to end up with the same problem.

Or am I good to just tape to the wood and have a slight gap in the barrier? How good is wood at stopping drafts? I can put the tape as close to the edge of the existing drywall as I can and put a tab of black glue, will this work ok? When I put the new drywall it will squish it all together and it should seal right?


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## ront02769 (Nov 28, 2008)

Overall vapor barrier is important. A small leak in same such as you are trying to square away is a nothing.....for example, did you go through and seal the inside of all of your electrical boxes so that warm moist air from inside doesn't make it out through those many holes in your barrier?? Ron


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## Gary in WA (Mar 11, 2009)

Red S. you are thinking air barrier, a vapor barrier is area weighed, it can have holes/rips/tears and part that is still intact is the percentage, as said; *pp.5*;http://buildingscience.com/document...retarders/view?topic=/doctypes/researchreport

Be sure to plug any holes in top/bottom plate to those cavities.
ADA the drywall (your air barrier); http://www.buildingscience.com/documents/information-sheets/air-barriers-airtight-drywall-approach/


Gary


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## Red Squirrel (Jun 29, 2009)

Actually now that you mention it I don't think they wrapped it properly around electrical boxes back in the day, so I probably should go shoot some foam in those, without getting some in the actual box. 
So is proper way in my case to just cut flush and then try to tie it in as best as I can on the wood only then? I guess I can go and do that, it just feels wrong. I may be able to slightly lift the drywall to shove the new plastic under with some glue so guess that will have to do. New drywall will then press both together along with the tuck tape.


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## Nestor_Kelebay (Jun 17, 2008)

Red Squirrel:

What I do is cut up to the studs and flush with them.

Then I fasten pieces of spruce 2X2 to the studs on both sides of the hole with 3 inch long drywall screws. I generally cut the 2X2's a bit longer than the length needed so that they can be pulled tight to the back of the drywall at the top and bottom of the hole. That way, I know the front of the spruce 2X2 is flush with the front of the stud. (I countersink the holes with a 90 degree countersink to better accomodate the trumpet shaped head of the drywall screws.) I use 2X2's to give myself a generous amount of room on both sides of the hole. If you try to cut the drywall down the middle of the stud like some people will tell you to, then when you go to screw the new drywall into place, you get "breakouts" on the edges of your drywall patch where the trumpet shaped heads of the screws push the gyspum core of the drywall outward, making for what I'd consider a less secure fastening of the drywall patch. Using spruce 2X2 "nailers" allows you to place the screws further from the cut edge of the drywall to avoid those breakouts.

Then I staple vapour barrier to the pieces of 2X2 (without tying it in to the existing vapour barrier because there really isn't any good way to do that).

Then I use 1 5/8 inch long drywall screws to attach my drywall to the 2X2's.

Then I use fiberglass mesh tape over the joints being careful NOT to overlap it at the corners.

Then I use a "curved trowel" to spread joint compound over the butt joints. A curved trowel looks like an ordinary trowel until you set it on a flat surface or sight along it's edge. You then notice that the blade is curved so that it arches up about 1/8 of an inch in the middle. Since you hold the trowel at a comfortable angle to the wall when spreading joint compound with it, a curved trowel allows a total Newbie to spread a perfectly symmetrical "mound" of joint compound over a butt joint that's only about 5/64ths of an inch thick in the middle. That's more than thick enough to bury fiberglass drywall joint tape, but not thick enough to leave a visible "bump" on the wall even if you have wall mounted light fixtures.

Spread your joint compound and scrape any high spots off with a sharp paint scraper. (Hold a light close to the wall to make the high spots stand out.)

Spread joint compound and SCRAPE smooth until your repair looks not too bad, and then SAND the final application of joint compound smooth.

Prime and paint.


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## Red Squirrel (Jun 29, 2009)

Ok so you don't tie it with existing then? That was mostly my main concern as to try to minimize amount of air leakage I create. But guess I'll just go ahead and cut flush then. I cut drywall half way to the stud so wont have to put a 2x2, but I've done that before in indoor applications as well. Was my first time doing an outside wall, so there's a bit more to consider to avoid compromising thermal efficiency.


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## Nestor_Kelebay (Jun 17, 2008)

Red Squirrel:

Yes, that is correct. If you repair an exterior wall, there's really no good way of connecting the new vapour barrier with the old. So, you do the best you can by just having vapour barrier under all your drywall even if it's discontinuous at the repairs.

If you want to minimize air infiltration into your walls, then just use two or three coats of an alkyd primer when priming the repair in preparation for painting. A coat of alkyd primer will prevent air infiltration at the repair seams.


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