My water heater blew while I was on vacation and left my equipment room under an inch of water for several days. Needless to say, the drywall got moldy and I had to tear it out. Now I'm trying to replace the drywall behind the HVAC blower, but it's a super tight space (see picture).

butt joint behind HVAC

If drywall tape were adheasive on my own I could align the tape from either side of the HVAC unit, but sadly it's not. so I'm wondering, how should I fill this butt joint?

I was considering using duct or masking tape, or even spray foam, but assume that would just make a huge mess.


  • See my answer Method 3 will probably be what the doctor ordered..
    – Ken
    Mar 7, 2018 at 6:55
  • The picture is confusing me as to just what kind of joint is happening there. It looks like maybe this is an outside corner in which case it is conventional to use a corner strip before trying to use joint compound. Can you possibly sketch an overhead figure of the joint location to provide a better sense of how this is laid out?
    – Michael Karas
    Mar 7, 2018 at 13:39
  • 2
    The OP is talking about the horizontal but joint behind the air handler.
    – Ed Beal
    Mar 7, 2018 at 15:13
  • Can't you just leave it the way it is? Reach in a few inches with tape, but don't worry about the rest?
    – JPhi1618
    Mar 7, 2018 at 16:53
  • I cut my dry wall such that it would fit in my car and provide overhang on either side of the HVAC / Duct work (3.5' x 6' sheet). There's about a foot overhang on the other side that allowed me to get enough extra screws in to secure it.
    – virtualxtc
    Mar 7, 2018 at 20:54

2 Answers 2


I am sorry to hear about your water damage issue - stinks for sure.

You mention if it were adhesive you could do it from either side of the hvac - sounds like you have access to the back side of that unit ?? If So see method 1 if not see method 2.


Use the dry wall tape on a flat stick and push to the back side so you can get to it. Then use Push Pins (like thumb tacks) to hold it in place. Pull it up into position on the front side and secure it with pins - you can then run some joint compound over it a long handled putty knife (probably need to make your own handle here) or use a 'paint roller' to try and spread it flat.


To repair that tight space dry wall butt (amazing you installed the dry wall there). What I would do is make a jig that can hold my dry wall tape on one end like a long flat bar (much like you have sandpaper in a small sander , place joint compound on the end and slide that in place and get the end in the correct position. Set your jig in the proper position that the tape would be able to be pulled across flat . Wait for the joint compound to dry - now it is secured to the back side and you can add compound with a roller or a long handled putty knife while pulling the tape tight and flush.


Some drywall tape actually does have an Adhesive Backing - look for the WEB TAPE with Adhesive..

Self Adhesive Drywall tape

EDIT 8 MAR 2018

Modified Putty Knife

  • 1
    Method 3 is the way to do. I use this mesh tape for ALL drywall joints. The adhesive backing keeps it in place before any wet drywall mud is applied. Several parallel overlapping runs of the mesh tape can even be used to bridge a fairly wide gap.
    – Michael Karas
    Mar 7, 2018 at 13:32
  • I was aware of mesh tape, but the holes in it are problematic as I am uncertain of my ability to actually cover all of the mesh with joint compound given the small clearance as it would extend past the reach of the recommended barbecue spatula method mentioned below. The sand paper stick method didn't seem practical due to the small amount of compound it can hold, but perhaps combining the two methods will work.
    – virtualxtc
    Mar 7, 2018 at 20:52
  • 1
    @virtualxtc - you will need to make your own putty knife with 4 inch space you have a difficult job. Even sealing from the backside of the wall the compound will not flow through - sealed yes. From your comments I am guessing this is backside and you removed side wall to get access? I would recommend an 8 inch wide putty knife (with a 1/4 inch slow bend in the blade ) attached on the end of a broom stick. I will try to post an example later.
    – Ken
    Mar 8, 2018 at 23:40

The best tool for this job is a barbecue spatula. I has a long, light handle, usually very simple form, with a smooth and flexible blade on the business end of it. You might have a machete laying around which would be just as useful.

alternate taping knife

  • My picture was shot though a wall I had opened in order to replace more drywall. This method seems ideal for this wall which has a bit more clearance (~4") from the unit; though, I'm considering just sealing it from the inside portion of the wall.
    – virtualxtc
    Mar 7, 2018 at 20:56
  • 1
    If it’s fireproofing that seam that you’re most concerned with, how about slipping a 1/4” cement board behind the furnace. You can butter the backside of it with compound and overlap existing seams. A galvanized steel sheet might even be better.
    – herb guy
    Mar 7, 2018 at 21:46

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