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In trying to diagnose/fix a leak (details here), I cut a hole in the drywall near the shower. I found that immediately behind the drywall was some sort of barrier that was both rubbery and a little paper-like. For what it's worth, this barrier appears to be only on the back of the drywall -- I see bare wood on all other surfaces when I peer into the hole.

Someone suggested that this was a humidity/moisture barrier, but it doesn't look like the thin plastic sheets when searching that online. Any ideas on what this barrier is and how to patch it (or if I even need to)?

Edit: Based on an answer below, I added an Image 3 schematic of what I think the cross section looks like...

Image 1: Location of hole (green arrow):

location of hole

Image 2: closeup of hole:

drywall backing closeup

Image 3: cross-section schematic: cross-section schematic

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    Being in a shower, probably a water/leak barrier for showers/tubs, much heavier than simple vapour barriers. Unknown if patch kits are available but imagine so. If so a larger hole will need to be made by very carefully removing the drywall.
    – crip659
    Dec 16, 2022 at 1:06
  • Can you confirm that the tar paper is, in fact, adhered to the inside of the wall in which you made your hole? If so, its purpose is probably an additional layer of defense for this side of the wall should there be a leak in the shower pan. Additionally, it will probably be rather difficult to make a full, seamless replacement without opening your hole stud-to-stud, top to bottom and laying another piece against the wood then covering the whole thing with more drywall. You could, IMO, get away without it.
    – FreeMan
    Dec 16, 2022 at 14:32
  • I can confirm that there is no adhesive between the tar paper and the drywall -- it looks like the tar paper was first affixed to the studs, then the drywall over that. After a second look, I can also say that the tar paper ends where it meets the floor and does not continue on to any horizontal surface. Because the studs (or some solid wood) is just an inch to either side of the existing hole, I actually wouldn't have to expand very much to do the patch. But yeah, would rather not if it wasn't critical...
    – Andrew
    Dec 16, 2022 at 18:53
  • For what it's worth, I went ahead and patched the drywall without doing any sort of repair/replacement of the tar paper. I suppose time will tell if that was a smart move, but unless there is another update below, future readers can assume that it is working out just fine!
    – Andrew
    Dec 21, 2022 at 20:53

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Looks like an old-school shower area install with roofing felt (not plastic, at least looking at the pictures.)

"Rubbery and a little paper-like" - felt soaked in tar. Probably still smells like tar, a bit. The same product is called tarpaper, I'm not placing bets on the actual material of the "felt" before tar is applied. Out in the weather it will get brittle with age, in a protected interior application it will remain pliant. It's a moisture barrier, but not much of a vapor barrier. It would have been applied to the studs before drywall was applied, not applied to the drywall.

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  • "Tar paper" sounds like a good description. When I hit it with a heat gun, it turns softer and a little sticky/waxy. I don't quite get what this moisture barrier is supposed to do, because presumably I have a separate shower pan liner that is preventing moisture from getting in this space (right?)... I added a cross-section schematic with my guess of what's behind there. But in any case, is this something you think I need to repair before doing the drywall patch?
    – Andrew
    Dec 16, 2022 at 6:20

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