drywall over old plaster

Hello everyone!

This is my first post, and I believe it may be a doozie!

My wife and I just bought a 1930 house that has been completely renovated inside in 2007, however the plastic tub surround was not installed properly and there was quite a bit of splitting of the silicone where the tub surround meets the tub. I expected to find some wall damage, but did not expect to find what I did. Upon cutting into the drywall (which is just plain old drywall) to remove it, I discovered the original lath and plaster walls are still behind the drywall! We removed the drywall layer about 3 feet up from the tub until the wall felt dry, and the walls behind the drywall look and feel to be in good shape. We do not want to get into tearing all the lath and plaster down to the studs and starting over even though that is probably what you all will suggest. I think we will just re drywall the cut out spots with greenboard and reinstall a new plastic tub surround. However, here is the big question that I have. The tub is on an outside wall, and the person that did this put a vapor barrier between the old plaster and the new drywall only on the outside wall and this was also the wettest of the 3 walls. Should this vapor barrier be there, or should I remove it and screw the drywall right to the plaster like the other two inside walls? Could I then use a vapor barrier primer on the greenboard to seal everything before installing the tub surround just to insure no future water penetration?

phew.. sorry this was so long, but I am at a loss and need help!

I think you are asking for problems with the vapor barrier there. The surround is (at least in theory) waterproof, then you have another layer sandwiching the drywall. Anything that might have gotten in there had no way to dry out.

Greenboard is no longer approved (at least in the national codes) for use in damp areas, primarily because it really doesn't work well. Yes, the paper covering is more water resistent than 'normal' drywall, but once you put all those fastener holes in it, it doesn't really buy you much of anything. There are some panels that are decent, though. Some surrounds are designed to attach to the studs, and no wall is required behind them. Verify which type you have.

So, I gather cement board instead of greenboard.. I just figured I only needed cement board if I was going to do tiles. But, do I leave the vapor barrier in place to do cement board? also, the other 2 inside walls do not have this vapor barrier, I would have to add one, How do I add a vapor barrier to an existing wall? just tuck tape it to the lath wall under what I am cutting out?

Well, if the preformed walls are doing what they are supposed to, it wouldn't matter what you put back there. As noted, some walls are designed to stand alone and need only to be attached to the studs. The advantage of using cbu is, even if it does get damp or wet, it won't soften or swell or degrade. But, because it could get damp if there's a problem, you want some plastic film behind it to prevent that moisture from getting into the wooden structure and rotting it out. If there is a lip on the tub, run the plastic film over it, then trim it off after you get the walls in place. Then, if anything gets behind, it will have a way to get into the tub, rather than going down the wall.

Ok so I assume just taping a vapor barrier to the plaster and then installing the cbu over top is fine then, and as you mentioned it will only be an issue if the tub surround fails. The only reason I am concerned is because of just how wet the drywall was that I took out. I was thinking the vapor barrier was somehow trapping moisture and making it worse, but this would not hurt cbu.

I'd be tempted to just put up some plastic then the surround. But, it sounds like the tub may have been installed such that the space needs something to align things. If you can put the surround up properly, and support it properly without adding the extra layer, just put up the plastic sheeting. The surround should be waterproof in itself, so, unless it wasn't fitting properly, moisture got in there some other way. It's possible it may have gotten in from the top, depending on how tall you are, how high the water pressure is, and where the shower head is...it could splash on the wall above the surround, and get in behind there. A good bead of caulk along the top edge might be called for. Generally, you don't want to sandwich anything between two vapor barriers, if you do, then the thing can't dry out. The good thing about cbu is, it won't swell or degrade if it does. But, constant moisture, if any organic material happens to be there, will support mold.

Jim, I want to thank you for all your help.. you seem to be the only one that has an idea of how to help me.
The silicone on the old tub surround was pretty degraded where it meets the tub, so I am thinking that is how the drywall behind it got wet, but I cut out the entire first layer of the back wall about 3 feet up from the tub and it was all wet to the point that there was mold on the polly that was between the 2 layers. This seems excessive to me, and this is why I am worried that the vapor barrier was somehow trapping moisture between the drywall and the tub surround. I don't think I can put up the surround without replacing the layer I removed because the surround would either have to go inside where I cut out leaving a very ugly unfinished drywall edge, or overlapped onto the finished wall and then there would be no support where the half inch of drywall was removed.. I would love to insert couple of pictures I took if it would help you see what I mean, but I don't know how.. It asks me for a url when I try to insert.

I have been doing some reading about paintable vapor barrier sealers that sound like they may do the trick.. Could I not cut off the vapor barrier I have exposed, lay the new cbu back into the cavity back on top of the old wall and paint this vapor barrier onto the cbu, effectively creating a vapor barrier right behind the tub surround?

Scroll down further when entering a reply, and you'll come to a section with a 'manage attachments'. That is where you insert your pictures. There are resolution and file size limitations.

thanks. here are some pics:

update:

drilled a hole through the inside wall to see what was in there. from outside to inside looks like this:
-exterior wall - wood shaving insulation -3/8" plywood - drywall - then the vapor barrior you can see in the picture, so on top of this will be the cement board.

I have a similar tub layout.....with the window. I looked at the popular tub surrounds and could not see how I could cut the surround for the window and seal it properly. The one I wanted to use was the Accord tub and shower surround set....

How do you plan on cutting out the surround and finishing it around the window so there is no leaks? The window makes no sense to me as to the location......I have 2 bathrooms like this and really wish the windows were not there......I would have been done as the surround is way faster and easier........

I put up backer board...tub to ceiling and am about to tile the entire bathroom.....floor and walls.....

rich, the old surround was cut to the window frame and then a nice wide trim moulding put around it overlapping the surround, then the moulding was painted with a high gloss paint and every seam siliconed. This seemed to work as there were no signs of leakage around the window at all (the stains on the wood below the window were obviously there from a long time ago because everything is dry around there now)