no studs in my plaster walls?

arnold4321November 2, 2010

I live in a 110-year-old row house. The shared walls on either side are plaster. The sound insulation provided by the shared plaster walls is terrible. I have read that plaster is supposed to be more sound-proof than drywall, but that's not my experience here. You can hear normal speaking voices through the walls. I can just about follow along with my neighbors' conversations on one side in particular, though they are not being particularly loud. The other side is a bit better, which may be due to different wall construction on the interior of that neighbor's house.

My master bedroom spans the width of the house and thus shares a plaster wall with the neighbors on both sides, and like I say, on one side in particular you can hear normal speaking voices. I want to leave the plaster in place and screw 1x3 furring strips where the studs are, and then screw two layers of drywall to the furring strips. I will also take additional soundproofing measures like Green Glue between the drywall layers and caulk around the edges, etc. I'll lose a couple inches on either side of the room, but I'm fine with that. For starters, though, I have to find the studs, and I am starting to think there are none. I have put 2 1/2 inch drywall screws all over the place in the wall trying to find them, starting with logical places like 16 inches from the corner. There are indeed some places where the screws grab pretty strongly, but there is certainly not as much resistance when I'm turning the screws as there would be with studs. In some places I find reddish dust in the walls all the way to the surface of the plaster. In some places my screws hit some kind of hard masonry product about an inch deep and won't go any further. In some places it's grey-white plaster all the way through.

I'm not finding any hollow spaces except behind an outlet. The outlet box is not attached to a stud -- it's just floating in the plaster. Through the holes in the outlet box I can see what looks to be the back of my neighbor's wall, and it is a light greyish color and is a flat, rough surface. It's not as bumpy as the back of a plaster wall would be -- it's more flat.

The good news is that the plaster wall appears to be very strong -- strong enough that I would almost consider gluing and screwing the furring strips wherever the screws seem to hold well. I can hang a 2-gallon can of primer from one of the screws and it doesn't budge, and I definitely cannot pull the screws out with my hands. That said, I'd certainly prefer to find studs.

Any guesses as to what's going on inside these walls? The thought crossed my mind that the red dust is rotten wood, but it doesn't appear that way to me. It's almost clay-like, but dry. It's very fine dust and very uniform.

Downstairs, I opened up one of the original interior walls and can see full 2x4 studs, 16 inches on center, along with the old lathe that was left inside the newer drywall. So I expected to find studs in the exterior (shared) walls as well. Wrong assumption?

Thanks for reading my long post : )

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Try an electronic stud finder first.

IMHO the best approach is to remove the plaster so you can seal the back of the outlet boxes and the perimeter framing at the other side of the wall (if it is really a stud wall) then fill the cavity with insulation and then install resilient channels and two layers of 5/8" type X gypsum board to provide a 2-hour fire resistance rating between the properties.

If you seal the edges of the perimeter framing before installing the drywall you should nave a very sound resistant wall.

The red stuff might be hollow structural clay tile used for fire separations 100 years ago.

An alternative is to use molly bolts to secure strapping or resilient channels to the existing plaster surface. I prefer the IsoMax system. This approach also avoids modifying a fire rated wall which would require a building permit.

Materials like QuietRock and Green Glue are intended to achieve an improvement in sound isolation in the thinnest possible space. If you can give up an additional 2 inches of space the resilient mounted channel system will provide better results.

    Bookmark   November 2, 2010 at 4:27PM
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If there is any type of masonry in the wall is was very common to apply the plaster directly to the masonry.

Special blocks with a square keying surface were used.

I wouyld not be surprised at all for a 110 year old row house to have plaster directly on masonry.
It will not have good sound isolation when installed this way.

Plaster has excellent sound isolation when it is on wooden studs.
The mass of the plaster limits the amount of sound that can be coupled to the studs.

You may need to simply build a wall just inside the existing plaster wall, and then consider isolation mounting of the drywall, along with some sound absorbing fiberglass fill.

    Bookmark   November 2, 2010 at 5:05PM
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I have a little Black & Decker stud finder. It does OK on drywall, but not great. It does worse on the plaster, but that may be because there are no studs in there. I think it's quite likely, as brickeyee suggests, that the plaster is directly on the masonry and there is some kind of raised surface in places -- the square keying surface -- and this is what is stopping some of my screws.

The reddish stuff could be some kind of patch applied long ago -- who knows. Or some weird chemical reaction.

I don't want to give up 4+ inches on either side of the room, and the plaster seems very strong, so I'm now inclined to glue & screw furring strips to the plaster and then add two layers of drywall as I described. This is not going to eliminate all sound and won't allow me to blast a home theatre system or anything like that, but should be adequate for privacy. I'll be able to dig the outlet boxes out and insulate behind those.

As for removing the plaster, it's not clear how much space that would net me, since those square keys seem to be only about an inch deep in the plaster. Though perhaps I could work around those with framing so they wouldn't be a factor. Another consideration is that I don't want to cause too much disruption through this process to the neighbors, so that also dissuades me from breaking off the plaster.

Thanks to both of you for your replies.

    Bookmark   November 2, 2010 at 5:29PM
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"I don't want to give up 4+ inches on either side of the room..."

There is no requirement to build a full thickness wall.

Steel studs turned on the flat work well, and you would only give up ~1.5 inches (they need to be barely clear of the old wall surface) plus drywall layers.

    Bookmark   November 2, 2010 at 8:22PM
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I've lived in and owned a number of 19th Century homes built roughly the same way--plaster directly on masonry party walls. Trying to stop the sound transfer with drywall on resilient channel was never very successful. That's because there are so many flanking pathways for sound to travel through, particularly common attics, crawlspaces and sub-floor planking Plus, researchers have commented that just one misplaced screw can short-circuit the whole system. I've used Quiet Rock to good effect, but it is very costly in small quantities. Green Glue has similar tested results, but I have no experience with it.

(I've found growing old helps, as your hearing diminishes. And if you're "lucky" enough to get tinnitus along with it, you're always so awash in a sea of noise emanating from your own head that the neighbours hardly matter.)

    Bookmark   November 2, 2010 at 9:10PM
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The hollow structural clay tile I mentioned was a red brick-like/terra cotta material with grooves on its face for the direct application of plaster or stucco. Entire houses were constructed of it instead of wood studs. I suspect in your case it might have been used by itself for a fire resistant party wall or as an infill material between bearing studs. Natco was the major producer in the US.

Here is a link that might be useful:

    Bookmark   November 2, 2010 at 9:17PM
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latest thought is to rough up the plaster with sandpaper and use liquid nails to glue the furring strips to the wall, without screws. liquid nails doesn't dry hard so I would think it would not be great at conducting sound. That, in combination with the Green Glue between the drywall layers, should suffice, if I caulk everything well. Yes some sound will go up through their ceiling and down through mine, and a little bit from below the floor, perhaps, but not much.

I'll get short enough screws so they don't go through to the plaster.

I'm going to seat the bottom course of drywall in a strip of foam pipe insulation and then cut the excess. That should also take a little weight off the wall.

    Bookmark   November 3, 2010 at 12:26AM
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re: Natco, I think there's a good chance that's what it is, or a similar product.

    Bookmark   November 3, 2010 at 12:30AM
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Why not green glue the drywall to the existing plaster? Try a sample first.

    Bookmark   November 3, 2010 at 7:06AM
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I don't think Green Glue is intended to be used as an adhesive in place of screws, despite the name.

    Bookmark   November 3, 2010 at 8:19AM
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Green Glue application.

    Bookmark   November 3, 2010 at 10:21AM
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yes, the sheets must be screwed down because the compression spreads the Green Glue thin. "Drywall sheets must be screwed together while the material is still wet."

    Bookmark   November 3, 2010 at 11:56AM
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You may have old gypsum Pyrobar block, or could be black iron with a stretched metal lath between the upright iron. In either of those scenarios case there's no studs. Very common in NYC (not sure where you are).

At this point, you could laminate 5/8" drywall and Green Glue as suggested. Also seal the outlets.

Here is a link that might be useful: Wall Soundproofing Options

    Bookmark   November 4, 2010 at 10:02AM
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I wanted to report that the addition of furring strips plus two layers of drywall with green glue between the layers did not have any effect. The approach does not work, since all I achieved was addition of a relatively small amount of mass to an already massive wall. More likely what I needed was to create a larger air cavity by using studs rather than furring strips (along with insulation in the cavity). I erred in thinking that the green glue product would help make up for the lack of a larger air cavity, but it definitely did not. For much, much more detail about my (failed) project, see this thread:

    Bookmark   January 16, 2011 at 4:15PM
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You need to decouple the new wall form the old surface completely.

Steel studs turned flat and held off the wall, along with sound absorbing fiberglass and 5/8 drywall will go a long way.

Part of the problem with solving problems like this is that you generally need to overdo things since going back later when a fix is not adequate more than doubles the work.

The sound insulation will help with sneak paths through the existing wall, and decoupling so the new heavy surface is not attached to the wall (floor and ceiling only) also helps.

You are likely to have to pull electrical boxes forward into the new wall.
Since the old wall is going to be concealed damage is not a big deal.

    Bookmark   January 17, 2011 at 9:48AM
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