How to soundproof a wall against the neighbors?

Dear permies,
a friend of mine wants to soundproof her living room wall. It's an old spanish house with relatively thick walls that are shared between neighbors.In her living room she can hear them talk and can hear the TV, without making out what exactly they're saying.
She would like to soundproof the wall and also reduce the wifi coming through from the neighbors. The wall has to be replastered anyway.
My brain instantly went to clay, as in heavy and thus reducing sound. Would adding a 5 cm clay sand (lime) plaster help?
Do you have other ideas involving (mostly) natural material?
Thanks in advance

Every little bit would help, but I don't know if 5cm (about 2in) of clay would be enough.

The only things I can think of that would help would also eat into the room space. Straw bales and rockwool insulation. Unfortunately to get a decent amount of sound proofing, I suspect you would need 6-10in or more. That would be something like 13-24cm?

Thanks Matt.
So you would go and put Rockwool on the existing wall and then put plaster over it? Or another new wall in front?

How would you attach the Rockwool to the old wall to get the best soundproofing?

Just keep in mind that I am not an audio expert, nor am I a building expert :)

I also do not know what products you have available over there, so I will have to go off what I'm familiar with... in hopes that there is something similar over there. The rockwool brand of insulation has two lines... one that is just insulation which would help with audio, and then a line that is for insulation AND sound deadening which would definitely help. Over here it comes in batts or 4x8 sheets for slightly different use cases.

https://www.rockwool.com/north-america/products-and-applications/products/safensound/
https://www.rockwool.com/north-america/products-and-applications/products/rockboard/

I think what I would do is get some 4" of the rockboard and lay that up against the old wall with no supports... it should stand by itself with friction. Then build the thinnest frame you can on the outside to hold the new plaster. You could build a wall using the safensound batts... but I think the wood frame might help bridge sound in the same way it bridges heat. So why not a continuous wall first, with a thin frame on the outside for plaster.

Soundboard.  It’s a drywall product, you’ll have special order it.

There’s a recycled blue Jean insulation that is used for sound deadening.

Sound waves travel through direct contact.  An easy way to dramatically reduce sound transmission is simply to offset framing on your side.

LeroyMerlin sell sheets of acoustic and thermal insulation made of recycled cotton textiles for 8 euros per square metre - leroy merlin acoustic insulation

I have no idea how effective it is...

Thanks for all the tipps so far. I'll consult with a builder here.

And I think I have some of that material left over here, Burra.

I'll keep you posted once we move forward

I have no affiliation with Leroy Merlin by the way. They are just a place that sells in both Portugal and Spain with a reasonably usable website. There are likely other, better builder's merchants near you.

I've no idea on the wifi, but hanging a couple rugs/blankets/quilts/fleeces and the like on the walls can help with the sound migration. It might buy her a little peace until the supplies for another solution arrive.

Dane Geld wrote: Sound waves travel through direct contact.  An easy way to dramatically reduce sound transmission is simply to offset framing on your side.

I would absolutely research this aspect of it, as I've read that an air gap/offset framing/sealed sheet of plastic are key to interrupting sound. It's a wave, so you need one or more things that will either stop the wave, bounce the wave back, or disperse the wave so it's more of a "white noise".

I can ask my engineers later, but they're both at work right now.

Benjamin,

Is the wall solid? How thick is it?
High mass walls have excellent sound insulation values. Is the roof structure shared with the neighbor? Maybe this is how the sounds gets in. I would investigate it first, before adding anything - especially vulgar synthetics to an old house.

I used to have a diesel engine under the kitchen sink on my sailboat, and the admiral ordered me to "soundproof that thing!" I learned that the art of soundproofing is "decoupling" - so you alternate high density and low density; what will transmit easily through one density will often be attenuated or blocked by another density. If the wall is dense, add a foamy layer and then another dense layer. But if you also want to block wifi signal, then before anything, simply wallpaper the whole wall with aluminum foil; radio signals will not go through it. If there is any signal after a wall of aluminum in place; it is going over the top or reflecting from outside.

Hi Benjamin, I am a musician and have tried everything for sound abatement. Based on my experience, the gold standard is mechanical isolation of the wall surfaces. Yes, more mass in the wall is good but isolation is even better. The problem is that you have a wooden medium on the inside connected to large, flat surfaces on the outside that collect and transmit sound vibrations very effectively. In fact this is exactly how a piano soundboard works. If you can isolate one or both of those wall surfaces from the wood studs inside the wall, that will stop nearly all of the transfer of vibrations from one surface to the other.

If time, space, and money are not issues you can just build two parallel wall systems - two sets of studs running parallel to each other with wallboard hung on the outside of each set of studs. But you can get nearly the same benefit without all of that work by using a product called resilient channel to hang the wallboard. This is a metal strip that is folded over so you can screw one side to the studs, then screw the wallboard to the resilient channel. This way the wallboard is isolated from the studs by a 12mm/0.5 inch gap. That is enough to stop nearly all of the transfer of vibration from stud to surface.

I just looked up resilient channel and found a bunch of pages from soundproofing companies claiming that resilient channel does not work. Well of course they would say that, they want you to buy their consulting services and proprietary toxic gick. I have built wall systems with resilient channel and it works great. In fact I believe it was invented specifically to provide sound isolation at low cost. I have been told it is used in hotels for this purpose.

Here is an example (no connection to the company, just the first search result):
https://www.buysuperstud.com/products/clips-and-accessories/resilient-channel
The stuff I used was similar to the RC1 or RC-SUPER from that site.
Home improvement stores do not carry it but it is a standard item at specialty drywall/plaster vendors.

Of course this is geared towards a conventional modern gypsum board wall, but if your friend wanted to try to get the same benefit using more natural materials she might try adapting the concept to creating a lath matrix that is similarly isolated from the studs and plastering the lath with the medium of her choice. I might worry slightly about the finished wall flexing and cracking though (the resilient channel gives just a tiny bit - not an issue with solid wallboard). Or wood paneling would also work although the screws would have to be dealt with visually (you cannot use nails with resilient channel). Maybe use tongue and groove boards.

Bear in mind that there may also be significant vibrations, especially in the low frequencies, coming through the floor if the two sides rest on the same joists or subfloor. Treating the wall will not help with that. Perhaps a covert operation to disconnect the neighbors subwoofer is the best strategy.

-Aaron

Regarding WIFI blocking, is your friend concerned about the effects of radio frequency transmission or is she just annoyed that their network is interfering with her own network? If the second, she may be able to configure her own WIFI router to use a different frequency than her neighbors. Some routers are supposed to do this automatically but they also allow manual frequency selection.

If the first, I can also speak from experience that a sheet of metal will block WIFI frequencies very effectively, especially in the newer 5GHz band. I have a family member who is a sheet metal engineer and has several walls in his house made of stainless steel and copper. They look great but you can forget about using the WIFI! I had not thought about using aluminum foil. It might be possible to get incorporate a faraday cage-like element into the wall for this purpose that would use less metal, but that is way beyond my knowledge.

-Aaron

Thank you all for your insights and ideas. I’ll update you when we move forward.