Acoustically Insulating Party Wall

bemak

We recently bought a house and prior to getting the keys we noticed that we could hear a decent amount of sound from the neighbour on the other side of the party wall. We really liked the house and decided to take it on with said issue.

Fast forward a few months and we have the keys and I lined up a builder to do some remedial work, primarily the lining of the party wall. From experience on hotel projects I specified a Gyproc 70mm Acoustud, with 50mm Isover acoustic insulation and 2 layers of Soundbloc plasterboard, skimmed, all of which was to be set off the existing block wall by 25-30mm. All good. I even verified this with Gyproc themselves and they thought it was a belt and braces specification.

So now the wall is in, yet to be skimmed but on checking it out last night I noticed to my disappointment that we can still hear some noise from next door. It's not as bad as it was but I was expecting a much bigger improvement considering the buildup proposed which can improve the acoustic insulation of a wall by up to 19db.

Anyway, we think the main cause of the noise is from the floor/ceiling joists. The house has a suspended timber floor and both the ground and first floors are supported by the party wall. The joists in the attic run parallel.

The party wall is a block on the flat and we think that our joists and the neighbours joists are spanning from the same positions in the wall, so in theory they're 25-40mm apart but I would assume that many are touching. Also, there are some gaps around where the joists are supported in the party wall.

So the plan is to fill the holes with a dense mortar and then fill the floor joists will acoustic insulation along the party wall (500-600mm section).

It's very hard to stop acoustic transfer from the joists so the hope is that by filling the cavity in the floors with insulation that we might prevent a 'drum' effect which might be amplifying the sound in the first place. To do this properly we should cut the joists at the party wall and install new supports but this is just too much surgery and cost for us to take on.

Just wondering if anyone else in a similar situation had any other solutions as to how we might improve the sound transfer from the joists? I think we're doing as much as well can bar cutting the joists which isn't an option.

Would be curious to hear yer thoughts!

Gumbo

My experience is that sound proofing a party wall that was built many years ago never works unless both sides carry out substantial, invasive works on both their respected sides of said wall.

bemak

kceire wrote:

My experience is that sound proofing a party wall that was built many years ago never works unless both sides carry out substantial, invasive works on both their respected sides of said wall.

Absolutely. Our plan is to be the noisy neighbours and make them do some upgrading!! In reality all our work has probably done more for them than us

imp1

A 19 dB reduction would be quite an achievement, but it really depends on where you start, if the wall was very poor to start with, then a 19 dB improvement would be noticeable, but when you take into account a 10 dB improvement would be perceived as about halving the noise, a further 9 dB would be another halving (roughly) so the overall perception would be about 1/4 of the starting noise - assuming the noise was all coming through the wall, an important assumption.
If the wall was middling to good to start with, then a 19 dB reduction in noise transfer through the party wall would probably result in a very nice level of insulation. Though improving a good wall by 19 dB would be quite a feat.
Timber joists are a problem, and not easy to resolve, it's not for nothing the TGD states they should be run parallel to party walls, or if necessary use joist hangers, not saddle type hangers, and not to build them into the party wall.

bemak

Another issue is the fact that the front wall is built of hollow block - and there's no break between us and the neighbours so sound has a nice route through that way as well. The lining of the wall has helped a bit but it definitely hasn't reduced it by half unfortunately. The rooms in the house are empty at the moment so furnishings will help a small bit but we'll just have to accept where it's at now - because I've no interest in throwing more money at the problem

bemak

So the walls are skimmed, painted, second fix is in and skirting is on and it's actually all made a huge difference. When I pulled up to the house last night, I could hear the baby next door crying (before I even got out of the car) so I thought, now we'll know if the lining has worked. Went into the house, checked all the rooms which adjoin the neighbour and nothing; not even a murmur. I'm actually amazed at how well it's worked since I put up the original post. I guess the wall just needed to be finished properly before passing judgment.

Just wanted to give a quick update in case anyone else is thinking of doing this down the line and found this post.

Meself

bemak wrote: »

So the walls are skimmed, painted, second fix is in and skirting is on and it's actually all made a huge difference. When I pulled up to the house last night, I could hear the baby next door crying (before I even got out of the car) so I thought, now we'll know if the lining has worked. Went into the house, checked all the rooms which adjoin the neighbour and nothing; not even a murmur. I'm actually amazed at how well it's worked since I put up the original post. I guess the wall just needed to be finished properly before passing judgment.

Just wanted to give a quick update in case anyone else is thinking of doing this down the line and found this post.

Hi Bemak,
Thinking of getting my living room done along the same lines.

Moved in before Christmas to mid terrace house. Sound is from neighbours tv. It coming from party wall between side of my chimney breast and front wall.

How much mm space did you loose in total ?
Do you have electrical sockets on the wall ?

Was it expensive.

Thanks

bemak

Hi Meself,

We lost about 120mm as follows: 25mm offset from existing wall. 70mm acoustud (filled with 50mm isover acoustic roll). 2 x 12.5mm layers of Soundbloc Plasterboard skimmed.

We have electrical sockets in the wall. No problem with this as the acoustic roll is behind them. They are also fixed onto a timber grounds.

In terms of cost, we did the gable wall on ground and first floor and filled in 500mm of the cavity in the floor joists along same. It cost in the region of €4-5k. That includes the moving of existing sockets and the reinstallation of same onto the new wall.

My advice would be to understand what buildups you're dealing with in terms of the party wall (ours is solid block on the flat), the external walls to the front and back adjoining the party wall (for example ours are hollow block which must be the worst), and the floor joists - are they spanning into the party wall, are they embedded into the wall or on hangers. If they're embedded are there gaps between them and the wall. Ideally you'd want the joists spanning parallel with the party wall. Once you understand what the existing situation is like, you can look at how to insulate it properly.

I was speaking to Gyproc when our lining was being done (about a different project) and they said that the buildup we had specified on our wall was "probably the highest spec'd party wall in the country :rolleyes:" so maybe I went a bit overkill on it. They did mention a product that I was unfamiliar with though - "Airtite Quiet". It's a skim that is applied to blockwork prior to the render. It's relatively cheap too and I was told that this product, on a blockwork wall, with a final sand & cement render coat can provide up to 50db+. That's excellent. Had I known I would have put this on mine as well because at the time I was being quoted 1k to put a skim coat on before lining the wall. I couldn't justify the cost so I didn't bother. Thankfully it hasn't been an issue - but had I known about this product I would have definitely put it on for the sake of 200e.