Soundproofing Party Wall & under floor

[JohnD]

We have recently bought a 1930s semi-detached house which we believe has double skin party wall.

It is probably 9 inch brickwork, and 4 inches between adjoining flues.

Cavities between semis and terraced houses are AFAIK quite a modern innovation to reduce noise.

 

[endecotp]

the joists run into the party wall.

That’s unfortunate. There may be gaps around the joists that you could fill, but even then you are still conducting sound from the wall into the joists.

Honestly you are unlikely to fix it.

 

[Smalltowngirl_95]

Yes, the joists are probably pocketed into the brickwork, and there are probably gaps. You would to take up at least some of the floorboards for access. Clean out loose material by scraping and using a builder's canister vacuum (not a domestic vac). You can pack them with sand and cement mortar but you will probably find expanding foam easier. Remove all the dust and spray the hole with water to make it stick. Use the plastic extension tube to get it deep into the hole. I use the pink fire foam which blocks smoke and flame, it chars but does not burn. Most likely you will never have a fire, but, why not?

In some cases there is a separate sleeper wall just inside, but I think I have only seen that on exterior walls where it protects from damp.

Gaps in the brickwork are worse in unplastered areas ( plaster hides them ) between floors and sometimes inside cupboards. Builders traditionally used the best bricklayers for the exposed external walls at the front, and hidden brickwork is often very shoddy.

Afterwards, you can pack between the joists with dense mineral wool which muffles noise (loft insulation is much lighter, so not as good).

The relevance of the chimneybreast is that the wall, between your fireplace, and next door, may be very thin, just a flue, and this is not as good for blocking sound. If it was just plastered, plaster is less dense and not as effective as dense thick brickwork.

This is most helpful John, thank you ever so much. We will take up a few boards downstairs to be able to see the joists and take it from there.

 

[Smalltowngirl_95]

Yes, the joists are probably pocketed into the brickwork, and there are probably gaps. You would to take up at least some of the floorboards for access. Clean out loose material by scraping and using a builder's canister vacuum (not a domestic vac). You can pack them with sand and cement mortar but you will probably find expanding foam easier. Remove all the dust and spray the hole with water to make it stick. Use the plastic extension tube to get it deep into the hole. I use the pink fire foam which blocks smoke and flame, it chars but does not burn. Most likely you will never have a fire, but, why not?

In some cases there is a separate sleeper wall just inside, but I think I have only seen that on exterior walls where it protects from damp.

Gaps in the brickwork are worse in unplastered areas ( plaster hides them ) between floors and sometimes inside cupboards. Builders traditionally used the best bricklayers for the exposed external walls at the front, and hidden brickwork is often very shoddy.

Afterwards, you can pack between the joists with dense mineral wool which muffles noise (loft insulation is much lighter, so not as good).

The relevance of the chimneybreast is that the wall, between your fireplace, and next door, may be very thin, just a flue, and this is not as good for blocking sound. If it was just plastered, plaster is less dense and not as effective as dense thick brickwork.

As an after thought would we be best insulating the whole downstairs suspended floor or just so much after the party wall. Don't particularly want to rip up all the floorboards as will be a nightmare of a job

 

[accelerator]

What you probably have is some open gas where the joists go into the wall. Then the cavity under the floor is acting as an amplifier. The fact you only have wooden flooring and no carpet also means you have no sound damping.

If you don't want to do work in the cavity, then adding sound insulation and carpeting above the floorboards may be an option.

 

[StephenStephen]

That’s unfortunate. There may be gaps around the joists that you could fill, but even then you are still conducting sound from the wall into the joists.

Honestly you are unlikely to fix it.

Reading the original post, the nature of the sounds heard are airborne, rather than structure conducted, so less of a problem than you might think.

 

[StephenStephen]

I would concentrate on insulating the sound coming from next door into your underfloor (sideways), and not worry about stopping sound coming up into the room from under your floor (upwards), so I wouldn't be trying to sound insulate anything other than the edges of the underfloor