Cement rendering of alcoves in Victorian property (+soundproofing)

[aufladen]

Hi all, as a DIYer doing my first ever rendering task, hoping for some advice from you guys!

Background - we have a Victorian terraced property and getting excessive noise in our lounge coming from next door (TV, conversation, loud yawning etc!). The wall has a chimney breast and 2 alcoves and we are about to embark on a soundproofing project, as we are having the entire room renovated anyway. Sound is primarily coming from the alcoves, where I believe the building structure can be weaker, but also from the chimney breast. In the alcoves, we will be building an independent stud wall (separated/detached from the rear wall of the alcove by 1-2cm) and using a combination of soundblocking materials on top of this, having researched different solutions. Might also use soundproofing panels on the chimney breast after we see the results from the alcove work.

The rear wall of the alcoves currently have a 2cm thick layer of what I believe is the original lime plaster - which in areas is flaking away to powder - this is a very low density material and of no use for blocking sound. I was going to build the new stud wall in front of this, but then started thinking about replacing those 2cm of old plaster with something like cement render to increase the density of the wall thus bolstering the overall soundproofing.

So my question is should I just use a standard 4:1 or 5:1 sand:cement render mix, or should it be weaker, and do I need to add lime? Bear in mind this render layer will never be visible as the stud wall will sit in front of it.

I think the mortar in the brickwork is lime mortar (also crumbles to powder). So I'm concerned about putting anything onto the wall which is too strong for the underlying brickwork.

Also does lime have the effect of 'aerating' the render? I know it makes it more 'breathable' but that sounds like it would decrease the effectiveness of the render from a soundblocking perspective as it would reduce the density?

I'm just weighing all this up at the moment, but it seemed like a simple and cheap way to improve the soundproofing we are doing for minimal extra effort/cost.

Any thoughts most welcomed!
Cheers.

 

[cdbe]

Sounds like a plan to me. Will also fill any gaps in the mortar joints. I would advise you to open up the ceiling as even if the joists run front to back, you may well have trimmers pocketed into the party wall which will cause massive sound transmission.

 

[Tigercubrider]

is it possible that the chimney is linked?
a mate’s house had that issue.
very early Victorian but the next door smoked and the smell came through.

 

[aufladen]

Hi cdbe, thanks for your reply.

Yes, filling in any mortar gaps is an added benefit of rendering and would help to limit sound transmission. I have yet to remove the plaster, so have not yet seen the state of the mortar, but it has been sketchy in other areas. I'm leaning more to the 5:1 mortar mix, so that it's not too strong.

I have full access to the ceiling void as the loft is directly above the lounge (we're on the top floor). I have also pulled up some floorboards and plan to extend the render, and some of the soundproofing to below floorboard level, where space permits as sound can resonate and amplify in these voids..

Interesting you mention trimmers - there are a couple around the bottom of the chimey breast as the joists do run front to back, so the trimmers help to tie in some shorter 'joists' in the alcoves and (I think) help to support a tiled plinth in front of the fireplace. The trimmers don't seem to be pocketed into the party wall though from what I have seen so far, although the bottom half of the trimmers where they meet the wall are a bit obscured. I think (hope) that they sit on top of the wall of the floor below, as I know the wall below is 3 bricks thick in places. Probably not much I can do if they are pocketed into the wall though, although I was planning to fill the floor voids in that area with acoustic mineral wool which might help dampen any resonance at least.

 

[aufladen]

Hi tiger,

Good point - as far as I know the chimeys are not linked, as we don't get any smells coming through. Unfortunate for your mate!

I understood that the chimney stack in these old buildings was built strong and that it's usually the alcoves which can be the weak points and hence transmit most of the sound, so I was thinking maybe I am getting some flanking sound through the alcoves into the chimney breast.

I usually only hear noise from the chimney breast if I put my ear right onto to the wall. If I put my ear down at fireplace level, into the void, I can't hear anything, thankfully!

This is why I plan to soundproof the alcoves first, then reassess noise levels and go from there, as I'm not sure how much noise, if any, is coming directly from the chimney itself into the room at this stage.