Fitting rock wool in interior walls - how to fit pre boarding?

[DownSouth]

We’re getting a plasterer to board out the internal walls while we get on with other jobs. 
After some advice please on what’s the best way of fitting it within a stud wall so that it’s still in place when the plasterer comes to board out? Any top tips? 

[crispy_wafer]

Usually friction fit between the studs, nothing special required.  Just cut ever so slightly oversize 

[marshian]

6 minutes ago, DownSouth said:

We’re getting a plasterer to board out the internal walls while we get on with other jobs. 
After some advice please on what’s the best way of fitting it within a stud wall so that it’s still in place when the plasterer comes to board out? Any top tips? 

are you doing this for sound reduction or heat insulation?

[Redbeard]

Just now, marshian said:

are you doing this for sound reduction or heat insulation?

 

+1, plus I think we need to know more about the building and the wall build-up (is it already an insulated cavity wall, for exampler? Is it old and retro-filled? Is it new. to Bldg Regs?

 

You may need a vapour control layer in the 'sandwich'.

[ProDave]

Board one side, fit insulation, board the second side.

[JohnMo]

Or just use rockwool Flexi for internal partition stud walls, for acoustic insulation. If a 75mm or 100mm stud use 50mm and fit in the centre of the stud, so you have an air gap both sides.

 

Make it a good interference fit (not huge so it bulges) it will stay put for as long as needed.

 

[DownSouth]

Doing it for sound reduction in walls, heat insulation in the ceiling between ground and first floor. Internal new stud walls in a new build timber frame which has factory fitted PIR with an additional 50mm PIR which we fitted to all exterior walls, followed by  membrane and battens. We can’t board one side first because we are doing rockwool ourselves and it has to be done first because of our timings.

Edited June 4, 2025 by DownSouth

[Kelvin]

Friction fit worked fine for us you can tap some long nails in on the studs to hold it too. 

[Iceverge]

Pay special attention to sockets etc. 

 

Air paths are the easiest way to transfer sound through a wall. 

 

For added mass consider putting scrap plaster in there loose. Better than throwing it in the skip or spending a fortune on higher density insulation. 

[JohnMo]

8 hours ago, Iceverge said:

For added mass consider putting scrap plaster in there loose. Better than throwing it in the skip or spending a fortune on higher density insulation.

Our last house built in 1830, use the same principle between floors. The joists were filled with builders waste in a uniform fill.

[-rick-]

I've seen other people (mostly renovating) who having to deal with the builders waste left in joists. It's not a nice job, even small changes (electrical/plumbing) at a later date are gonna bring up a lot of dust (and one thought the waste between the joists was causing dust problems even before they started renovating though how accurate this is IDK).

[ProDave]

I have found this joists filled with rubble thing.  I bet 100 years ago nobody calculated how the joists would react to this extra dead loading?

[Super_Paulie]

there's no way id be leaving a load of knackered plaster between my floors or walls. Just spent the last few years cleaning all that crap out. Technically i see the positives for it in sound reduction, but id go for a real product, i used acoustic wool all round. 

[JohnMo]

15 minutes ago, Super_Paulie said:

but id go for a real product, i used acoustic wool all round

Me too. But in my example from 1830, such stuff didn't exist.