Insulation between floors

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On my extension I want as good acoustic performance between floors as possible, I was planning on using rockwool slabs, the joists are 6inch, I was planning on doing a layer of 100mm rockwool and a layer of 50mm. Alternative is to try and squeeze 2x100mm in, or have a single 100mm and a 50mm air gap.
Anyway anyone who's done similar with any advice much appreciated.
On my last extension I just used loft roll but I'm hoping for better soundproofing with the rockwool acoustic slabs.
 
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Normal wool worked OK for me, unless you want to get into isolated floor or something to stop impact noise. Or carpet. The heavier the better, would sand work? Only half joking!
 
Yeah it's more whether it should be compressed or exactly the depth of the joist, or whether an air gap is beneficial, pretty sure it isn't
 
At a guess I'd say pack it in as much as you can without damaging things. More mass. If you put cotton wool in your ears you pack it tight.
 
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Yep, I know where you're coming from but also heard that it can become less effective when compressed (acoustic wise)
 
Sound travels through buildings in two ways (highly simplified explanation!).

Structure borne sound - i.e. the thump of footsteps impacting on the floor above and vibration from floor mounted speakers!
Airbourne sound - i.e. what passes through the holes/gaps such as the airbourne component of the footsteps above and other airbourne sound like music from speakers and speach

What you are doing will insulate mainly the Airbourne sound and it is usually mass that helps stop that but certain materials designed for the job also help. The way to stop structural sound is by isolation construction techniques which you will not be able to do.

In short pretty much anything you pack into the floor space will help. - but specialised materials will be the most effective. Consider this - tightly packed insulation will fill the void which is partially isolated contruction and therefore more structure bourne sound may pass through, although you will improve against airbourne sound - Without testing I suspect it is a trade off so your comments about tightly packed insulation may be true in a way.

It may be of interest that when building a travel lodge a few years back the construction dividing rooms were stud partition walls, but of a rather special make up. In fact two stud walls back to back to isolate the construction and then each room facing side was double boarded with 20mm thick wallboard and finally conventional wall insulation filled between the studs of the partitions without bridging the back to back gap.
 
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Thanks guys, really useful info
In my case I don't care about sound travelling downwards, only upwards, which again suggests filling the void is the way to go.
 
You wont compress Rockwool. Earthwool or Dritherm will compress about 20%.

It'a all a bit academic though, as you can put down a thick underlay and you wont notice the difference to the floor being packed with Rockwool. Sealing any gaps in and around the floor deck (and joists) will be much more beneficial.
 
Cheers woody, sealing gaps was thinking either expanding foam or shoving loads of dritherm or something in there? Like the 2" gap between last joist and wall
 
Rockwool will do for any gaps between the joists and wall, but its more the gaps between the floorboards and walls, and skirting and floorboards, and along the bottom of any dry lining.
 
Thanks. I was going to look at using that glue that looks more like expanding foam to fix the chipboard, seen it used on sites but not really sure what it is, assuming it's better than just screwing them down and PVAing the joints which I've done in the past
 
That glues seems to be the fast setting Polyurethane type that expands a little before drying.
 

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