Floor Joist Insulation. Is this the right approach

[NBW]

As of today, the builder has finished onsite and plaster boarded the ceilings on the ground floor, i have just found out that there is no insulation installed in the joists...

I have original floor joist in place in my 1960 house.  Large refurb and UFH has been laid in routed chipboard for upstairs.  A lot of pipework and electrical runs really make it difficult for PIR.  
So my plan is to use 100/150mm RWA45 from rockwool.  I am also considering double plasterboarding.

Question - is it worth double plasterboarding. I dont want to buy soundeadening plasterboard and replace the existing as thats a lot of wastage

[Redbeard]

I am not clear what the issue is. Is this a house which you lived in before the works, which has a proven record of noise transfer issues from GF to FF and vice versa? If so do you get the impression it is (a)impact noise which is the main one, or (b)airborne noise. If the former I would not necessarily expect a big change, as much of the 'noise' could be transmitted through the structure via joist pockets. If the latter it may help. If there was not much of a problem anyway why bother? (Unless you are using the insulation for both sound and heat *and* you want to 'thermally compartmentalise' the house).

[NBW]

@Redbeard
Thanks for the reply.
Having UFH and no carpets upstairs i am keen to improve the thermal and sound insulation in the house.  Kids running upstairs sounds like a herd of elephants.  I am also keen to try and improve the thermal insulation in the rooms as well
 

[Temp]

  In an ideal world you wouldnt have joists to conduct  noise through the floor, you would have a separate floor and ceiling or false ceiling.

 

 

In short wool insulation and double plasterboard is probably the best way to go. That will reduce transmission of things like music or TV but might not reduce impact noise/foot fall as much as you would like.

[Nickfromwales]

23 hours ago, NBW said:

@Redbeard
Thanks for the reply.
Having UFH and no carpets upstairs i am keen to improve the thermal and sound insulation in the house.  Kids running upstairs sounds like a herd of elephants.  I am also keen to try and improve the thermal insulation in the rooms as well
 

If you have the height, you could install resilient bars and then a layer of 15mm sound block plasterboard onto those. The RB's deal with foot traffic and impact etc.

 

The builders wouldn't normally install insulation in a makeover / refurb unless building control had stipulated it, and it really should have been part of the spec that they tendered to so not really anyone's fault. Reality is that the void there is within the heated envelope, so insulation in that void would add nothing re thermal performance anyways, but you'd obvs have preferred it for acoustics.

 

Avoid penetrating the PB for spotlights, or go for a very good quality unit which is sound rated (as well as fire).

 

If this is life or death, you could drill the existing plasterboard and pump the voids with cellulose (Warmcell from PYC), then over-board and skim.