Insulation between floors - top or bottom?

[SuperPav]

Sorry if this is a silly question but... we have 254mm posijoists between floors. If I put a 50 or 100mm layer of acoustic wool in the cavity, does it go up against the underside of the upstairs floorboards, and then run services below it, or do I run all services and then the acoustic wool sits below, just above the downstairs ceiling? The first way would be more practical for me, does it make any difference?

 

 

[nod]

Run all your services first 

Then put the insulation in beneath

[Nickfromwales]

3 hours ago, nod said:

Run all your services first 

Then put the insulation in beneath

We're opposites again lol.

As the top chord of the posi-joists takes up a good 50-60mm depth or more which is dead space anyways, I always recommend putting the insulation up in between, tight to the underside of the upper deck, and to hold it up with either fishing line or a nylon string and a staple gun. Then we run all the services underneath the insulation. Helps a lot with keeping most of the service void open and clear, and I prefer that for electrical etc so you're not cutting or compromising insulation where there are MVHR vents / downlights / speakers and so on. The insulation can go in early and be done with, and IMO it's a far easier install all round. 

Much of a PITA when the insulation fitters have pushed a cable off point and you need to go digging through acoustic insulation to find it.

[crispy_wafer]

47 minutes ago, Nickfromwales said:

We're opposites again lol.

As the top chord of the posi-joists takes up a good 50-60mm depth or more which is dead space anyways, I always recommend putting the insulation up in between, tight to the underside of the upper deck, and to hold it up with either fishing line or a nylon string and a staple gun. Then we run all the services underneath the insulation. Helps a lot with keeping most of the service void open and clear, and I prefer that for electrical etc so you're not cutting or compromising insulation where there are MVHR vents / downlights / speakers and so on. The insulation can go in early and be done with, and IMO it's a far easier install all round. 

Much of a PITA when the insulation fitters have pushed a cable off point and you need to go digging through acoustic insulation to find it.

 

Good logical approach, allows us to get the job done and materials into the fabric of the building instead of sitting on the floor getting in the way.

[Nickfromwales]

1 hour ago, crispy_wafer said:

 

Good logical approach, allows us to get the job done and materials into the fabric of the building instead of sitting on the floor getting in the way.

Yup. And less pressure on you to out-perform the guys waiting to close the areas off later downstream too   

[nod]

23 minutes ago, Nickfromwales said:

Yup. And less pressure on you to out-perform the guys waiting to close the areas off later downstream too   

My site experience kicking in again

If we insulate before electricians or plumbers 

They either pull it down or aske us to remove it 

If your plumbers and sparks are happy to breath the fibers in Go for it 😁

It’s easier to put it in first as Nick says 

But check with your sparks and plumber first I bet at least one has an allergy to the stuff 😷 

[ruggers]

Interesting to read the pro's and cons of fitting insulation before or after MVHR, plumbing and electrics. I've been thinking about the same. I see nods point of people having to or not wanting to work around pre installed rockwool but also the pro's of pre fitting it so that full slabs can be installed and not having to later cut around plenums and down lights etc. I will be the one installing 85% of the plumbing and MVHR but not the wiring.

 

If I'm passing semi rigid ducting down from upstairs to the ground floor plenums, the ducting will have a nice sweeping bend on it into the 253mm posi joist void, so I'm thinking the ducting will naturally sit better in the lower half of the joist with insulation above?

 

For maximum acoustic performance, is there any best position for the rockwool to be placed. Eg does having is hard up against the underside of the 22mm flooring work differently with no air gap either side?