Insulating and sound proofing vaulted roof

[Steveb21]

Rafters are 9x2so approx 225mm depth
I was planning to insulate with rigid board at 165mm leaving a 60mm air gap and then plaster boarding.

Is rigid board such as celotex good at keeping sound out?

Other options
Add a acoustic plaster board
Could I incorporate sound proofing in rafters and reduce my Insualtion down from 165mm? If so, what product? Rockwool acoustic slab?

Advice please

 

[Mr Chibs]

From what I remember standard practise is to apply another layer of PIR foam, say 50mm over the rafters, before you plasterboard.

This is what I have done before. Not needed sound proofing, but you could always double board with plasterboard.

 

[Mike13]

Soundproofing requires care to prevent conducted sound, so board over the rafters will be needed. You need to decide which frequencies you want to keep out (or in) most. It might need a heavy fibreglass set between the rafters for some of the thickness available

 

[foxhole]

From what I remember standard practise is to apply another layer of PIR foam, say 50mm over the rafters, before you plasterboard.

This is what I have done before. Not needed sound proofing, but you could always double board with plasterboard.

This is what I did but used insulated plasterboard.

My loft

• foxhole
• 3 Dec 2015
• 1

100mm insulation .

You can see the first sheet fixed on the left rear.

 

[Steveb21]

Is it a necessity to insulate over the rafters as well as in between them? As mentioned earlier my plan was to use 165mm celotex and the plaster board straight over the rafters.

 

[Mr Chibs]

For building regs, yes it is, think it used to be 150mm total i.e. 100 in-between and 50 under, (will stand to be corrected) it's to negate cold bridging from the wooden rafters.

If you are not doing building regs, then you can please yourself.

 

[Mike13]

If you want to soundproof properly, then yes you will need to overboard too.