Replacing Roof - Insulation

[Steveb21]

I am replacing the roof on my dormer bungalow this coming week.

New membrane, battens and tiles.

I have drawn image below.

Orange = Dormer room

Where i have drawn red lines is where i will remove current insulation and replace. The joists are 200mm/8" deep. I will use either mineral or glass wool here.

The area in question is where i have marked with blue arrows. The rafters are 75mm. I was thinking to add in 25mm acoustic insulation.

https://www.travisperkins.co.uk/cav...25mm-11-10m-4-x-600-26-64m2-per-pack/p/848595 , leaving a 50mm air gap. Reason for this is to reduce road noise coming from that area. Currently if i put my head against the slopes in the dormer i can hear the road traffic noise. In theory currently there is plaster board, 75mm of space and the membrane and roof tiles.

Is this the correct way of tackling this issue? Is there any other product I could use?

 

[Steveb21]

Any one?
Rockwool RWA45
or

https://www.travisperkins.co.uk/cavity-and-internal-wall-insulation/knauf-acoustic-roll-ready-cut-25mm-11-10m-4-x-600-26-64m2-per-pack/p/848595

Which can i use in the sloping area pointing with the blue arrows.

 

[^woody^]

Add 50mm, a 25mm void will be OK for that short distance, or even full fill with no gap if you use breather membrane that does not need a void.

Fit a vapour check layer to the underside of the rafters in that location, and you could also use acoustic plasterboard - its blue.

 

[Steveb21]

Woody
I stripped roof and found a white polystyrene type Insullation. 50mm in between the rafters were the dormer slopes were. Is this type any good? I expect it to have been installed back in the 80s.

 

[Steveb21]

Could I leave the polystyrene in place (50mm) and add 25mm rockwool on top? 75mm in 75mm rafter. Then membrane over - using Klober Permo Forte.