Hello,
This is my first post!
Re planned loft conversion. We live near a main road and I'm keen to incorporate some soundproofing in my plans for converting the loft. Current build is concrete tiles, non breathable felt and 100mm rafters with trusses.

I can't afford to replace the whole roof so will have to make do with a 50mm ventilation gap.

My architect has suggested extending the rafters to 150mm, then 100mm PIR between rafters 60mm PIR below rafters then airtight membrane (plan to install MVHR) then 25mm battens for service void.

Now I know that PIR is a poor sound insulator and mineral wool is better so I wondered about:

a) using some "rubber joist strips" between the existing rafters and the new rafter and
b) extending the rafters to 170mm (70mm happens to be the depth of the trusses which perhaps could be re-used if they can be screwed directly underneath the existing rafter) allowing 120mm PIR between then counter battens with 50mm (or more) mineral wool between. Then airtight membrane then acoustic plasterboard.

I thought the rubber strips might help decouple the outer timber from the inner structure (except for the screws). I can't afford a full noise isolation clip/furring channel system which would also take up more space.

I thought the counter battens could also serve as the service void given the mineral wool can be compressed.

I don't really know how much difference the above changes will make compared to the original plan but we have a lot of lorries going past and I'm keen to do everything I can to reduce the noise and am willing to lose a bit of thermal performance for this (cold spots at counter battens I know).

Regarding building regs I don't know how to do a proper u value calculation but adding the R values of the PIR and mineral wool seems to meet requirements (in England)

I would be grateful for anyone's thoughts or experience with anything similar.

I'm just going through the sound insulation process on a wall and my research gave me the following
For sound insulation I wouldn't use rigid board insulation. Wool insulation is better and dense wool is needed
What I am using is 100mm normal insulation, 50mm dense wool (steprock) then heavy plasterboard either acoustic plasterboard or if this difficult to obtain then fireproof plasterboard
The plasterboard is held on a false wall of plasterboard framing separated from the real wall with the 100mm insulation between and the 50mm steprock within the framing.
You can get Resilient bars to separate the additional framing from the rafters to help better sound insulate.
Unfortunately sound insulation is more expensive (and more difficult) than heat insulating and attention to detail is more important.

+1 for Ubakus. Use the demo version if you don't want to create an account.

Had a play on Ubakus for build-up as suggested by Architect:

Thanks everyone, that's really helpful and thanks to Dominic for those calculations.
My builder is a bit reluctant to "mix insulation" but I will have a chat to him. The only possible issue I can see is that if I use an airtight membrane below (warm side) of the rockwool then if any moisture did get to the rockwool it would have no where to go as there would be PIR on the other side.
That u-value of 0.2 doesn't quite meet building regs in England (0.18) but if I make the rockwool 75mm it should do and I wouldn't lose any height if I don't have a separate service void.
acoustic plasterboard definitely makes sense too
thanks again

Yep, c4, p.80.

https://assets.publishing.service.gov.uk/media/63d8ed5de90e0773d8af2c97/Approved_Document_L__Conservation_of_fuel_and_power__Volume_1_Dwellings__2021_edition_incorporating_2023_amendments.pdf

thanks for pointing that out Nick,
If anyone else is considering it does appear to be possible using the same calculator. I was helped by having 600mm centres for my rafters.

Ha! I guessed 450 centres for your rafters, but we too have 600 (on our barn conversion) whereas the bathroom in the farmhouse was more like 400. A second layer of plasterboard (which was suggested for noise) also seemed to help the U value a tiny bit when I put it in the calculator.

Note that the wool insulation sold for acoustic purposes is a bit denser than that sold for thermal purposes. Each works a bit for the other purpose but isn't quite as good as the one that is designed for it. So double check the numbers in your calcs if necessary.