Green Building Forum - mixing insulation to balance thermal and acoustic performance in a cold pitched roof conversion Tue, 19 May 2026 16:49:42 +0100 https://www.greenbuildingforum.co.uk/newforum/ Lussumo Vanilla 1.0.3 mixing insulation to balance thermal and acoustic performance in a cold pitched roof conversion https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18077&Focus=305119#Comment_305119 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18077&Focus=305119#Comment_305119 Mon, 29 Jan 2024 19:12:25 +0000 mattjcates 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.]]> mixing insulation to balance thermal and acoustic performance in a cold pitched roof conversion https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18077&Focus=305120#Comment_305120 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18077&Focus=305120#Comment_305120 Mon, 29 Jan 2024 20:22:04 +0000 Dominic Cooney mixing insulation to balance thermal and acoustic performance in a cold pitched roof conversion https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18077&Focus=305121#Comment_305121 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18077&Focus=305121#Comment_305121 Mon, 29 Jan 2024 23:02:56 +0000 Peter_in_Hungary 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.]]> mixing insulation to balance thermal and acoustic performance in a cold pitched roof conversion https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18077&Focus=305122#Comment_305122 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18077&Focus=305122#Comment_305122 Tue, 30 Jan 2024 08:36:51 +0000 tony
I would go double plasterboard, two different thicknesses, say 12.5 and 15mm on walls and ceilings, forget the rubber as that helps with impact noise,

Don’t let any air or noise in under the flor]]> mixing insulation to balance thermal and acoustic performance in a cold pitched roof conversion https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18077&Focus=305124#Comment_305124 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18077&Focus=305124#Comment_305124 Tue, 30 Jan 2024 11:51:45 +0000 Dominic Cooney mixing insulation to balance thermal and acoustic performance in a cold pitched roof conversion https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18077&Focus=305125#Comment_305125 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18077&Focus=305125#Comment_305125 Tue, 30 Jan 2024 12:02:55 +0000 Dominic Cooney mixing insulation to balance thermal and acoustic performance in a cold pitched roof conversion https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18077&Focus=305126#Comment_305126 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18077&Focus=305126#Comment_305126 Tue, 30 Jan 2024 16:52:16 +0000 mattjcates 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]]> mixing insulation to balance thermal and acoustic performance in a cold pitched roof conversion https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18077&Focus=305128#Comment_305128 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18077&Focus=305128#Comment_305128 Tue, 30 Jan 2024 19:16:54 +0000 Nick Parsons
Bldg Regs in England is no longer 0.18 for sloping ceilings, I am fairly sure, since July 2022. Pretty certain it's now 0.16 for all elements of roofs.]]> mixing insulation to balance thermal and acoustic performance in a cold pitched roof conversion https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18077&Focus=305129#Comment_305129 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18077&Focus=305129#Comment_305129 Tue, 30 Jan 2024 19:19:50 +0000 Nick Parsons
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]]> mixing insulation to balance thermal and acoustic performance in a cold pitched roof conversion https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18077&Focus=305130#Comment_305130 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18077&Focus=305130#Comment_305130 Tue, 30 Jan 2024 20:43:30 +0000 djh Posted By: mattjcatesThe 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.(a) Use a vapour control membrane rather than an airtightness membrane.
(b) PIR panels are vapour tight, but the gaps between them and the joists aren't, and neither are the joists themselves.
(c) The rockwool is on the warm side of the insulation, so vapour shouldn't condense anyway.

Bottom line - I don't think you need to worry too much about vapour and the rockwool]]> mixing insulation to balance thermal and acoustic performance in a cold pitched roof conversion https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18077&Focus=305131#Comment_305131 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18077&Focus=305131#Comment_305131 Tue, 30 Jan 2024 21:20:49 +0000 mattjcates 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.]]> mixing insulation to balance thermal and acoustic performance in a cold pitched roof conversion https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18077&Focus=305132#Comment_305132 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18077&Focus=305132#Comment_305132 Tue, 30 Jan 2024 21:42:12 +0000 djh mixing insulation to balance thermal and acoustic performance in a cold pitched roof conversion https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18077&Focus=305133#Comment_305133 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18077&Focus=305133#Comment_305133 Tue, 30 Jan 2024 22:21:59 +0000 Dominic Cooney mixing insulation to balance thermal and acoustic performance in a cold pitched roof conversion https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18077&Focus=305135#Comment_305135 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18077&Focus=305135#Comment_305135 Wed, 31 Jan 2024 10:14:01 +0000 Peter_in_Hungary Posted By: djhYou definitely don't want a breather membrane on the warm side.
+1
Posted By: djh(b) PIR panels are vapour tight, but the gaps between them and the joists aren't, and neither are the joists themselves.

For effective thermal (and sound) insulation there shouldn't be any gaps. It is usual to foam in sheet insulation between rafters.

Whilst wool insulation will be a bit worse than PIR for thermal insulation it will be better for sound insulation (and easier to get a good fit to the joists).

Also wool type insulation is vapour open so any moisture creeping through the VCL will have an easy route out.]]> mixing insulation to balance thermal and acoustic performance in a cold pitched roof conversion https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18077&Focus=305137#Comment_305137 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18077&Focus=305137#Comment_305137 Wed, 31 Jan 2024 11:13:56 +0000 djh