Pitched roof insulation options

[andreas]

I have had to remove the plasterboard on one face of our pitched roof due to water damage and would like to put some insulation up at the same time as there is currently none.

The roof is going to be re-tiled and a breathable membrane fitted during summer 2025 but at the moment it has the black bitumen type non-breathable membrane. There is no ventilation. The rafters are 100mm deep.

What are the best options?

It would be best if any solution can consider that work is going to be done in the not too distant future and capitalise on upgrading again later. Could I fit 50mm PIR between the rafters and then use 25mm PIR below the rafters before boarding over with soundbloc plasterboard, then when the breathable membrane is fitted in the future back-fill will another 50mm PIR? Will anything done now be required to meet building regulations?

[jayc89]

How much headspace can you afford to lose? 50mm PIR between the rafters and as thick as possible insulated PB under them. 

[andreas]

1 hour ago, jayc89 said:

How much headspace can you afford to lose? 50mm PIR between the rafters and as thick as possible insulated PB under them. 

very little which is why I looked at 25mm insulation below.

Is there any reason not so use separate plasterboard and insulation? I already have the sound blocking plasterboard so it would be most economical to buy only the insulation.

[bmj1]

8 hours ago, andreas said:

very little which is why I looked at 25mm insulation below.

Is there any reason not so use separate plasterboard and insulation? I already have the sound blocking plasterboard so it would be most economical to buy only the insulation.

 

Mannok do an insulated plasterboard that is 17mm PIR + 9.5mm plasterboard. Works a treat. Used it on our project.

[andreas]

9 hours ago, bmj1 said:

 

Mannok do an insulated plasterboard that is 17mm PIR + 9.5mm plasterboard. Works a treat. Used it on our project.

I can get away with 40mm total, so 25mm insulation and then 12.5mm plasterboard is fine. Any more than that and the plasterboard will start to cover the windows. Will building control accept that the original design of my house means that is all that can be done?

I am also keen to know if there is any reason not to put separate insulation and plasterboard up as I already have the plasterboard.

[Buzz]

9 hours ago, andreas said:

I am also keen to know if there is any reason not to put separate insulation and plasterboard up as I already have the plasterboard.

No reason whatsoever, just a bit more faffing around as the 25mm will need to be pinned first then then the plaster board fixed afterwards and make sure you mark all of your rafters as you go along with the 25mm otherwise it will be a very frustrating game of find the fixing point .

[Redbeard]

Back to your original post - you said: 'Could I fit 50mm PIR between the rafters and then use 25mm PIR below the rafters before boarding over with soundbloc plasterboard, **then when the breathable membrane is fitted in the future back-fill will another 50mm PIR? Will anything done now be required to meet building regulations**?' (** my emphasis **)

 

If you go now with the 50 between and 25 below you may need BC approval if it is more than 50% of the relevant thermal element (the sloping ceiling) *in the room you are stood in* (Regulation 23 of Part L). As 75mm would not achieve 0.16W/m2K you would have to argue practicability with the BCO.

 

You also cannot automatically assume that you can fully-fill the remaining 50mm rafter depth when you subsequently re-roof. The BEIS retrofit Best Practice guidance for insulation over rooms-in-the-roof says 50mm vent space req'd, common practice since 2010 Bldg Regs has been 25mm, and only a select few membranes claim to be suitable for full-fill unless you are counter-battening on top, effectively putting the ventilation gap *over* the membrane rather than under.

 

 

Edited October 11, 2023 by Redbeard
spelling

[andreas]

21 hours ago, Buzz said:

No reason whatsoever, just a bit more faffing around as the 25mm will need to be pinned first then then the plaster board fixed afterwards and make sure you mark all of your rafters as you go along with the 25mm otherwise it will be a very frustrating game of find the fixing point .

Thank you, I agree it will be frustrating but also frustrating would be losing the money I spent on the sound block plasterboard and then disposing of it haha

[andreas]

5 hours ago, Redbeard said:

Back to your original post - you said: 'Could I fit 50mm PIR between the rafters and then use 25mm PIR below the rafters before boarding over with soundbloc plasterboard, **then when the breathable membrane is fitted in the future back-fill will another 50mm PIR? Will anything done now be required to meet building regulations**?' (** my emphasis **)

 

If you go now with the 50 between and 25 below you may need BC approval if it is more than 50% of the relevant thermal element (the sloping ceiling) *in the room you are stood in* (Regulation 23 of Part L). As 75mm would not achieve 0.16W/m2K you would have to argue practicability with the BCO.

 

You also cannot automatically assume that you can fully-fill the remaining 50mm rafter depth when you subsequently re-roof. The BEIS retrofit Best Practice guidance for insulation over rooms-in-the-roof says 50mm vent space req'd, common practice since 2010 Bldg Regs has been 25mm, and only a select few membranes claim to be suitable for full-fill unless you are counter-battening on top, effectively putting the ventilation gap *over* the membrane rather than under.

 

 

Thank you. Fortunately it is well below 50% as there is a large window on the elevation that needs repair, so it is probably more like 25%. 

If common practice has been 25mm ventilation space, could I fit 75mm PIR between the rafters safely and then the 25mm over the top. I don't think that would meet the 0.16w/m2k either but it would be closer.

[Redbeard]

No it wouldn't meet the U value target, but if headroom is at a premium you could argue the toss if yopu needed to, but if Bldg Regs do not apply as it's not 50% or more of the relevant thermal element, arguably just do your best (though personally I would be trying to meet the target).

 

Common practice has been 25mm, and some problems have arisen due to inadequate ventilation. In my view the gov't Best Practice guidance (https://www.gov.uk/government/publications/room-in-roof-insulation-riri-best-practice). Is well-advised. If you decide to go for 25 only then, when you do have the roof replaced, ask for written confirmation from your roofer that the membrane to be used is certified for use with a 25mm ventilation gap (and then check that they actually used that one). It goes without saying that ventilation feeding that gap (the 'ins' and the 'outs' should be checked and confirmed to be sufficient.