Pitched roof insulation

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Hi

Did do some trawling through the forums and all I could find was answers to flat roof insulation or pitched roof insulation but in the rafters.

My question is on a pitched roof but insulation in the joists. I’m looking at using PIR as I’ll be using a lot of that already (converting a garage). I’ve already put some tile vents in and will get a vented soffit so ventilation is covered. However, I’m struggling to find out what the minimum requirement is for PIR insulation in a roof. It always seems to be about rafters. I’m assuming it’s not different but just trying to tot up what I need to order

Thanks
 
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It would depend on the joist depth. You would not want any air gap between the ceiling and insulation or between the insulation layers.
 
It would depend on the joist depth. You would not want any air gap between the ceiling and insulation or between the insulation layers.
Joist depth is 125mm. So will be that thickness all the way through. Would using 30mm over the joists be sufficient at that point or does it need more to cover the loss through the joist?
 
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Which regulations are you working under, old or new? (old would be pre June 2022)
 
Which regulations are you working under, old or new? (old would be pre June 2022)
Good question. I think I got my application in for May so assume that’s old? At a high level is there much difference?
 
Good question. I think I got my application in for May so assume that’s old? At a high level is there much difference?
Old standard u-value was 0.18 - new standard is 0.16. Seems small but does make a difference. To achieve 0.18 the basic specification is 100mm PIR insulation between rafters and a continuous layer of 50mm beneath. There are variations in some materials that appear to be the same so you might come across a different make up. You don't say the type of roof membrane you have. Is it a breathable membrane? If not you'll have a problem with that spec because the minimum ventilation void above insulation is 50mm. If that is the case you should go for the best alternative you can reasonably achieve. Most inspectors would accept a reduction due to the difficulty of the existing structure.
 
Old standard u-value was 0.18 - new standard is 0.16. Seems small but does make a difference. To achieve 0.18 the basic specification is 100mm PIR insulation between rafters and a continuous layer of 50mm beneath. There are variations in some materials that appear to be the same so you might come across a different make up. You don't say the type of roof membrane you have. Is it a breathable membrane? If not you'll have a problem with that spec because the minimum ventilation void above insulation is 50mm. If that is the case you should go for the best alternative you can reasonably achieve. Most inspectors would accept a reduction due to the difficulty of the existing structure.
Yeah it’s one of the old non breathable ones. So I’ve installed some vent tiles at the back and a vented soffit at the front.

Won’t be insulating in the rafters, it’ll be between and above the joists.
 

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