Hi, hoping for some advice as my indecision is doing my head in! Vaulted ceiling in 300 Yr old loft where rafters are less than 50mm in some areas so it's pretty impossible to insulate between rafters, even with 25mm stuff. I can get the min u value with 100mm pir so planning to attach the whole lot under rafter and nothing inbetween so have required air gap. Is this a bad idea, and do I still need to put something behind the purlins or risk any condensation finding it's way between purlin and pir butted up against it? Thanks in advance!
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PIR under rafters only, not inbetween as no depth, what about the purlins!
- Thread starter 1dav
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Is this a loft conversion or are you just improving what's there? Presumably you're having the bottom of the purlins exposed? I suppose a layer of aerogel or similar on the top of the purlins would complete the insulation bubble but it's £££s for a single sheet but then you may only need one sheet.
We're improving what's there. Bitumen felt due to bat license so non breathable. Happy to go over rafter with pir but yes purlins will be exposed (well maybe 25mm by the time pir goes up). Still not sure how to avoid heat loss or condensation where pir meets purlin unless I try and put a vcl behind the purlin but haven't seen any Google images where anyone has done this so guess it's not necessary and I'm overthinking it?
Or use some aerogel over the top of the rafters, as just mentioned. 
Why the rolling eyes? You asked me if the purlins would be exposed and if its a loft conversion. I answered those questions for clarity.Or use some aerogel over the top of the rafters, as just mentioned.
Because you were offered a method of minimising the cold bridging through the purlins and seemingly ignored it.
Surely if it's just a loft you would insulate between ceiling joists.
It's a lived in space, with vaulted ceiling. But it was already a lived in space with a vaulted ceiling, hence saying I'm improving - there was no insulation previously just the felt. And I am conscious of air flow hence why I didn't want to put pir between rafters as couldn't achieve a 50mm gap. Anyway I prob just need to get some local advice in person as I'm obv not explaining it very well. Thanks for your help.
I'll do a sketch ....
So this any clearer, use the aerogel on top of the rafters to connect the PIR and make the insulation layer continuous?
Thanks yes that's really clear now, really appreciate your time and will see where I can get that aerogel from as not heard of it before. Cheers
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