Pitched roof - PIR between+ above or under rafters?

[low_and_there]

Redoing our rear sloping rear addition pitched roof and on stripping have realised that the timbers - including the wall plates - aren't sound enough to reuse, so now have the opportunity to rethink the insulation a bit.

 

I had been planning on PIR between and above the rafters (200mm total - aiming for 0.12u (in line with the Mannok design referenced in several places on this forum) and felt that was better anyway than between and below rafters - given that the rafters essentially become internal to the thermal envelope - but wanted to sanity check if I'm missing something? Is there any obvious better approach - above+between or between+below?

 

We could move the Rafters 100mm higher and go with between and under instead. I have done a warm flat roof already and understand the debate there about cold vs hybrid vs warm but with pitched roofs it seems to be a different kettle of fish.

 

In case relevant, the roof is pitched to meet a firewall parapet rather than a ridge, and I'll be insulating the wall internally (it's a party wall to a cold loft at the top).

 

Related question - my builder has said he always leaves a 25mm cavity on the inside of the PIR before the PB layer to avoid condensation there. I'm placing a AT/VL in front of the PIR so had thought no need. No need for a service void on this roof.

 

Also - have considered using mineral wool rolls instead of PIR between studs as I'm aware it's hard to get a good fit - but can't find 0.032 u value rolls anywhere online (excepting in vast quantities).

 

Thanks

[Iceverge]

1 hour ago, low_and_there said:

case relevant, the roof is pitched to meet a firewall parapet rather than a ridge, and I'll be insulating the wall internally (it's a party wall to a cold loft at the top)

 

A sketch would be valuable here. 

 

Avoiding PIR between the rafters is best. 

 

How much roof thickness do you have to play with. 

 

U-Value is only one component of a house buildup.

 

Don't forget airtightness, windtightness, decrement delay, fire performance, longevity, noise protection, material availability, cost, thermal bypass and condensation risk.