DonnaP123

Excellent advice! And I guess I WILL have to be extremely fastidious with the VCL and just make sure we get it right! We will also have MVHR which should (I guess) also help to mitigate excess moisture. Or at least it can’t hurt.

Thank you - you are correct, the roof is built with the sarking above the rafters, tiles etc on top. We couldn’t increase the ridge height to have a full warm roof so have compromised with the hybrid version. Counter-battening would work and is a good idea, thank you.

Thanks everyone! Food for thought….dew point analysis and seeing if we can find a way to make wood fibre alone work.

Advice needed! For our woodcrete extension we have built a warm pitched roof with 80mm wood fibre sarking boards over the rafters. We planned to put approx 190mm flexible wood fibre bats between the rafters, but now realise many of the rafters are not deep enough to accommodate this. So my question is, is it okay to put PIR between the rafters (as would need less depth than wood fibre for the equivalent U value), and how much would we need to reach a U value of 0.15 overall, including the wood fibre sarking already there? I can’t find an online calculator that allows me to mix wood fibre with PIR! We would lose the breathability of the wood fibre but should still hopefully benefit from not overheating in the summer (which was the main reason for using it) given we have 80mm wood fibre above the rafters, so I don’t think there is an inherent problem with mixing the two, but could be missing something?