Flyingbartman

Ian, I was advised by the installer 50mm PIR would be able to cope with the pressure of the cellulose, however 25mm had the risk of blowing out. Hence no allowance for additional OSB.

I was very interested in reading this topic as we are doing the exact same exercise. My architect planned for 140mm frametherm 32 between studs and 25mm PIR over the studs followed by 15mm fireboard. I looked into blown cellulose insulation and received a quote for just over 100/m3. The best price I can find for the frametherm 32 is about 93/m3 so for me it is an easy choice as the cellulose is an installed price. It wouldn't even be worth doing the frametherm as a DIY job. Hence I find it interesting to read most people opt for mineral wool due to the cost. Our house is on the large side at 490m2 which may make a difference, however I wouldn't imagine it would make that big a difference. My plan now is for 140mm cellulose between studs with 50mm PIR over the studs taped to form a VCL. On top of this would be a 25mm or 38mm service void finished with 15mm fireboard giving us a U-value of 0.16. Any thoughts would be appreciated.