Today I went to the house because I'd agreed to meet the electrician on-site, to go through a few issues regarding cable routes etc.

Whilst I was there, I noticed that the builders had completed the insulation in the front attic room, ready for beginning the taping on Monday morning (the back rooms still need another layer). However, although they had mainly done a good job, where they had stuffed the void behind the purlin with quilt insulation, it had pushed up the 25mm PIR boards, compromising the air gap for ventilating the roof:

Something to take up with them on Monday, no doubt. They've also taken delivery of a pallet-load of 9mm plasterboard, which seems a bit premature to me considering they haven't even started the taping yet, and still have the bottom layer of PIR to put on at the rear.

Meanwhile, the costs are in for the VIP - significantly cheaper than Spacetherm. I've been reading various things about VIP and the main concern (apart from vulnerability to puncture both during installation and in service - both of which I think we can mitigate on this project) is degradation in performance over time. Need to clarify the situation with the suppliers on this issue.