Wrong insulation used, Small claims court or not ?

[Tom]

How well would the PIR have worked with the open web joists anyway? Sounds like it would be a pain to fit to ensure no gaps - and chances are, left to their own devices, the builders would have done it badly. So perhaps mineral wool isn't such a bad substitute after all. 

[Iceverge]

Annoying yes, very but it's not the end of the world. 

 

I'm of the opinion PIR is a bad thing between timbers anyway and wouldn't have seen it as a good spec. Differential shrinkage between the foam and timber leaves gaps and you'll get thermal bypass. It's really tricky to fit well and most builders can't afford the time to do it well.  Practically mineral wool will perform closer to the theoretical U value than PIR . Not to mention fire, sound, off gassing. 

 

I wouldn't panic about the lower U value.  Installed value is lightly to be about 0.25 worst case. Cheapest mineral wool and solid joists. 

 

PIR theoretically might have been 0.16. This would translate into about £75 more in heating per year at the new gas price of 15p/unit on a 100m2 roof. Not a nice pill to swallow but hardly catastrophic. 

 

Overboarding with 50mm Pir would take you back to 0.16 assuming worse case for the existing roof.