Ken
I think that Celotex require a 25mm residual cavity even if you do acheive the required U value. I know a builder had a simlar issue near me and was forced to pull it down.
Sorry.
C
I have just fitted 2 big Velux GPL roof lights into my loft conversion. 1st Velux opens and shuts perfectly, it didn't need any balancing. 2nd Velux did not have the hinge mechanism engaged with the spring. So I do as the instructions say and lift it fully open until the spring and catches click...