insulating the outside of a pitch roof

[hallega]

Up to roof level and about to install the timbers. 

We are having hand made clays and I want as much insulation as possible as we have used Durisol blocks as main construction due to their U value.

Husband is being told to use multi layer insulation and I want as deep as possible warm roof. We have put this material under our barn roof and it does make a difference, but I don't live in it!

Has anyone else used a thick warmroof under tiles, is this possible at all due to possible fixing problems?

Any info at all would be helpful please.