So my extension has a part vaulted ceiling which requires insulating. Going from the drawings the ceiling was to have 100mm insulation boards between the rafters which are 147mm x 47mm C24, then another layer of 60mm insulation over that to get the U value. Now I have been able to source some 120mm insulation boards at an unbelievable price could I use these instead of the 100mm boards this would then reduce the air gap between the board an roof to 27mm is this sufficient? I will still then use the 60mm over that as I got these also at a very good price.