Insulation between rafters

Hi
I am in the process of having a new roof fitted. I am told I need a U value of 0.18. The rafters are 75mm deep. I have to leave a 25mm space between insulation and breathable membrane. This leave me with only 50mm space for insulation. Due to height restrictions I only want to use the minimum depth possible to achieve the required U value of 0.18 by means of a 50mm PIR board and a multi foil insulation. Any advice would be much appreciated.