Alterative option to 150mm celotex + 50mm celotex?

[Iceverge]

Give me a cross section please of the house. 

 

I think I have an economical solution in mind. 

 

 

[Iceverge]

 

 

Raise the tie for the rafters, add 125mm PIR on the underside of the sloped sections, 

 

Full fill the rafters with mineral wool/cellulose and pur 400mm above the flat section. 

 

OSB above the rafters and batten and counter batten above for ventilation. 

 

Add a 100mm service cavity below the rafter tie for MVHR. 

 

Forget about any storage in the roof. It's simply much too small. 

 

Even with your original design it's only 1000mm high at the apex.