Bedroom vaulted ceiling insulation - thinnest possible?

[ashthekid]

Hi,

 

I am currently in the middle of an old conversion project whereby the new timber roof structure has been built over the bedroom and now we have the floor joists in the head heights are a lot tighter than we thought so I'm thinking about the vaulted ceiling and how it's insulated to see if there is a better and more efficient way to thermal and sound insulate it. We have two Velux windows going in because we do not have any side wall windows whatsoever.  

So the current layers are for a cold roof:

Slate tiles, breathable felt membrane, 150mm rafters with 100mm PIR insulation between allowing for 50mm ventilation gap, then 50mm PIR across the underside of the rafters, 25mm battening, 15mm soundbloc.

It cannot be changed to a warm roof as the box gutter on the sides has already been built.

 

Is there a better way of layering it underneath the rafters for better sound and thermal insulation?

In another room where we have much higher ceilings we have decided to add an additional layer of 50mm underneath the rafters so 100mm in total along with the 100mm in between the rafters. Sadly we just don't have the headlight in the bedroom for this though which is where we have the issue.