Insulation of extension ceiling.Pitched roof/cathedral ceiling

[portlouis]

Hi,
My ceiling developed a leak from the bathroom drain which runs above it.
As the pitch was too low to crawl above the ceiling we decided to break away the ceiling to make life easier.

Would now like to insulate the ceiling with celotex/kingspan rather than mineral wool and also would like to do away with the ceiling, so kitchen has sloping ceiling aka cathedral ceiling.

How to achieve the u value or exceed it. The rafters are 150mm deep and there is a breathable membrane then clay roof tiles.
Thanks.

 

[jeds]

100mm between rafters and 40 or 50mm continuous layer beneath.

 

[Scarlet Pimpernel]

If you really wanted to push the boat out, you could insulate above the rafters.
Obviously cost would depend on the size of the roof.

 

[Mag2.0]

If you really wanted to push the boat out, you could insulate above the rafters.
Obviously cost would depend on the size of the roof.

that would require removal of the roof covering?

Id' go with as Jed's suggested. 100mm between and then 50mm over.
Or you could reduce is down and have insulated plasterboard, although im not a fan of the stuff.