PIR insulation to outside of timber frame

[Pappa]

Hi guys

 

I'm building a small timber frame extension.

This is what I want my wall to look like from inside to outside based on other designs I've seen around the internet:

 

15mm skimmed plasterboard.

Vapour control layour

89mm stud with 70mm PIR in between

9mm OSB

50mm PIR.

Breather membrane

Render board

 

What is the most thermally efficient way to attach the 50mm PIR from the outside into the OSB/Timber frame?

 

How necessary is the breather membrane? Bare in mind I've got a VCL on the inside wall.

 

How do I mount the render board on to the Breather membrane (or directly on to the PIR if breather membrane not required)?

 

Can I skip the Render board, and render directly on to the Breather membrane (or directly on to the PIR if breather membrane not required)?

 

I'm trying to do it this way because I have very limited space and I want to maximise the U-value per mm of wall thickness.

 

Thanks in advance

 

Pappa