Timber cavity wall for dormer with insulation inside, between and outside

[nghakrmyk]

Hi all - unsure if best to put this here or in heat insulation.

 

What are people’s thoughts on a timber cavity wall build up (with hung slates) where I may need to add insulation to the outside of the wall as well as in between and in front of the studs.

 

I’m looking at roughly 62.5mm insulated plasterboard inside, 100mm PIR inside 100mm studs and 50mm PIR outside. I might be able to get away with just the insulation outside (and then a proper separate VCL inside) ie do away with the insulated plasterboard but TBC.

 

The issue I’m facing is that a small section of a steel beam supporting the outermost wall of an L shaped outrigger dormer will be above the existing roof line and therefore essentially exposed to the outside elements if not insulated (see red box in the picture below which hopefully makes sense). It might be overkill but just trying to make sure potential condensation is managed.
 

So far I had the following questions:

- Is PIR ok to use as the outside insulation? Foil backed?

- if so, what’s the correct build up for the outside element? 

- do I need to cross batten for slates?

 

Any input would be greatly appreciated.

 

 

[nghakrmyk]

I have since spent a bit more time on EWI and am realising this is maybe not a simple question.

 

It seems wood fibre and EPS are the more common options but I will have to manage the build up carefully as there is only c.100mm total for insulation and tiles etc ie between the outside of the internal skin of the existing brick wall below (which is what the new stud wall is being built off) and the external brick wall.
 

Either way any insight / assistance would be greatly appreciated.