1930s home - insulating older parts of the house

[Jothetaxi]

We are renovating and undergoing a major extension of our 1930s home.  The exterior will have an insulated render. There are some minor parts of the original where we are unable to have an external render, such as between the top and bottom of two projecting bay windows at the front of the house and a projecting inglenook style fireplace on the ground floor.  

 

In these areas, the plan is for the insulation to be added internally. The bay window insulation is coming in at around 85mm deep with plaster.  The contractor would like to do the same spec in the inglenook fireplace.   The issue here is that at 85mm deep, it eats significantly into the space around the internal chimney breast and we would like to install some fitted shelving etc.  Plus, with a cavity it would make it tricky to install any joinery in this area and potentially create issues with fixings.

 

The architect has suggested a 50mm insulation, that would equate to 60mm when finished.

 

I wondered whether anyone could recommend any other insulation options here? or whether the architect's specification is the only option.

 

Thank you .

 

[Redbeard]

Why not do it in an insulated plaster such as Diathonite? That way there is no separate insulation and plaster layer. If your proposed nIWI is PIR then Diathonite will not give you as much insulation value, but at least every bit is insulating, unlike a system with battens and plasterboards. PIR thermal conductivity 0.022W/mK and Diathonite around 0.039 IIRC.

[Jothetaxi]

Thank you for the reply. That is really helpful; I had not heard of Diathonite before. I am going to look into this, as that sounds like a good alternative. 

[Jothetaxi]

Can Diathonite go over existing plaster walls, or should it be applied to bare brickwork? 
Thanks