Bay Tiling and Insulation

[wingit]

I'm renovating the inside and outside of my bay window, it's a 1930's house. I'll be using YBS BreatherQuilt and solid PIR foil insulation.

I've stripped away the plasterboard on the inside of the bay, and plan to fit the BreatherQuilt against the inside and outside face of the rafters, for two reasons. firstly, I'll have enough on one roll to do so, secondly it seems that more insulation is better.

So going from outside in, it would be BreatherQuilt, then 70mm PIR, then on the inside of the timber, BreatherQuilt, then 25mm PIR.

I'll be leaving at least a 25mm air gap between the outside BreatherQuilt and the 70mm PIR insulation between the rafters.

Question: Will it create a condensation or ventilation problem by having a breathable insulation membrane on the inside and outside of the timber rafters?

Edited to save time for anyone reading; Breathable insulation needs space to expand into, to my understanding, this is achieved with counter battening.
In the 1930's vertical bay I planned to fit in, on the outside, there is not enough space to fit it without going past the sills. On the inside, TLX Silver, for example, is suitable but requires battens to allow for expansion, so PIR foil insulation is the way to go.

 

[wingit]

I'll be using Redland Rosemary clay tiles on 38 x 25mm battens, can anyone recommend suitable screws to fix the battens to the rafters?

 

[datarebal]

We have used the ybs product, however

Superfoil do a similar product. Email their tech support for advice.

 

[wingit]

Having spoken with a couple of technical departments, that type of breathable membrane is not suitable for internal insulation.
TLX Silver, for example, is suitable but would require 38mm battens over it to allow for expansion of the insulation, negating the space-saving I was hoping for.

For the external application, under the bay tiles, I was told that for TLX Gold it would need to be the insulation against studs, then 38mm battens to allow for expansion of the insulation membrane, then battens on top to tile onto. Is this called counter-battening?

 

[datarebal]

https://www.superfoil.co.uk/breathable-multi-foil-insulation/

Good product and not expensive..check the u values against others.

Yes you describe counter battening, but some multi foils can be used without if there is room to drape