External wall upvc

[DonDonson]

Hi, I'm building an external wall from 4x2, I've been advised by celotex of the correct insulation that is needed to comform to regs which is 100mm celotex between joists and 12m plasterboard with 1" celotex ( havent got the paperwork infront of me of correct thickness but im sure it was 1") attached to it to fit on the inside.
On the outside I am going to put 12mm ply and then wanted to put upvc hollow soffit on top.
The question I'm asking is should I put building paper on top of the ply then battens and then upvc?

 

[noseall]

Instead of building paper use breathable membrane.

 

[DonDonson]

Thanks for the reply, still apply the membrane straight to the ply and then batten?

 

[DonDonson]

Anyone have any other ideas about battening on top of the breathable membrane to leave an air gap or just nail the upvc cladding straight to it?

 

[^woody^]

You can't use hollow soffit, as it is not designed for vertical use and will let water in. You need proper shiplap

Fixing straight to the ply will not allow the membrane to breath and will not allow condensation between the membrane and the back of the cladding to evaporate

If this is via b/regs then it is not a approved way of construction

 

[DonDonson]

Sorry I misled, it will be using upvc ship lap cladding, I was informed by celotex of insulation requirements when discussing the external wall, the wall can't be double skin because of space that I've got, how do you suggest i do it then? I was going to use 4x2 with 100 mm celotex between, another 1" of celotex on the inside plasterboarded and skimmed, on the outside would be 12 mm ply, breathable membrane, batten and then the cladding!! Would be very grateful for your recommendations

 

[^woody^]

What you have stated there is the way to do it

In practical terms, 4x2 is not 100mm so 100mm insulation will project past the timber. Use 90mm.

Better still, use 90mm CLS as it will be straighter

 

[DonDonson]

You stated before that it was not a approved way of construction via building regs? What's wrong with this way if there's adequate insulation, breathable and watertight?

 

[^woody^]

It is not breathable unless there is an air cavity between the membrane and the cladding

 

[DonDonson]

But this is ok right? Because there will be battens on top of the membrane and then the cladding leaving an air gap, does this conform to regs?

 

[^woody^]

breathable membrane, batten and then the cladding!!

What you have stated there is the way to do it