What insulation to use for external timber wall

[danchapmanme]

My 1970's house has external timber walls upstairs. The walls consists of the following elements, from outside in:

1) PVC cladding
2) Felt (Looks like roof felt)
3) 4x2 timber studs
4) 50mm polystyrene (as insulation)
5) 6mm plywood

I was considering replacing the polystyrene insulation with Rockwool RW5 Insulation Slab (50mm) and replacing the plywood with Fermacell (12.5mm).

Am I right in thinking that you shouldn't fill the complete cavity with insulation because of condensation?

Is the Rockwool/Fermacell a good solution or is another product combination better? e.g. Celotex etc. I'd like the walls to be better insulated and sound proofing.

Thanks.

 

[AronSearle]

Technically you may already have a condensation risk, unless their is a vented cavity behind the PVC cladding, which you don't describe.

Vapour resistant materials on the warm face, breathable materials or vented cavity to the cold face.

Likely poor fitting or gaps in the PVC, and poor levels of insulation prevent condensation actually occurring.


The plywood maybe providing racking resistance, fermacell may not provide an adequate structural replacement.

If you can provide a vented cavity behind the cladding, then you can fit insulation between the joists and use insulated plasterboard, if you can't I would not want to advise you.

 

[danchapmanme]

Thanks.

I'll check if its a vented cavity behind the PVC.

If it is, how much insulation would I use between the studs? Do I fill half the cavity e.g. 50mm? Should I use Celotex (GA4000)?

I have just read up on Fermacell. It can replace timber sheets, providing racking strengh.

So new wall construction would be:

1) PVC cladding (assuming cavity)
2) Felt (Looks like roof felt)
3) 4x2 timber studs
4) 50mm Celotex? Is this the correct thinkness?
5) Fermacell

Do I need a vapour control layer between the Fermacell and Celotex or will the foil covering on the Celotex provide this?

BTW - Its only the front and rear facing walls that are timber, the side walls are brick/cavity walls.

 

[AronSearle]

If you have a cavity behind the PVC cladding, full fill, if you don't, leave a 25mm airspace.

At the very least ensure the PVC cladding has some kind of overlap that allows ventillation.

Avoiding condensation is most commonly done by ensuring the warm side is 5x more vapour resistant than the cold face.

Putting a VCL on the warm side will help, but you can see its hard to say you have a 5-1 ratio with PVC cladding.