Which insulation

[Mareille]

I have a single story passage which is made out of concrete blocks I am planning on plastering it with insulation behind however the builder wants to use metal stud and line the back with silver foil insulation which seems to cost a fortune my idea is to use plaster lined with polystyrene which appears to be a lot more cost effective, this idea is met with lots of huffing and puffing from the builder and warnings about damp. my question is how does the insulating properties of the two methods compare and why would the polystyrene let in damp another consideration is I want to keep the depth of insulation as small as possible as I don't want to make the passage too narrow

Thanks Chris

[Chancer]

You need to give more details.

I am assuming that you mean parpaing by concrete blocks?

What if any exterior finish is there to prevent moisture ingress.

I speak to you now from a converted outbuilding that has double thickness solid brick external walls, when I bought the place it had been done as you propose with doublage and the walls were thick with black mould from floor to ceiling, the plasterboard was sodden and could be pulled off by hand, the polystyrene behind had degraded into millions of individual granules which then had to be swept from the floor, the majority just flew away with the wind. Amazingly it was in use as the Hôtel kitchen [:-))]

I have redone the walls with the rails with panneau laine de verre revetu between them and a tiny airspace and no longer have any damp.

[La Guerriere]

... and metal stud is 48mm plus 13mm of plasterboard = 61mm thickness from the wall