Multifoil insulation (eg Triiso 10) supposedly has wonderful insulation properties (equivalent to 200mm rockwool or fiberglass) when lining a pitched roof. I am told it is not suitable for insulating an external wall. Why?? It would seem to me that if it is such a wonderful insulator then tacking it to battens on the (external solid) wall and then fixing the plasterboard over the top should be as effective as 200mm rockwool whilst taking up a minimum of space. The multifoil insulation would act as a vapour barrier behind the plasterboard and there would still be a small ventilated gap between insulation and outer wall.
thats why you use a breathable membrane on a roof,and with a ventilated soffit board condensation does not occur. If you use it to line a wall the moisture is trapped and has no way to disperse, causing damp.
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