Multifoil insulation on solid wall

[Det]

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.

Can anyone through any light on this issue?

[welsh brickie]

the multifoil tends to create condensation more than fibreglass insulation.

[Det]

Surely if it creates condensation then it is not an effective insulation. You only get condensation if the surface is cold.

[welsh brickie]

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.