How do I install a vapour barrier when insulating from the outside?
First: Tyvek is _NOT_ a vapor barrier – it is an infiltration barrier to reduce air infiltration. Were it an actual vapor barrier, it would be an absolute disaster as it would trap moisture inside your walls causing eventual rot at the sill-plates. Been there, seen that condition far to often for it to be funny any more. But installing Tyvek after all the work is done won’t hurt in any case. Most spray-foams are inherently vapor barriers, so no additional material is needed. It is simply not practical to install a separate vapor barrier from the outside. First, you cannot wrap your studs – so – they would be left ‘within’ the warm space. This could put the dew-point within the warm area at each stud – not a good situation. Cutting to the chase, this seems like a massive, expensive undertaking for what may be a simply solved situation. I do not know the nature of your problem nor who is paying for the removal and repair. But if just formaldehyde infiltration is the issue, there are quit
