Wall Insulation - combining 3 types..!?

[john61ct]

Yes continuous vapour barrier is needed

at least facing the inside

but if your outer skin may leak, or places like PNW where EVERYTHING is humid most times I would do the outside layer too.

Note a thin skin of spray foam both helps prevent condensation and helps the dropcloth if that is your VB.

XPS is only needed facing outside arctic conditions, helps buffer the polyiso.

and really important, you can't just go in between the metal studs & beams,

the R-value **over** the metal bits is important, between them and your living space.

So if you want 2.5" that is best as a continuous sheet. Then the "within the metal" is infill, yes helpful but that is just supplementary.

Just giving all the metal a thin covering of spray foam, plus slabs over the metal would be better than just filling within the metal, if costs forced a choice between the two.

Plus sprayfoam to seal gaps / glue between and behind the boards, striving for a 100% sealed envelope.

2.5" continuous, that's all you have over the metalwork, adding lot$ more in the gaps behind, useful yes but will definitely be diminishing returns.

But if funds are available, best is a professional coming in doing all the metal, scraping to get a flat plane say intruding .75-1" into the interior, then you only need 1-2" boards over that depending just how arctic, boondocking in snowy mountains ski areas?