Two (air/vapor) barriers ?
When building a sauna you always have a foil barrier on the inside of the studs. Typically either FF-PIR or paper backed foil. Then an air gap and then interior cladding like T&G spruce. See ‘Notes on Construction’ here: http://localmile.org/trumpkins-notes-on-building-a-sauna/
Something like JM FF-PIR has a vapor perm of 0.05 and air barrier of <0.02 L/(s•m2)
What should the exterior be? Tyvek is, supposedly, an air barrier but not a vapor barrier. If any moisture/vapor were driven in to the wall structure would it realistically be able to get out? Or best for the exterior to have no Tyvek or other air/vapor barrier?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part