Vapor Barriers and Alaska

A homeowner in Anchorage, Alaska, is re-siding a 1983 house and asked for advice on a wall moisture question. The house has 2x6 walls with plastic vapor barrier on both the inside and outside of the studs, fiberglass insulation, plywood sheathing, and a thick coat of polyurethane over the original cedar siding. The contractor plans to wrap the whole house, including a new addition, in Tyvek (a water-resistant barrier, or WRB, that keeps out rain but lets some vapor pass) before installing new LP SmartSide siding over the old cedar. The homeowner is worried that adding a second vapor-blocking layer outside an already vapor-blocked wall could trap moisture inside the wall cavity, even though no mold or rot turned up when they opened up sections of the wall during the addition work.

This is a specific building-science question posted to a contractor forum, not a program announcement or new product. No answer had been posted yet at the time of writing, so there is no recommended fix to report.

For homeowners in cold climates with older walls that already have a vapor barrier, the takeaway is a general one: adding new exterior layers over existing siding during a re-siding job can change how a wall dries out, and it is worth understanding what materials are already in the wall before wrapping it in something new. If you are planning a similar re-siding project on an older home, especially one with a plastic vapor barrier already built in, it is worth having someone look closely at the full wall assembly before adding another layer outside.