Vapor retarder needed?

This is a homeowner's question posted on a building science forum, not a news story, so there's no policy or program update to report. Someone renovating a 1973 home in Vermont is rebuilding their exterior walls with fiberglass or mineral wool insulation, rigid foam board on the outside of the sheathing, and a weather-resistant wrap, and asking whether they still need to add a special interior layer called a vapor retarder (a material that slows moisture from passing through a wall).

The question draws on years of expert writing from the site's building science columnist, who has generally argued that adding an interior vapor retarder is often unnecessary, and can even trap moisture and cause problems, when a wall already has enough rigid foam insulation on the exterior side. That foam layer keeps the wall sheathing warm enough to avoid condensation problems, which is the main job a vapor retarder would otherwise do. The columnist has also warned that skipping the exterior foam without expert calculations, or installing a true vapor barrier (a stricter, less breathable material) on the interior of a wall that already has exterior foam, can cause trouble.

For homeowners planning a similar wall upgrade, the takeaway is that pairing continuous rigid foam insulation on the outside of the sheathing with air sealing measures can eliminate the need for an interior vapor retarder in most cold climates, but the right approach still depends on how much foam is used relative to the total wall insulation and on local climate conditions.