New addition vented roof abutting a non vented roof

This is a homeowner's question posted to a building forum, not a news story, so there's no program or rebate update here. The homeowner is building a 10x20 addition in Anchorage, Alaska, and running into a tricky roof detail: the existing roof is unvented (foam sprayed directly against the sheathing), while local rules require the new addition's roof to be vented and insulated to at least R-49. The two rooflines meet at the same ridge, which raises the question of how to transition between a vented and an unvented roof without creating moisture problems.

One forum member suggested a workaround: build the new roof's sheathing about two inches higher than the old roof's sheathing, then add strapping over the old roof so both surfaces end up level and share one continuous vent channel running from the eaves to the peak. This would let the homeowner leave the old roof's existing insulation alone rather than tearing it out.

For most readers, this is a reminder that additions aren't always a simple matter of matching materials to an existing house. When old and new roof or wall assemblies meet, the way they handle moisture and airflow has to line up too, especially in cold climates like Alaska where trapped moisture in a roof cavity can cause rot or mold. Anyone planning an addition that ties into an older roof should expect this kind of detail to come up in design and permitting conversations.