For a cost-effective build or major renovation here, a targeted hybrid approach gives the best balance of performance and budget. Rather than foaming the entire envelope, it applies the more expensive closed cell spray foam strictly to the two zones where thermal leakage actually happens, and uses conventional materials everywhere else.
Seal the roof plane. Closed cell foam on the underside of the roof plane or at the attic ceiling stops the stack effect, where rising warm air forces its way out through the top of the house. Given the snowfall and the near-freezing daytime highs, this is the single best defense against the warm air leaks that melt rooftop snow and build ice dams.
Seal the rim joists. Where the home meets the foundation is where the freezing ground-level drafts enter, and on a hillside lot with exposure on the downhill side that junction works harder than it would on flat ground. Foam here is what ends cold floors.
Use batts in the walls. With the top and bottom of the house sealed against air leakage, the main 2x6 exterior walls can move to rockwool or high-density fiberglass batts, which deliver excellent thermal resistance at a fraction of the cost of filling the whole cavity with foam.
Add a vapor retarder. Standard wall batts do not block air or moisture transmission on their own, so this assembly gets finished with a dedicated interior vapor retarder, either 6-mil poly or a smart membrane, installed over the studs before drywall. That prevents indoor humidity from condensing inside the wall cavity through the freezing stretches of winter.
The logic is straightforward. You spend the foam budget where air moves, and you buy R-value where it is cheapest.
A Complete Envelope Strategy
Roof, wall, rim-joist and vapor-control details must be coordinated as a complete assembly. Exact materials and placement depend on the project and applicable code.