We are building a new home in Nova Scotia Canada. It’s 2×6 walls with osb shielding and we are spray foaming the stud cavities with 4in of close cell foam and installing 1in rigid foam board on interior side to create a thermal break. There will be an air gap between the spray foam and the rigid foam, will this cause moisture problems in the walls and also would it be wise to reduce the amount of spray foam by 1in to offset the cost of the rigid board.
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You will not have moisture issues. That said, I don't believe that this is a particularly smart wall assembly for a new home, but maybe there are details that I'm not aware of that call for this assembly.
Rigid foam is commonly installed on the exterior, and though it complicates many details, it also makes more sense outside of the walls for many reasons than inside, where it also complicates many of the details.
You may be choosing closed cell spray foam for its air sealing properties, but there are better wals to air seal, like taping your seheathing. Most closed cell foam isn't particularly green. You have some options for your wall insulation, from what you are planning to no foam at all to something in between, like a flash-and-batt or flash-and-fill approach.
Whatever you decide to do, keep in mind that air sealing is super important and should be tested. Here are some articles that I think you will find helpful: