If you have a 6" stud cavity, its very common to not completely fill the stud cavity with spray foam. However, the closed cell foam will completely seal and insulate the wall, so it will still be very effective.. With the stud bay open they can get good adhesion on the sides so that it really seals the wall. There is some transmission of cold/heat through the studs, but its not as significant. My opinion is that keeping out cold air is more important than the depth of the insulation. You need to also ensure that your windows are installed and sealed to the wall properly as part of the project.

If it was my house, I would spray the wall insulation first, and then have the electrician come in and run the wiring and install the boxes in the remaining space in the stud bay. I don't really like to have all the electrical buried in foam insulation where its not easy to access.

Bruce

Closed cell spray foam is approx 6.5 R per inch, and eliminates need for vapor barrier. It is excellent. Yes! You can definitely use spray foam and then add batts on top.

If you literally can see daylight, then if you have the siding off consider using an air-sealing tape on the exterior over all gaps, with suitable primer.

If you have the siding off consider wrapping the outside with 1-4" of rigid foam then filling the stud bays with blown in cellulose. This is a very cost effective approach to get both good air sealing and high r value, you can get to the r-40 range. Thermal bridging across the framing is very significant-- 20% or so of the wall will have only negligible r value with a straight spray foam approach, though the air sealing is excellent.

If you take a "flash and batt" approach the spray foam needs to be sufficiently thick that you don't create a condensing surface inside the wall (similarly with the rigid foam plus cellulose). There are tables that show the required values for different climates. Right offhand I'm not sure R15 is going to be enough for Quebec

Thanks very much for the comments - Rwiegand: we have found a firm (with excellent referrals) that uses a 'better/different' brand of foam which has an R7 value per inch, so with 3.5 or 4 inches, I think we'll be ok. we keep reminding ourselves that this is a cottage, not our principal residence, if we need to wear an extra sweater in February that's fine. Bruce: yes, the electrician has said that he prefers to do his thing after the foam has been applied, so wiring etc are not forever buried in foam. thanks again -