how important is spraying foam in the corners between a stud and the sheathing?

You are a smart homeowner. I always have 1/2" of foam sprayed on the interior side of wall cavity as soon as the electrician/plumber have done their rough in. This foam will get into the smallest of gaps and is meant to seal out air, however it does have a high R value also. Now you don't want to buy cans of spray foam- this is not the same stuff.

You need to call a insulating contractor who has this equipment required to do this correctly. It is one of the most money saving things you can do to reduce the heat/cooling cost for your home. It will save you money each and every day you live in this house.

It also adds structural strenght to the framing as it adheres to everything. If you have the finances, you can fill the entire cavity, but I usually just have the 1/2" sprayed on and then they install the fiberglass batts over the foam. It's worth every penny.

The spray foam won't really add to the R value very much. Cold air can still come through and come around places where you have used the foam. I think that there is a spray insulation that turns blue and it is sprayed between all of the studs. It will turn blue once sprayed and will increase R value a lot. Meanwhile, use the fiberglass insulation between the studs and on outside walls, you may want to consider stapling a plastic sheet on the studs. It will create a barrier against anything that may seep through the insulation and create a warm and comfortable home.

Here's a few things to help...First , wear gloves, because the stuff is next to impossible to get off your skin, and it will be all over you like Oprah on a glazed ham. Good idea to put on a pair of safety glasses too, because you're working overhead. If it's the high expansion foam, you want to try and foam about 2" or so thick. If a few spots are light, then hit them afterwards to even it up. The lower expansion foam should be about 3 inches as it doesn't expand as much. Make sure you get any openings where things like wires, cable TV or outside garden faucet pipes go through the outside rim joist of the house. While you have the foam out, do a quick inspection of the outside, and see if any openings need filling. Let the foam cure 24 hours and if any dripped down or is bulging out too much, take a utility or bread knife and slice it off. It should be covered with drywall later as it emits toxic fumes. Good luck with your project. Dr. Harvey Glimebinder

If you are going through the work, I would just spend the money and have someone come in and spray foam the whole wall. It is hard to seal a wall completely without using spray foam.