Painting - Filling large gap in wood, exterior

I have some boards that appear to be tongue n groove on the front of my house and they have been placed diagonally. As time has passed, they have begun to pull apart, leaving a gap from l/4 to l/2 in places. This is allowing water inside and also, the celotex board is "shining" through the cracks. Since they are somewhat large, can I use the spray foam into those cracks, smooth off the excess before it sets, sand it and paint over that. wouldn't that work better than trying to fill a l/2 gap with caulk?

You could do that, but smooth it off before setting, then put a layer off caulk over that after its dry. Caulk will withstand the elements better.