Can I use spray foam to insulate the underside of the open roof on my 600 sq. ft. cottage?

Yes, is the short answer to your question. Foam can be sprayed on the underside of a roof like yours, and as long as it’s done correctly there’s no need for ventilation. That said, there is more than meets the eye when it comes to applying foam the right way.

1. First of all, you need to choose a type of foam that’s impervious to air infiltration. Some spray foams are actually designed to allow air to pass through them, and if you use one of these types, condensation will develop between your roof boards and the foam. I’ve had good luck using two-part polyurethane foam for applications just like yours.
2. Second is the application technique. There is sometimes a tendency for foam to shrink a week or two after application, and this might create condensation-promoting gaps between your rafters and the foam that sits between them. I’ve found it best to spray thin layers of foam around the perimeter of the space between rafters first, let it harden for a few minutes, then slowly fill in the entire rafter space in layers no more than 1 inch thick. Foaming like this is also one of the only options for insulating around a skylight that’s surrounded by an open roof like yours.

One side effect of insulating your roof with foam in this way is increased roof temperatures during the summer. Your roof deck will get considerably hotter with foam insulation in place and this will probably shorten the life of conventional organic asphalt shingles. Fiberglass shingles, however, are much more heat resistant. They stand up very well for long periods of time even on extremely hot roof surfaces.