I am having a home built and I'm using open cell spray foam against the roof deck. My insulation guy says there will be no insulation on the attic floor because the attic will become a conditioned space. My question is: Should I still try and air seal around can lights, and other air passageways which may leak into the attic, or do I not have to worry about it?

I own a hot roof, but used closed cell. You should do the same.

You insulation guy is right about the air seal, just selling the wrong product for this application.

http://www.greenbuildingadvisor.com/blogs/dept/musings/open-cell-spray-foam-and-damp-roof-sheathing

I think he was concerned that closed cell could cause problems if I ever developed a roof leak. The leak would not be noticed, and rot could become an big problem.

Do people wait for roofs to leak before they replace them.

OK, but smart people don't.

I am not worried.

Do people wait for roofs to leak before they replace them.

OK, but smart people don't.

I am not worried.

I'm not sure that when exactly a roof will leak can be accurately predicted.

I wonder if one could put something against the underside of the decking before spraying closed cell foam. Then it should be possible to replace decking if it ever rots (as you would on an uninsulated roof).

Don't try to isolate conditioned space from other conditioned space - that's asking for humidity/condensation problems.