Steve: If your spray foam installer leaves the vents open, he will be committing the 2nd of the 4 problems I described above. You will most likely have comfort and efficiency problems. You may well have condensation problems. You will be spending a lot of money on a product that likely won't perform as it should. Don't let him leave the vents open. If the installer you've chosen doesn't understand this, you may want to choose someone else. 
That’s $58.3 Billion dollars. That’s roughly how much money we wasted in 2010 in the US according to the Department of Energy because of air leaking out of our building envelopes. If your building leaks energy, you’re paying more for energy than you need to. Clearly you’re not alone. If you’re ready to claim your share of the savings, it’s quicker and easier than you think.

Roof Coating Seal