Kory,
One word of caution with spray foam insulation in your attic. Although I enjoy the insulating properties of the spray foam in my relatively cool attic (79 degrees, 56% humidity in Seminole, FL) I did have and still do have an annoying problem. I have a recently built house with a stone-coated metal roof installed over a radiant/reflective barrier and also standard roof felt. The roof leaked around one of the perforations (where else?) and I believe the well known open cell spray foam insulation may have retained some of the leaking water around one of the upper cords of the roof truss. I did not notice the hidden leak until the water soaked through the insulation and the stained the drywall of the bedroom ceiling below. The roof perforation probably leaked because of poor flashing and the ever-present evil (and excessive) caulk failure. Even though the insulation is open-celled, it is my guess that it retained enough water to allow the truss cord and adjacent sheathing to stay wet and rot into oblivian with the help of the omnipresent bacteria/fungi. When I ripped away the adjacent foam, alot more water poured out. I discussed this with the manufacturer who of course said this senario could not happen with an open cell product. Gee; That answer was a surprise.
The roofer and I performed a quick and dirty non-scientific test by pulling out a wide piece of intact and well-cured foam, hollowing out a shallow divot and pouring some water into the depression. To my amazement, most of the water eventually did soak through. So I guess most of the water does go through the product, but maybe not all of it.
Simply put, make sure your roof is absolutely rain-proof (water-tight, waterproof) prior to spray foam installation if you intend to spray it under the roof sheathing. I think maybe I would consider using one of the peel and seal membrane products under the roof covering. At the time I built the house (prior to recent hurricanes) the sticky membrane was usually only used as the lower sheath covering in ice prone areas to protect against ice dams back leaks. Now it seems that the builder magazines are pushing it for everyone's roof. I know the arguements about vapor barriers and no-vapor barriers on roofs and exterior walls. After reading lots o' literature and anecdotal reports, its my impression that the only consensus of opinion concerning what to do about water vapor is that there is no consensus, other than get rid of it. Everyone thinks they are right and the other guy doesn't understand the physics of water vapor permeability. Its amazing how people can twist a relatively static science to prove their point. The only guy in Florida that seems to know anything about leaky houses in Florida is Joseph Lstiburek who has numerous must read articles at http://www.buildingscience.com/.
Best of luck in this uncharted territory (it ain't so simple),
John in Seminole