Spray foam insulation is a great product. Homes insulated with it can be some of the most efficient and comfortable homes built. I've been in plenty of these homes and can tell you that when spray foam is installed properly, they outperform 99% of fiberglass batt-insulated, stick-built homes. (I can also tell you that 73% of all statistics are made up on the spot, so please don't ask for documentation of that statistic.)
What do you mean by "our AC units require fresh air ventilation"? Fresh air doesn't come from the attic. If there's an atmospheric combustion appliance in the attic, such as an 80 AFUE furnace, then you shouldn't be encapsulating the attic anyway. If that's the case, you don't want spray foam on the roofline at all. You can either change out the furnace to a sealed combustion unit or do your insulating and air-sealing at the flat-ceiling level.
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. https://www.youtube.com/watch?feature=youtu.be&v=ggLAUsiuI_o

I was hesitant to buy this because many reviews stated that it didn't cover as advertised, but after reading/following all directions, everything turned out perfectly. It even seemed to apply slightly more than the 650 bd/ft that was advertised. The application process was very easy to learn and I applied all of the product in about 1 hour and 10 minutes- prep time included. I couldn't be happier and I purchased another kit this morning.
Closed-cell foam has a higher density and offers a higher R-value per inch. It also adds strength to walls, ceilings and roofs and can contribute to a structure’s overall durability. Additionally, closed-cell foam’s low water vapor permeability offers protection from moisture when installed in places where high humidity—from climate, occupants, or flooding—meets the structure. 
We are turning our roof into an unvented roof assembly by raising the roof and blowing in SPF. We are planning to leave the existing vapor barrier down but remove the fiberglass batting and then adding 6" of SPF in all the cavities, to completely seal and insulate the house. Should we have any concerns about doing it "upside down" and not spraying the foam directly to the underside/sheathing of the roof?

What more is that we also think it is one of the most durable among liquid roof coatings on the market. It can last for several years, working continuously in protecting your roof from outside elements, such as dust, leaves, twigs and debris that would otherwise damage your roof. When it happens, your roof might have leaks and cause discomfort inside your RV.
The product is an acrylic coating, which is packed conveniently for ease of use. As it is also in one-gallon size, you can rely on its supreme area coverage reaching up to 125 square feet. The Dicor rubber roof coating is also very easy to apply. When shopping around for a roof coating, we believe it pays off to find an easy to apply product for fast results. It is one of the best qualities of the RV roof coating that makes it a top pick in the category. 

Roof Coating

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