In the early days of insulating attics, people believed the insulation was a heat source. That’s not the case, as the insulation is actually a barrier between two areas of different temperatures. As for the “hot roof” theory, the unvented attic system could make the temperature of the roof go up a little, but this increase won’t cause damage to your shingles.
While the R value will vary, most ocSPF products have an R value of around 3.8 per inch. Unlike medium-density closed-cell SPF, ocSPF is not a vapour barrier. When installed at 5.5 inches or more, ocSPF does act as an air barrier. It is often used for interior walls because it provides sound reduction by blocking and absorbing air leakage. It is usually only recommended for indoor applications.
One thing I wanted to mention was that I think your price point is a little high and that might deter people from considering spray foam. As a project manager for a spray foam company for years I can tell you that we offer 0.5 Lb. open-cell foam at around $.25/board foot. We even offer a 1.0 Lb. open-cell foam for less/the same cost as you quoted the 0.5 Lb. foam for above. The 1/0 Lb. open-cell foam has an R-value of almost 5.
Carefully examine the roof membrane (surface material) for cracks, tears, blisters, evidence of ponding, exposed foam, and open seams. If already coated, evaluate the condition of the coating. Pay particular attention to areas around roof penetrations (vents, skylights, pipes, etc), ponding areas, cracks in parapet walls, and the attachment of roof membranes to parapet walls. Roofs should be in good condition to warrant coating.
For maximum durability and leak resistance, we recommend that two coats of topcoat be applied at 100 sq. ft. per gallon, or a minimum of 20 mils dry coating. If two complete topcoats are not applied, always apply two thick coats to problem areas such as scuppers, drain areas, ponding areas, seams, and repaired areas. Avoid using Elastek Solar Magic™ or The Shield™ on ponding roofs. Clean skin and tools promptly with water.
a) Make sure that your house has been well heated prior to installation because the foam expands at different rates as it hits surfaces with different temperatures. If it is cold outside the sheathing may be much colder than the studs consequently the foam may expand more from the sides of the wall cavity creating air pockets in the wall cavity. This can be minimized by an experienced installer. http://www.youtube.com/watch?feature=player_embedded&v=ggLAUsiuI_o