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.
Spray foam insulation, when properly installed is both an air barrier and when closed cell foam is used at the proper depth a vapor barrier! Before hiring any contractor you should establish the brand of foam being used and verify that with the named manufacture. Please ask for ESR reports and MSDS sheets on any foam being installed in your home. A couple areas where you might consider spray foam insulation include: http://www.youtube.com/v/ggLAUsiuI_o&feature=youtube_gdata
The United States has adapted to using sprayfoam insulation and a new technology called Wall Injection to retrofit existing wall construction by drilling small holes between wall studs in the structures framing and filling the void with a less aggressive expanding water based foam. This allows existing home and business owners to conserve energy by creating a thermal envelope in their existing structure.
The Canadian National Building Code references the CAN/ULC S705.2 National Application Standard which must be followed during all installations of 2lb medium density closed cell polyurethane foam. Every installer of CAN/ULC-S705.1 compliant medium density, spray applied foam must be licensed in order to spray foam and hold valid photo ID issued by their Quality Assurance Program (QAP) provider showing their license is in good standing.