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:
These operate connected to a high pressure pump commonly found using 300 to 7,500 pounds per square inch (2,100–51,700 kPa) pressure to atomize the coating, using different tip sizes to achieve desired atomization and spray pattern size. This type of system is used by contract painters to paint heavy duty industrial, chemical and marine coatings and linings.
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:
Blocking heat buildup is a complicated task. Heat comes in three forms: ultra-violet (UV), visible light, and infrared (IR). A quality ceramic coating will block all three, especially IR, which is responsible for roughly 57 percent of heat load on a building. “Some ceramic paints claim to block all heat caused by UV,” says Pritchett, “but UV only accounts for three percent of heat load on a building.”

OK, Peter, I finally went back and read Alex Wilson's article on what he perceives as a serious problem. I haven't seen the full report, but based on the summary he wrote on the Green Building Advisor website, I question the science. It seems to me that he's chosen the wrong metric and he's basing his conclusion on too many assumptions because he doesn't have enough data. You can see my comments at the end of his article. Coating Services
There are exceptions to the thermal barrier rules. If the spray foam insulation is used as roofing or covered by concrete whichis, at least, an inch thick, then thermal barriers are not necessary. Additionally, you do not have to use a thermal barrier if the spray foam insulation is used on the interior of sill plates and rim joists, so long as the spray foam is 3 1/4 inches or less. Thermal barriers are also not necessary if the spray foam insulation is used in an attic or crawl space, as long as they are not used for storage or as living areas. In attics and crawl spaces where thermal barriers are not required, the use of ignition barriers is necessary. Insulation Spray Coating
This is in great contrast to my friend's experience. Her attic spaces were sprayed and the foam is actually pretty, and I was in her attic 6-7 days after it was sprayed and could hardly smell anything when I stuck my face up to the foam. And she did not have to clean up one speck of dust during or after the spraying. I got the name/number of the guy from her who managed her attic project, but between the time he did my house and the time he did her house, he was let go from the company who did her attic and hired by another company. He assured me everything would be the same, etc., etc., and I had no reason to believe anything had changed, but the sprayers who did my house later admitted they had never sprayed an attic before and I also found out that the foam used in my attic was from a different manufacturer. Neither my friend nor I had any idea this guy was with different company then, until the week after they filled my attic with off ratio foam, and my home with toxic vapors.  
Spraying paint with compressed air can be traced back to its use on the Southern Pacific Railway in the early 1880s[1] In 1887 Joseph Binks, the maintenance supervisor at Chicago's Marshall Field's Wholesale Store developed a hand pumped cold-water paint spraying machine to apply whitewash to the subbasement walls of the store.[2][3] Francis Davis Millet, the decorations director for the World's Columbian Exposition in Chicago in 1893, used Binks and his spray painting system to apply whitewash consisting of a mix of oil and white lead to the buildings at the Exposition, taking considerably less time than traditional brush painting and tuning it into what has been called the White City.[4][1][3] In 1949, Edward Seymour developed a type of spray painting, aerosol paint, that could be delivered via a compressed aerosol in a can. Insulation Spray Coating
I've seen this only once, and it was with closed cell foam, but I've heard of it happening with open cell foam, too. I don't know the details, but I've heard it could result from a bad batch of chemicals, improper mixing, or too high a temperature. Whatever the cause, it's not a good thing. The photo below shows how the spray foam pulled away from the studs. A little bit of uninsulated area like that adds up to a lot of heat loss/gain when the whole house has that problem, as it did here. Spray Coating
The process typically operates at 39–120 °C to avoid thermal damage. It can induce non-thermally activated surface reactions, causing surface changes which cannot occur with molecular chemistries at atmospheric pressure. Plasma processing is done in a controlled environment inside a sealed chamber at a medium vacuum, around 13–65 Pa. The gas or mixture of gases is energized by an electrical field from DC to microwave frequencies, typically 1–500 W at 50 V. The treated components are usually electrically isolated. The volatile plasma by-products are evacuated from the chamber by the vacuum pump, and if necessary can be neutralized in an exhaust scrubber. Spray Coating
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