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 Services

Ever wonder why it's so hot and wet inside a metal building that has spray foam insulation, in the summer and so cold and wet in winter? The metal skin magnifies the heat of the summer and frigid temperatures of the winter. Metal is a very good conductor of heat and cold. In hot weather, metal framing and sheeting rapidly radiate the sun's heat into a building. In cold weather it rapidly releases the heat out of your building. When warm air comes in contact with the cool metal sheet, condensation occurs. 

The long answer is more complicated. There are instances where ignition and thermal barriers are required, and others where they are not. It is all dependent on the location of the insulation. Additionally, the materials that satisfy the requirements of ignition and thermal barriers vary. In some circumstances, such as the use of an intumescent coating, an ignition or thermal barrier may not be necessary. It is important to check with your local code inspector to make sure that your use of spray foaminsulation and ignition barriers is correct.
A rule of thumb puts two thirds of the coating on the substrate and one third in the air. True HVLP guns use 8–20 cfm (13.6–34 m3/h), and an industrial compressor with a minimum of 5 horsepower (3.7 kW) output is required. HVLP spray systems are used in the automotive, decorative, marine, architectural coating, furniture finishing, scenic painting, and cosmetic industries. Spray Coating