Chris, don't know the answers to your questions, but I have heard lots of horror stories from reliable sources about the kind of foam that is sprayed in to wall cavities via drilled holes. Seems that the tendency is for the foam to setup just a little way inside the hole leaving most of the space uninsulated. This kind of defect can be seen clearly with an infrared camera. If you are talking about the kind of foam that is sparyed on before the wall is closed up, that is a different story. If you must spray into closed walls consider dense packed celulose.