Flame Spread Index
When polyiso is used in a residential home, it must have a flame spread index or rating of 75 or less. This number is a reference to how fast and how far the flames spread in a test known as a tunnel test. In this test, a material sample 20 inches wide and 20 feet long is burned in a test chamber. The resulting flame spread index of a material divides it into one of three classes, with Class I or A materials having little flame spread, and Class III or C materials burning more readily. Numbers run from an index of 0 to 200. Polyiso foam must have a flame spread index of B (II) or better. Some polyiso panels do have a flame spread rating of A (I), which is more optimal.
Smoke Developed Index
When polyiso is used in the walls of a residential home, it must have a smoke developed index of 450 or less. This is another number generated from testing materials as they burn; in this instance, the number refers to the amount of smoke generated by burning the sample material. The lower the number, the less smoke is generated when the material burns, so lower numbers are better. Smoke inhalation is extremely hazardous, and smoke should be considered just as dangerous as the fire itself.
When polyiso foam does burn, it gives off some toxic gases as well as smoke, such as hydrogen cyanide, which some homeowners find concerning. However, it should be noted this is not a phenomenon unique to polyiso foam. Any material that contains nitrogen gives off some amount of hydrogen cyanide as a byproduct when burned. Polyiso foam also gives off carbon monoxide, another common toxic gas that results from fire. Polyiso foam doesn't provide unique hazards in regard to toxic emissions.
Depending upon where you live, there may be other requirements or stricter regulations for what is considered an acceptable rating for your insulation. For instance, California has some extra regulations because of the frequency of forest fires. It's therefore important to consult local building codes to learn if there are, in fact, any special regulations in regard to using this kind of insulation. In addition, polyiso foam typically needs a thermal barrier installed along with the insulation.