One issue that has confused many people is the code requirement for thermal and ignition barriers and how it relates to spray foam insulation when installed in an attic or crawl space.
Thermal Barrier - A thermal barrier, as far as building code is concerned, is any product that has been ASTM tested and is considered to have a "15-minute thermal barrier" or an "index of 15." An example of a thermal barrier is 1/2" sheet rock, 1/4" plywood or particleboard, and some fire proof coating.
Ignition Barrier - a product that prevents the ignition of the product which it is applied to from a spark, or from direct heat, but does not protect from direct flame over a period of time. Ignition barriers are usually spray on or brush on coatings.
The Fire Suit and the Leather Jacket
This analogy might help understand the difference. Lets say a firefighter is wearing his fireproof suit, his suit is our thermal barrier. We are standing next to him wearing a leather jacket. This leather jacket is our ignition barrier. The firefighter can walk though a fire without burning, but we can't. That leather jacket would burn quickly. But if we were standing outside the fire and a spark come in contact with the leather jacket, it's doubtful we would burn. The jacket would give us a small amount of protection, but nothing close to the amount of protection that the fireproof suit would give us.
In conclusion, a thermal barrier is a high-level of protection, and an ignition barrier is a low-level of protection.
When does code tell me I need to us a thermal barrier?
Simply put, everywhere foam is applied to the interior of the building, a thermal barrier must separate the foam from the interior of the building. For example, when foam is applied on the exterior walls your sheet rock on the walls is your thermal barrier. When foam is applied to the roof deck, sheet rock installed at your ceiling is your thermal barrier. If your ceiling is not sheet rock, or does not have a 15-minute fire rating, you must apply a thermal barrier directly to the foam.
When does code tell me I need to use an ignition barrier?
Code says that anywhere foam is applied in an attic or crawl space, it must also be protected from the attic space from an ignition barrier in addition to the thermal barrier that is at the ceiling. The way the code sees it is that once the foam is separated from the interior of the building with a thermal barrier, the foam does not need the same level of protection from the attic space. However, it does require some protection. That is when the ignition barrier, or low-level protection, is used.