Is Spray Foam a Fire Hazard or a Fire Barrier?

Both. Spray Foam only chars, does not self-sustain combustion, and will extinguish once an external flame source is removed.  Spray Foam is also thermoset, not thermoplastic; meaning that it does not return to liquid form once it is cured.  These properties make Spray Foam a great fire barrier.  For example, if the common wall between a warehouse and a showroom is sealed with Spray Foam a fire can burn in the warehouse all night and the showroom would remain untouched.  However, it is the super insulating properties of Spray Foam that make it a fire hazard.  The heat from a fire inside a building completely sealed with Spray Foam will be reflected back to the fire and cause the fire to burn hotter and quicker.  For this reason a code approved fire barrier must be installed over Spray Foam in living spaces.  ½” drywall is an approved fire barrier for this.