Like practically all organic building materials polystyrene foam is combustible. However, in practice, its burning behavior depends on the conditions under which it is used, as well as the inherent properties of the material. When installed correctly, expanded polystyrene products do not present an undue fire hazard. It is recommended that expanded polystyrene should be protected by a thermal barrier in specific applications as referenced in the International Code Council (ICC) and Canadian Materials Construction Centre (CMCC) building codes.
(Video courtesy of Bromine Science and Environmental Forum)
When burning, expanded polystyrene behaves like other hydrocarbons such as wood, paper, etc. If EPS is exposed to temperatures above 212° F (100° C), it begins to soften, to contract and finally to melt. Whether or not these can be ignited by a flame or spark depends largely on the temperature, duration of exposure to heat and air flow around the material (the oxygen availability).
Under certain fire conditions, EPS will ignite when exposed to an open flame. The transfer ignition temperature is typically 680°F (360°C). Although foam insulation is relatively difficult to ignite, in the case of ignition, burning will readily spread over the exposed surface of the EPS and it will continue to burn until all EPS is consumed. While the low density of the foam contributes to the ease of burning through a higher ratio of air (98%) to polystyrene (2%), the mass of the material present is low and hence the amount of heat released is also low. Ignited EPS will produce a dense smoke that will result in carbon monoxide, monostyrene, hydrogen bromide and other aromatic compounds. It is important to note that these gaseous compounds are emitted at variable rates depending on the temperature of the fire and are far less toxic than many ‘natural’ building materials, including wood.
Because of these characteristics, foams used for construction require a covering as a fire barrier. One half-inch thick (1.27 cm) gypsum wallboard is one of the most common fire barriers. Some building codes, however, do not require an additional fire barrier for certain metal-faced, laminated foam products. Check with your local building code/fire officials and insurers for specific information on what is permitted in your area.