Prussian General Carl von Clausewitz is known for his insights on war, but one of his lesser-known quotes hits home in the word of codes and standards:
“Not until terms and concepts have been clearly defined can one hope to make any progress in examining the question clearly and simply and expect the reader to share one’s views.”
In codes and standards, clear terminology is everything. Yet, confusion around terms is surprisingly common. That is why organizations like the International Code Council (ICC), ASTM, and the National Fire Protection Association (NFPA), start their respective codes and/or standards with a section dedicated to definitions.
In this article, we will break down how these groups define “combustible” and “noncombustible” and a third term used by the NFPA — “limited combustible.”
Noncombustible and Combustible
Let’s start with the first two, noncombustible and combustible. In the ASTM standards, a material is considered noncombustible if it is tested to and meets the criteria set forth in ASTM E136 Standard Test Method for Assessing Combustibility of Materials Using a Vertical Tube Furnace at 750°C.
Alternately, the material can be tested to ASTM E2652 Standard Test Method for Assessing Combustibility of Materials Using a Tube Furnace with a Cone-shaped Airflow Stabilizer, at 750°C, using the acceptance criteria as set forth in ASTM E136.
In both cases, noncombustible simply means the material won’t show significant signs of combustion (burning in layman’s terms) under the test conditions.
That’s a lot of test methods to think about. But looking at the ASTM definition makes it a little less daunting, where the term is defined as, “not capable of undergoing combustion under specified conditions, and those conditions are established in the aforementioned test methods.” Basically, if a material passes, it’s noncombustible.
The ICC International Codes (I-Codes), however, take a different approach to combustibility. Not only do they rely upon the ASTM definition, but they have an exception clause that states if a material has a structural base of a noncombustible material (such as gypsum) with a surface material not greater than 1/8” thick with a flame spread of less than 50 as determined by ASTM E84 Standard Test Method for Surface Burning Characteristics of Building Materials that material is considered noncombustible for the sake of the code. However you look at it, gypsum panel products are noncombustible per the I-Codes.
In terms of combustibility, both ASTM and ICC agree: if it does not meet the criteria of E136, or the code exception, then a material is combustible. The NFPA defines it similarly.
Limited Combustible
The NFPA codes, however, include an additional third category – “limited combustible”. A material in the NFPA codes is considered limited combustible if it meets all the exceptions for a material to be considered as noncombustible in the I-Codes, shows no evidence of progressive combustion (i.e. does not continue to burn and contribute to fire) and has a heat release rate at or below 3500 BTU/lb. when tested in accordance with NFPA 259 Standard Test Method for Potential Heat of Building Materials. Gypsum wallboard is considered a limited combustible material in the NFPA codes, and glass mat-faced panels are considered noncombustible.
So, what does it all mean? Here’s the bottom line:
- All gypsum panels are noncombustible per the I-Codes.
- Gypsum wallboard is considered limited combustible under the NFPA Codes.
Regardless of how they are defined, gypsum panels are still the premier code-accepted choice for passive fire protection in the construction of fire-resistant assemblies.