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Gypsum Board Fire Ratings

Please explain the different types of gypsum wallboard as related to fire rating.

First it must be noted that gypsum board, as a single product or building material, has no fire-resistive rating in and of itself. Fire-resistive ratings are based on the performance of an entire assembly, be it a wall, column enclosure, floor- or roof-ceiling assembly.


While ASTM defines two categories of gypsum panels, Regular and Type X, there is a third common category, enhanced Type X, which is often referred to as Type C. They provide varying degrees of fire resistance with the Regular core providing the least and the Enhanced Type X (Type C) the most.


Regular core is a gypsum board that is composed primarily of a gypsum mineral that is calcium sulfate and two molecules of water. It is the chemically combined water that provides the product with excellent fire-resistive properties. The Gypsum Association’s “GA-600 Fire Resistance and Sound Control Design Manual” states that the amount of water in gypsum is about 21% by weight. This water retards heat transfer through the material and, more importantly, through an assembly until it is driven off as a vapor at elevated temperatures. As the water leaves it generates fissures and voids, which degrade the structural integrity of the panel. This structural integrity is vital because when installed in a wall, the panel must withstand the rigors of the hose stream portion of the fire-resistive test. In a floor-ceiling assembly, the gypsum panel must have the capacity to remain in place through the fire test.


The core of the Type X product is modified to offset the effect of the departing water. What defines a Type X panel is performance based in that the panel must meet specific test requirements when installed as part of an assembly. The definition can be found in the various ASTM gypsum panel specification standards, such as ASTM C1396, Standard Specification for Gypsum Board. There the board is defined by its fire performance in a specific assembly tested in accordance to ASTM E119, Standard Test Methods for Fire Tests of Building Construction and Materials. The

specific performance and assembly for a 5/8 inch thick panel is defined as the following: “Not less than 1 hour for boards 5/8 inch thick (or 3/4 hour for 1/2 inch thick) applied parallel with and on each side of load-bearing 2 by 4 wood studs spaced 16 inch on centers with 6d coated nails, 1 7/8 inch long, 0.0915 inch diameter shank, 1/4 in. diameter heads, spaced 7 inch on centers with gypsum board staggered 16 inch on each side and tested in accordance with Test Methods E119.”


To achieve this level of performance was to negate the deleterious effect of the fissures and voids that occur in the Regular core. The effective solution was to add glass fiber strands to the gypsum core.


There is no formal definition for the third type of board, Type C. It has been termed “enhanced Type X,” yet no formal performance-based definition has been developed. However, published fire-resistive assembly tests reveal a marked improvement in fire resistance. A comparison between assemblies that differ only by the type of gypsum board shows the increased performance. Some assemblies where Type C was used can only achieve that level of performance with the Type C formulation. Most notably are floor- and roof-ceiling assemblies that have only one layer of gypsum board as a ceiling membrane. This single layer will be the Type C formulation and can only be attained with the Type X when an additional layer of board is added. Another area where there is a marked difference is in the fire protection of structural columns. Here more layers of Type X must be added to achieve the equivalent performance of the system with Type C.


The increase in performance in Type C is accomplished by addressing the voids and fissures created by the decreased water content. This is accomplished by the addition of core modifiers, most usually unexpanded vermiculite to the gypsum matrix. As the water turns to a vapor, the vermiculite must expand at the same rate and volume. A balancing act ensues in the formulation and distribution of the vermiculite. Gypsum boards tend to shrink in general during a fire, which is a cause for concern over the placement and finishing of gypsum panel joints. Consequently, too much vermiculite will negatively impact the performance of the panel. Too little will not effectively fill the fissures and voids, leaving the panel with diminished capacity.


From a contractor’s perspective, it is essential that the correct type of gypsum board be installed. Generally speaking, a Type C board can be used where a Type X was tested, but not necessarily the other way around. The installation of the incorrect panel may have detrimental effects on the performance of the assembly in a fire event.

Robert Grupe is AWCI’s director of technical services. Send your questions to [email protected], or call him at (703) 538.1611.

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