Fire Resistance

How gypsum delivers fire-rated performance

Jun 10, 2024

Explore the science and history of gypsum, plaster and drywall

From commercial to residential construction, ensuring safety against fire hazards is paramount.

Gypsum plays a crucial role in mitigating the spread of fire within buildings. The fire resistance of drywall and plaster is primarily attributed to the unique properties of gypsum.

Fire-rated gypsum board assemblies serve three critical functions:

  1. Protecting the lives of building occupants by giving them time to evacuate
  2. Safeguarding property by compartmentalizing fire and smoke
  3. Ensuring firefighters have time to rescue occupants and contain the fire

So, what makes this popular building material so effective in fire protection? Let’s delve into the science of drywall to better understand its composition, its properties and its history of safeguarding structures and lives.

How is gypsum fire resistant

While gypsum has often been called the “miracle mineral,” its chemical name is calcium sulfate dihydrate. The word “dihydrate” in the name refers to the presence of two water molecules bonded to each calcium sulfate molecule. The chemically combined water in gypsum comprises 21% of its weight.

Understanding calcination

When heated, the water within the gypsum is released as steam through a process called calcination. The result is calcium sulfate hemihydrate, or what is commonly referred to as plaster of paris, named for a large gypsum deposit at Montmartre in Paris, France.

The calcination process is advantageous for two reasons:

  1. Plaster can be formed into virtually any shape or be used as a facer for other materials. When mixed with water, plaster rehydrates as the water molecules chemically bond back into gypsum that sets and hardens.
  2. Walls constructed with plaster or gypsum board will not transmit heat in excess of 212 F (the boiling point of water) until all of the gypsum in the board is completely calcined. That is remarkable considering the temperature of the furnace in a 2-hour fire test can exceed 1,800 F.

The evolution of gypsum in construction

Gypsum plaster has been in use for over 7,000 years. Ancient Egyptians used it as mortar between blocks in the pyramids and as a smooth coating for the interiors, some of which still exist today.

In the late 1800s, Augustine Sackett discovered a method for controlling the setting time of plaster. This advancement led Sackett to patent a precursor to modern-day gypsum board in 1894, which consisted of alternating layers of wool felt and plaster, known as “Sackett Board.”

The first product standard for gypsum board, ASTM C36, Standard Specification for Gypsum Wallboard, was introduced in 1921.

Gypsum board gained widespread popularity during the postwar building boom in the United States as a cost-effective and efficient alternative to plaster. Commonly known as “drywall” because it avoided the humid environments and wet applications of plaster, gypsum board could be installed faster and with less skilled labor, providing jobs and homes for returning veterans.

In 1947, the first wall assembly to achieve a 1-hour fire rating was constructed, and in the 1950s, Type X drywall was defined in ASTM C36.

Over the years, additional standards were created as more gypsum products were developed, such as moisture-resistant products, soffit boards and shaftliner panels. In 2004, these various standards were consolidated into ASTM C1396, Standard Specification for Gypsum Board, the current standard for all paper-faced gypsum board products.

While modern drywall products provide valuable benefits, such as sound control and mold resistance, the fire resistance of gypsum is what continues to make it the premier choice for the construction industry.

How does drywall achieve fire ratings?

Type X gypsum board has special additives that give it increased fire resistance. Glass fibers, for example, are added to the gypsum slurry during the manufacturing process to reinforce the gypsum core as it calcines. Gypsum board assemblies are tested in accordance with ASTM E119, Standard Test Methods for Fire Tests of Building Construction and Materials.

To pass ASTM E119, the assembly must achieve the following:

  • The average of all thermocouples must not exceed 250 F over ambient temperature.
  • No single thermocouple can exceed 325 F over ambient temperature.
  • No through penetration or wallboard failure to the other side can occur during the hose test.
  • There must be no evidence of burn-through.
  • The structure must not collapse.

Resource Alert: Learn more about the testing standard for fire-rated assemblies, modifications permitted to those assemblies and the most common questions about fire-rated assemblies.

High-performance gypsum board and fire resistance

Gold Bond Building Products, LLC, an affiliate of National Gypsum Company, manufactures a wide assortment of Type X gypsum board products to meet a variety of requirements and help ensure code compliance, including:

Type X vs. Type C gypsum board

Type C gypsum board products incorporate special additives, such as vermiculite, that undergo expansion when exposed to elevated temperatures. This expansion helps reduce heat transfer and shrinkage of the gypsum board during calcination.

For fire-rated assemblies that require Type C drywall, National Gypsum provides four variations:

  1. 5/8" Gold Bond® Fire-Shield C™ Gypsum Board
  2. 5/8" Gold Bond® Kal-Kore® Fire-Shield C™ Plaster Base
  3. 5/8" Gold Bond® XP® Fire-Shield C™ Gypsum Board
  4. 5/8" Gold Bond® eXP® Interior Extreme® Fire-Shield C™ Gypsum Panel

Resource Alert: Learn more about the differences between Type X and Type C drywall.

Building solutions that prioritize safety

When it comes to building construction and fire protection, fire-resistant drywall stands as a critical safeguard against the devastating effects of fire. Its installation provides peace of mind to building occupants and owners, offering crucial time for evacuation and emergency response in the event of a fire while helping to ensure compliance with building codes and standards.

Gypsum board assemblies offer high fire-resistance ratings and gypsum board has low flame-spread ratings, making gypsum board integral to fire-rated construction. National Gypsum provides valuable information related to fire-rated assemblies via two of its industry-leading technical resources.

  1. The Wood Book™: A comprehensive construction guide for fire-rated assemblies in wood-frame construction, providing detailed assemblies for use in all wood-frame duplexes, townhouses, multifamily buildings and light commercial projects.
  2. The PURPLE Book®: A trusted resource for assembly details on the application of products provided by National Gypsum in fire-rated assemblies.

From its composition, structural design and rigorous testing, drywall exemplifies the fusion of science and engineering in creating solutions that prioritize safety and resilience of built environments for generations to come.

For more information, learn how National Gypsum provides fire-resistance solutions.