It's no secret that wood burns. To understanding how fire retardants in wood work, it's important to review the basics for fire.
Three things are needed for fire: fuel, oxygen and heat. Wood serves as the fuel for fire. When exposed to heat, the cellulose and lignins that comprise wood produce flamable gases that combine with oxygen to feed and expand the fire.
Infusing fire retardants into the fiber of the wood helps break that combustion cycle. When exposed to hear, fire retardants in the wood release carbon dioxide and water vapor to dilute the combustible gases. Retardants also convert the gases to char, which slows the fire growth and insulates the wood below.
Note that fire retardants do not make the wood non-combustible. FR wood exposed to fire and flame for an extended time will eventually burn. However, the fire retardants can slow the rate of fire growth to allow occupants to safely evacuate the structure.
What's in Fire Retardants
The fire retardant formulations used today are proprietary, but are mostly comprised of organic and inorganic salts or chemicals. The development of modern fire retardants for wood can be traced to work done by Joseph Louis Gay-Lussac in 1820. By the 1930s, the U.S. Forest Laboratory had investigated some 130 different fire retardant formulations.
Some of the components in fire retardants used today can offer protection against insects and decay fungi. Be sure to check with the manufacturer to confirm whether the fire retardant can provide this additional protection.
There are two types of fire retardants used, based on the expected exposure: interior and exterior. Some of the chemicals in current fire retardants for interior uses are water soluable and must be protected from exposure to liquid water. Exterior fire retardants must be able to withstand prolonged exposure to water; in fact, testing for exterior fire retardands include simulations of being exposed to 800 inches of rain over a 12-week period.
Strength of FR Wood
Because the infusion of fire retardants changes the chemistry in the wood, it can also impact its strength characteristics. As a result, design values must be adjusted for FR lumber and plywood products used in engineering applications.
Manufacturers for each fire retardant provide strength adjustment tables that should be applied to wood treated with their respective formulation. Refer to the guides for each fire retardant listed below for the proper strength adjustments:
Lonza Wood Protection
- Dricon - Fire retardant, interior use
- FRX - Fire retardant, exterior use
Koppers Performance Chemicals
FirePro - Fire retardant, interior use
VianceD-Blaze - Fire retardant, interior use
OtherPyro-Guard - Fire retardant, interior use
Exterior Fire-X - Fire retardant, exterior use