Wood is flammable, but its thermal behavior differs significantly from materials like steel or stone. This distinction is not due to pure fire resistance, but how wood reacts to intense heat, allowing it to maintain structural integrity longer than expected. Understanding this involves examining wood’s cellular structure and the chemical processes that occur when it is subjected to elevated temperatures. Wood’s ability to slow the transfer of heat is a primary factor in how it performs during a fire event.
Wood’s Unique Thermal Properties
Wood’s inherent cellular structure makes it a poor conductor of heat, classifying it as an insulator. This low thermal conductivity results from air-filled voids within its cells, which restrict the rapid transfer of thermal energy. Wood is approximately 400 times better at insulating than steel and 15 times better than masonry.
When wood is exposed to fire, the outermost layer burns, forming a layer of charcoal, or char. This char layer is a more effective insulator than the original wood, acting as a temporary barrier that significantly slows heat penetration to the deeper, unburned wood. This protective layer helps the inner core of a wooden beam or column remain cooler and retain its structural capacity for an extended period. The charring rate is a predictable factor engineers use to design fire-resistant timber structures.
The Pyrolysis Process
Pyrolysis is the chemical decomposition that occurs when wood is heated without oxygen, serving as the precursor to ignition. It involves the thermal breakdown of wood’s primary components: cellulose, hemicellulose, and lignin.
Initial heating (100°C to 150°C) causes internal moisture to dry out, consuming energy before chemical changes begin. Decomposition starts around 200°C to 300°C with the breakdown of hemicellulose. As the temperature increases, cellulose and lignin break down, releasing flammable gases, vapors, and tars (volatiles). The ignition of these volatile gases, not the solid wood, sustains the flame.
How Wood Species and Moisture Affect Performance
Wood performance under thermal load is heavily influenced by its physical characteristics, especially density and moisture content. Denser woods, such as hardwoods, exhibit better fire performance because they contain more mass that must be heated and decomposed. This higher density results in a slower charring rate, meaning the protective char layer is more compact and delays heat transfer to the interior.
Moisture content also plays a significant role in temporary resistance to ignition. Wood with higher moisture requires substantially greater energy to evaporate the water before pyrolysis can initiate. This drying phase acts as a heat sink, extending the time before the wood generates flammable gases necessary for sustained combustion. Therefore, seasoned wood ignites more quickly than green wood, which can contain a moisture content well over 30%.
Fire Retardant Treatments and Practical Safety
To enhance wood’s natural thermal performance, chemical fire retardant treatments are commonly applied, either as a surface coating or through pressure impregnation. These treatments typically involve compounds containing phosphorus or sulfate, such as mono- and diammonium phosphate. When heated, these chemicals alter the wood’s decomposition pathway, causing it to produce less flammable gases and more char and water vapor.
The chemicals encourage the formation of a thicker, more insulating char layer at lower temperatures. Some treatments also release non-flammable gases that dilute oxygen and flammable volatiles at the wood’s surface, inhibiting combustion. Practical safety is governed by building codes that mandate fire-rated assemblies, ensuring structural wood members maintain load-bearing capacity for a specified duration during a fire event.