Rotten wood does burn, but its characteristics differ markedly from sound wood, which is why it is often avoided as firewood. The decay process fundamentally alters the wood’s structure and chemical composition, significantly affecting how it ignites and the quality of the fire it produces. Burning rotten wood presents practical limitations in terms of heat output and specific safety concerns related to combustion byproducts.
The Role of Decay in Combustibility
Wood decay is a biological process driven primarily by fungi and microbes that break down the wood’s structural polymers: cellulose and lignin. Brown rot fungi, for example, selectively consume the structural carbohydrates, leaving behind a less dense, crumbly residue. This loss of mass leads to a porous, spongy texture, which directly influences the wood’s ability to ignite.
The reduced density of decayed wood means there is less solid material to heat, allowing for easier contact between fuel and air, which makes initial ignition faster than with solid wood. However, this same porous structure is highly problematic for sustained combustion because it dramatically increases the wood’s capacity to absorb and retain water. Decayed wood continues to hold this high moisture level long after sound wood has dried. The immense energy required to boil off this trapped moisture inhibits a hot, sustained burn, counteracting the advantage of its easy ignition.
Practical Burning Characteristics
Once ignited, decayed wood exhibits performance characteristics that make it a poor fuel source compared to seasoned, sound wood. The fire starts quickly due to the low density, but this is often followed by a quick, weak burn that fails to generate significant heat. A pound of wood, regardless of species, typically contains a similar amount of potential heat, but the low density of decayed wood means a given volume contains far fewer pounds of actual wood fiber.
This low mass per volume translates directly to an extremely low heat output, or British Thermal Unit (BTU) generation. The wood burns fast and cool, producing a large volume of smoke and steam as the high internal moisture boils off. Rather than producing a long-lasting, glowing bed of coals, rotten wood turns rapidly to fine ash and char, making it ineffective for any purpose requiring sustained heat, such as cooking or heating an enclosed space for a long period.
Health and Safety Considerations
Using rotten wood, especially in enclosed appliances like wood stoves and fireplaces, introduces several potential safety and health hazards. One concern is the potential for releasing mold and fungal spores into the air when the wood is handled or burned. These airborne spores, which are the agents of decay, can pose a respiratory risk, particularly for individuals with allergies or compromised immune systems.
The combination of incomplete combustion and high moisture content in decayed wood contributes significantly to the buildup of creosote in chimneys. Creosote is a highly flammable, tar-like residue formed when smoke and gases condense on cooler flue walls. This buildup restricts airflow and is the leading cause of chimney fires.