Temperature Down, Danger Up : Wood Heating Caveats From FPL

Most residential buildings in the United States employ wood as a primary construction material, and increasingly, commercial buildings are following suit. Although researchers at the Forest Products Laboratory (FPL) have spent the better part of a century formulating new treatments and methods for improving the fire durability of wood, fire safety remains a serious consideration, particularly during the winter.

Fire retardant test at FPL, 1940s.

A fire retardant test at FPL during the 1940s. FPL research has helped improve building codes, wood treatments, and testing standards.

This danger can be compounded depending on your method of heating. As the temperature goes down, if you choose to heat your home with wood, fire danger goes up. Proper precautions should be taken to ensure that the fire stays contained in the stove or fireplace, lest it spread to the surrounding structure.

According to The Wood Handbook: Wood as an Engineering Material, one of the most important problems associated with home fires is the smoke produced. The term smoke is frequently used in an all-inclusive sense to mean the mixture of pyrolysis products and air that is near the fire site. In this context, smoke contains gasses, solid particles, and droplets of liquid — but why is smoke so dangerous?

Smoke presents a potential hazard because it interacts with light to obscure vision, but the toxicity of combustion products is the primary concern. Fire victims are often not touched by flames but die a s a result of exposure to smoke, toxic gasses, or oxygen depletion. These life-threatening conditions can result from burning contents, such as furnishings as well as from structural materials involved.

The toxicity resulting from the thermal decomposition of wood and cellulosic substances is complex because of the wide variety of types of wood smoke. Composition and the concentration of individual constituents depend on such factors as the fire exposure, oxygen and moisture present, species of wood, any treatment of finishes that may have been applied, and other considerations.

The vast majority of fires that attain flashover (a fire’s sudden spread when an area is heated to its flashpoint) do generate dangerous levels of carbon monoxide, independent of what is burning. Carbon monoxide is a particularly insidious toxic gas and is generated in significant amounts in wood fires.

Even small amounts of carbon monoxide can be toxic because the hemoglobin in the blood is much more likely to combine with carbon monoxide than with oxygen, even with plenty of breathable oxygen present. Generally, two approaches are used to help deal with the smoke problem: limit smoke production and control the smoke that has been produced. The control of smoke flow is most often a factor in the design and construction of buildings.

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Wall test conducted in the large vertical furnace at FPL showing a wall panel at the point of fire burn through. Information of this nature is used in building designs to help ensure time for people to exit a burning structure and help contain the fire and smoke.

Draftstops are one useful control measure construction engineers implement. Draftstops are barriers intended to restrict the movement of air within concealed areas of a building. The are typically used to restrict horizontal dispersion of hot gases and smoke in larger concealed spaces such as those found within wood joist floor assemblies with suspended dropped ceilings or within an attic space with pitched chord trusses.

Doors can also be critical in preventing the spread of smoke and fire, even if they are made out of wood. Doors left open or doors with little fire resistance can easily defeat the purpose of a properly fire-rated wall or partition. Listings of fire-rated doors, frames and accessories are provided by various fire testing agencies. When a fire-rated door is selected, details about what which type of door, mounting, hardware, and closing mechanism must be considered.

Finally, keep in mind that smoke rises, and that when evacuating a burning building, clean air can usually be found closer to the ground. For more information on home fires, and tips to keep you and your family safe, visit Ready.gov.

For more information about the fire resistance of wood, please see Chapter 18 of The Wood Handbook: Wood as an Engineering Material.