In buildings where combustible siding has been installed, fire-retardant coatings could be used to reduce their vulnerability to wildfire exposures, particularly radiant heat and flame contact exposures. These exposures are more common in situations where close building-to-building spacing exists and a neighboring building ignites and burns, and where other combustibles (e.g., a wood pile or small out-building such as a tool shed) are located near the building. Commercially available coatings include fire-retardant gel products that can be applied by a resident or first responder. Since the length of time that a gel product is effective after application is on the order of hours, these would be applied only when a wildfire threatens. Other fire-retardant coatings, both film-forming paints and penetrating types, have been reported to have an effective service life on the order of years and would represent a more permanent, passive mitigation strategy. If effective, these longer-term coatings (both film-forming and penetrating types) would arguably be less expensive than, for example, removing combustible siding and replacing with a noncombustible type.
When used in exterior applications, the fire-retardant coating is subjected to changes in temperature, humidity, solar radiation and other weathering factors. This weathering can negatively impact the fire-retardant performance due to surface erosion or other forms of coating degradation, potentially resulting in a reduction of the fire-retardant properties before the end of the anticipated effective service life. For this reason, an experiment was designed to evaluate changes in the fire performance of coatings applied to an otherwise untreated wood substrate as a function of outdoor weathering.