Danger from California’s wildfires were thought to be restricted to individual homes and neighborhoods located in, or on the edges of, undeveloped land. Suburban tract homes located miles from brush or forest on a grid of streets and manicured lawns were thought to be burn-proof. Not so any longer.
The destruction of entire neighborhoods well outside any designated fire hazard zones in Northern California this month was as much a surprise to most as it was devastating. Many reasons for the increase in wildfire risk are well-known: California’s prolonged drought, and higher temperatures and increased wind energy resulting from climate change are a few. This can explain why wildfires are more frequent, more powerful, and spreading hot embers further from actual hot spots. But this still does not explain how houses with Spanish tile roofs, miles from wildland, are catching fire and burning to the ground.
Houses are like highly concentrated energy packages just waiting to ignite,” said Donald Falk, a wildland fire researcher in the University of Arizona School of Natural Resources and the Environment. But how are they igniting? In a firestorm, fire can spread from house to house, but this does not explain the extent of the recent destruction, or how multiple houses appear to light up simultaneously.
Apparently the typical attic may be the key, and it turns out that efforts to make homes more energy efficient may be partially to blame. One of the best ways to reduce air conditioning costs is to make an attic breath freely. Venting the attic at its low points (eaves) and high points (roof, gable or ridge vents) allows the hot air built up in the attic to flow out through the high vents. This air flow literally sucks air into the attic through the eave vents.
If the outside air is filled with smoke and hot embers, guess what gets sucked into the attic? Burning embers, dry wood framing, and strong air flow combine to make the perfect fire generator. Houses are burning down from the inside out!
Standard vent screens, even with very fine mesh, are not effective in stopping embers, which can be minuscule, from entering, particularly if the attic is acting like a vacuum cleaner. Fire-resistant vents utilize multi-layered mesh and/or temperature activated vent-closing vanes. We would not at all be surprised if new building codes require fire-resistant attic vents. You may not want to wait for new fire codes if you think your home may be vulnerable.