To Survive a Firenado, Run Away
Don't assume that even the most wildfire-ready house will stand up to the increasingly ferocious firestorms being whipped up by nature.
I've written before about the challenges of trying to build a tornado-proof home. Tornadoes, with winds of 250 mph, are more than just monster storms. Think of them more like miniaturized nuclear bombs.
Research has shown that yes, if you build a cement bunker with 16-inch-thick concrete walls, you might survive a direct tornado hit--if you leave out the windows. It's no accident that storm cellars are popular in much of the Midwest.
Extreme heat this summer has escalated the probability of what were once relatively rare occurences: the "Firenado." With giant plumes of smoke and dust, and flames leaping more than 100 ft., these whirling, flaming dervishes have terrified Californians who thought they'd seen every kind of wildfire.
What's a Firenado?
Not a lot is known about fire tornadoes (also called fire whirls or firenados). What we do know is that they can hit 2,000°F at the center of the flaming core, with flames reaching up to 150 ft. tall, and wind speeds of about 150 mph.
According to Wikipedia, firenados have enough strength to "uproot trees up to 49 ft. tall, and they also have the tendency to "propagate and start new fires as they lift burning materials such as tree bark."
How long do they last? Usually no more than 20 minutes or so. But that's more than long enough to leave most homes they strike in a roaring inferno.
Means of Resistance
Can any home survive these kind of conditions?
The short answer is, possibly, but not for long. To illustrate, let's look at standard fire resistance tests. ASTM typically tests wall assemblies with a large vertical furnace and gas burners. "The duration of the test ranges from 20 minutes to several hours, depending on the desired rating and the product or assembly being tested. Temperatures inside the furnace reach about 1700°F (~925°C) during the first hour."
Keep in mind, that's 300 degrees less than the temperature of a firenado.
Also, it's important to recognize that fire resistance tests are not intended to estimate the survival prospects of a building. They measure whether the materials can last long enough to give the residents time to escape.
Creating a building that can outlast a firenado (or even a less ferocious wildfire), in other words, represents a much higher (and difficult) benchmark to achieve than fire resistance merely aimed at buying time.
Layers Vs. Systems Vs. Wind
Certain building materials, of course, are inherently non-combustible, and thus have much higher heat resistance than others. Brick and concrete block are non-flammable and virtually impervious to heat. Metal roofing holds up well, and so do clay tile and cementitious roof shingles.
But in an actual home, each of these materials is just one layer in a larger system. A home clad with fiber cement siding, for example, might survive a few minutes of heat and stray sparks, but what happens when the insulation, structural elements (such as 2 x 4 framing) in interior finishes reach the ignition point?
Let me posit the fact, for instance, that some species of wood, such as Noble Fir, burst into flame after 20 seconds of exposure to temperatures of 430°C (806°F) You can find this and other Wood ignition data here.
Another factor to consider is the Category V hurricane-force winds possible with a firenado. I've seen many post-mortem hurricane and tornado sites. Catastrophic failures in homes typically happen at the soffits, or when a projectile breaks a window or garage door. What happens when that wind is also hot enough to instantly melt vinyl soffits?
While it's beyond the scope of this article to estimate how long a firenado might take to breach a well-built, fire resistant home, the point is that no "normal" home is designed to withstand this kind of hypercharged heat.
The best plan for Firenado defense is one that I'm currently writing about for the Tiny House industry. I'll be presenting at the Tiny House Jamboree in Austin next month on the topic of creating lighter weight, but sturdy and IRC 2018 code-compliant Tiny Homes that sit on foundations, but can be quickly unfastened and roll out when conditions turn bad--or giant storms threaten.
When the firenados come, and unfortunately, they are coming in greater numbers and intensity each year in the West, the safe bet is to pack up, roll out, and hope that this is the worst Climate Change will throw at us in coming years.