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Wildfire Resistance and Resilience: 7 Minutes of BS (+ Nine More Minutes!)

With 100 years of fuel on the floor and hotter climate, wildfire resistance matters almost everywhere
June 09, 2021

In this episode, we are joined by Daniel Gorham, a fire engineer and budding ecologist at the Insurance Institute for Business and Home Safety. We are going a little long in this episode because fire resistance and resilience are important topics for residential construction and climate change.

What wildfire is

wildfire | wīldˌfī(ə)r (n)

Wildfire-resistant construction is the design of your building, your homes, and your business—and the landscape around it—to reduce the vulnerability to the wildfire exposures that might ignite the structure.

Wildfire is different from wildland fire. Wildland fire is a regular part of a region’s ecology. Regular fires are literally part of the landscape.

A lot of plants and animals have evolved actually to require fire to survive and reproduce.

Lodgepole pine, found all over the American West, is one species whose seed coat is only broken down by a fire. Jack pine in the upper Midwest is another.

Kirtland’s Warbler is a bird species that also depends on fire: it nests only in the jack pine forests of Michigan, Wisconsin, and Ontario.

So that’s what wildland fire is.

Wildfire is a wildland fire that burns out of control.

There’s a key distinction there. Naturally occurring wildland fires are cyclical. At least, they used to be. Now, they’re pretty common due to 100 years of fire suppression and an expanding fire season.

Wildfire is different from wildland fire. It is also different from regular old house fires that we have in cities and the ‘burbs.

How wildfire works

When we’re talking about regular old house fires, we talk about fire inside the box.

We talk about fire inside the box. That’s your compartment fire, that’s your kitchen fire, that’s where sprinklers do a really good job.

But large outdoor fires, it’s kind of a different process, how they ignite structures. But the things that we know from interior fires do translate and can help us know more.

I’m a little bit of a cross-breed between fire engineer, with fire inside-the-box, and fire ecologist, people that typically look at a fire in the wildland in the ecosystem.

Buildings in wildlands ignite in one of three ways:

First is radiant heat.

We know radiant heat as the heat from the sun or a campfire.

In a wildfire scenario this is when you’d have a crown fire or a large flame, think about sitting around a campfire,

Radiant heat decreases with distance, so we can reduce that heat transfer by just increasing the distance between the big wall of flames and the structure.

If a campfire gets too hot, you can back up to cool off. If you move forward, it feels hotter. In fact, you can get burned from the sun or a campfire without ever touching it.

The second mechanism is direct flame contact.

Putting your hand in the fire.

And we can prevent this, or reduce the potential of this by managing the fuels around the structure with defensible space and in that home ignition zone.

The third mechanism, the really important one, is embers, or firebrands.

It’s the little things.

Small burning particles that break off from burning vegetation and burning structures, and get lofted up by the fire, and then blown out by the wind and it can travel for miles ahead of the fire front,

Can I just take a minute here to point out that this is a lot like the last episode where a building scientist talked about heat flow through a building (radiant, conductive, and convective)?

Those same three mechanisms were just described here by a fire engineer. Once again, convection is a big deal.

Yeah, so you know, the smoke starts to go up, and that’s the buoyant effect. And so larger fires can create more buoyancy and can lift not just smoke particles up, but they can lift up debris particles.

We’ve seen embers as large as pine cones, and for some of these really large fires, some of these mega-fires create their own weather essentially, and their updrafts can lift particles as large as full sheets of plywood.

And that burning fuel gets lofted up by the fire,

Once it’s in the air, wind or air movement from the fire will blow those embers out for distances that can exceed a mile where they...

Cross over good defensible space to land on or around the structure.

And they can ignite it in one of two ways:

Direct ember ignition

  1. They can land on flammable or combustible materials, like a combustible roof, or combustible siding. They can get into the vents and into the structure and land on combustibles inside. That would be a direct ember ignition.

Indirect ember ignition

  1. Or the embers can land on fuels that are on or around the structure, think pine needles in the gutter, or a bush you might have right near the front door. The ember ignites the fuel around the home, then the fuel ignites the structure. That would be an indirect ember ignition.

We have found that up to 90% of home ignitions are attributed to embers.

Because the little buggers can work directly or indirectly to burn through your combustibles.

And when you’re building in a wildland fire zone, there are a lot of combustibles surrounding your combustibles, so, yeah.

Why wildland fire-resistant construction matters

It’s a thing that you need to be aware of. Like hurricane areas, fault lines, flood zones, or extremely cold climates, wildland fire is a natural phenomenon that must be designed around.

Fire suppression does not work as a long-term strategy because too much fuel builds up, causing colossal fires.

And we as humans are choosing to build in areas where those natural fires are tending to happen, as people are starting to leave city centers or living out in communities where they have great views of nature and the environment, they're also putting themselves essentially in the perfect scenario of

  • Fuels around them
  • Topography to carry fire spread
  • The weather that we know is changing

So when those fires do happen, we now have structures and homes in a way that could be impacted.

Wildfire has always been a phenomenon, 2017 and 2018 in the state of California, some of those mega-fires, the Tubs, the Nuns, The Woolsey, the Camp fire, those fires were such large losses, has definitely brought everyone’s attention to it.

But the fact of the matter is, wildfire losses happen every year in the United States and across the world.

Sure, the mega-fires in the west get all the headlines, but really, it’s the smaller wildfires who are doing the everyday blocking and tackling that keeps firefighters on their toes and insurance adjusters in the essential-employee category.

On the east coast, we don’t think about this, but we saw that in the Pigeon Forge fire in Tennessee, you know, wildfire is not just a western state problem.

And, you know, as we have a changing climate, as fuels are starting to change, and as people are starting to live in areas that burned every x-number of years, we’re starting to have to think about wildfire resistance across the country, and in places that we might not normally have done so.

Wildfire has always been on the top ten list of the insurance institute’s risk-makers, but it was toward the bottom of the list.

Nowadays, it is on top.

Because 100 years of fire suppression has thrown a LOT of fuel on the potential fire,

Because a changing climate has lengthened the regular fire season from summer to year-round and because more and more people want to live in these exotic and exciting places.

...and that’s why it's really important that when we construct buildings and homes, and businesses, communities, that we learn that we are going to live with the fire exposure. And we need to build to resist that.

How to construct wildfire-resistant and resilient houses and buildings

Which is a heck of a segway into the ‘How to do it right’ portion of our podcast. What can we do to an existing structure to reduce its vulnerability to wildfire?

A couple of things. Really importantly, we want to reduce the exposure. That means the flames and the embers. What can we do to bring that down?

We can control the fuels and combustibles around the home. That’s the vegetative fuels, that’s having good defensible space so that we don’t have continuous fire spread to the home.

That’s also structural fuels

The shed in your yard, the patio furniture on your deck, all of those things are fuel to the fire, and you need to keep them from bringing the fire to your front door.

But really importantly, think about the five-foot zone around the home, the immediate zone, where we know embers are going to land, and we want to make that non-combustible.

Another thing that we can do is maintain debris that might accumulate on the structure. This would be on the roof, in the gutter, in and on vents

Reduce your yard exposure, keep your five-foot defensible space clear and noncombustible, and clear the debris from the building itself.

When it comes to breaking ground on new construction,

As a rule of thumb, using noncombustible building materials is always preferable.

Siding, roofing, deck—if they’re not combustible, they will not burn when the embers land on them.

The other key priority is

Openings in the structure where embers can enter.

Doors, windows, and vents need to be resistant to embers.

So for vents that means noncombustible metal-mesh screens ⅛-inch or smaller or specific vents that can resist both ember and flame intrusion.

Dan says that they chose ⅛ inch not because they think it will keep embers out. Embers are going to get in.

Whatever size mesh is used, embers get caught in it and burn down. When they burn down to ⅛ inch, they enter the house and look for combustibles.

It’s just that these embers don’t have much thermal power behind their shrimpy little ⅛-inch bodies.

Embers that can get through a code-approved ¼-inch mesh can pack a lot more of a punch. It’s another case where exceeding code is a good idea.

Smaller mesh size, like 1/16-inch, can accumulate debris, which can cut down on airflow or potentially be a direct-ignition hazard.

Soffit vents, ridge vents, and gable vents are all susceptible to ember intrusion, so it’s important to consider the appropriate venting strategy and products when designing the roof system.

We looked at gable vents, soffit vents and ridge vents for different wind speeds and different types of ember exposures. All three vent types are vulnerable to embers. The gable, because it is a flat surface, and if you blow embers right against it, can be particularly vulnerable.

But the under eave vents, the embers can still essentially blow up into that eave area, and for the ridge vents, again, embers coming at the peak of the roof can get into that ridge vent.

Of course, an unvented roof kind of removes the problem of the pathway altogether.

An unvented roof kind of removes the problem of the pathway altogether.

If you don’t understand how to build an unvented roof, there’s a podcast episode about it. It may be wise to substitute rock wool insulation for rigid foam board if using exterior insulation as part of the unvented roof strategy.

A couple of other aspects of roof design are pitch and overhangs. Pitch works the same way for debris accumulation as it does for rain.

The steeper the roof, the harder it will be for debris to accumulate on it. Just think of a ball rolling down a hill, the steeper the hill, the faster the ball rolls

And the faster the rain gets away from the roof. Overhangs in rainy areas are often a good idea because they shelter the walls and push rainwater away from the house farther. In fire design, they are a problem.

We know that that under eave area can be particularly vulnerable if there’s fire beneath it. If that bush under the eave does ignite from an ember and burns up into the eave area, that’s a worse fire scenario than if you didn’t have the ewave there at all.

Essentially the eave is creating a ceiling, trapping all the heat and flame from the burning bush, and that’s thermal exposure to the structure.

For windows, use multi-pane tempered glass. That’ll reduce the radiant heat and direct flame contact potential, which could break the glass, opening a hole in your defensive line that embers and flames can walk right through.

Skylights are like windows.

Skylights create another opening. Debris accumulates on the roof. Debris accumulates on and around the skylight. That debris can be ignited by embers, and that burning debris could break the skylight, and that creates the pathway.

Somewhat surprisingly, decks are not a deal-breaker. While it is probably a good idea to consider noncombustible decking, the framing choice is the more important one.

A deck is essentially vulnerable to two types of exposures:

  • Embers on top
  • Fire underneath

The embers that land on top often fall into the gaps between deck boards, where they can ignite the joists,

But also, it can fall between the boards and land in debris underneath and cause an under-deck flame.

In the Lab, they did fire testing on various decking products, both wood and composite, and they found that.

What we found was that they all create really bad fire scenarios for the structure that the deck was attached to.

And this is what it really kind of boils down to: It’s not about the deck board, but we think it's more about the joist, or the framing that the board is on.

They framed some decks with metal and blew another wall of embers at them and lo and behold,

You can still have a combustible walking surface, whether it's redwood or composite, and that deck with the metal joist and frame regardless of any of the other decks regardless of the walking surface.

So, we’re really starting to think about, like the building, decks as an assembly.

It’s not just the walking surface; it’s the joists.

The predictable pushback to this stuff is that it might cost more. But when you accuse engineers of something, they try to prove you wrong.

We actually did a study and we used a model home that we built from the ground up, this is new construction. In building A we chose traditional materials, and in building B we chose wildfire-resistant building materials.

I think we all know what’s coming next.

And we found that the cost difference is nominally the same, and in fact, wildfire-resistant construction can cost less than traditional construction.

The main point is that wildfire-resistant construction is affordable, and when you have the choice, the wildfire-resistant material can look better, can last longer, and can make your home more fire-resistant.

Now you know more about building better-looking homes that last longer and don’t burn. And that’s good because you get paid for what you do and what you know.

And now, you know more. Tell your boss ProTradeCraft says you deserve a bump in pay, and then run like hell.

I’d like to thank Daniel Gorham, and the Insurance Institute for Business and Home Safety for being a willing participants in our show.

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