In this Building Resilience field trip episode, we visit the Insurance Institute for Business and Home Safety (IBHS) research center in Richburg, South Carolina, to witness a controlled wildfire burn test. Researchers are studying how accessory dwelling units (ADUs) ignite and spread fire to neighboring structures, with implications for the housing density boom happening in wildfire-prone states like California. The episode covers test setup, construction variables, instrumentation, the burn itself, and key takeaways on fire-resistant design.

TRANSCRIPT:

Fasten your seat belts and place your trays and seat backs in an upright position, because we're going on a building resilience field trip. Today we're headed to Richburg, South Carolina, to the Insurance Institute for Business and Home Safety's research center.

I've come here in the past to witness a manufactured hurricane using their wall of wind, and we featured some of that footage during the inaugural season of Building Resilience. They also have a roof aging farm where they do longevity testing on various shingle brands. They take sections of those exposed roofs into the hail lab and shoot them with hailstones. A computerized micro scanner measures the hail's effect on the shingle, and one output from that research is hail-resistant shingle ratings. The hail is homemade for consistency and comes in different densities.

Today, the wildfire team is looking at accessory dwelling units. You know, like a granny flat in the backyard. A granny flat with test panels and a team of firefighters nearby.

Setting up the ADU burn test

In response to recent conflagrations, researchers want data on basically how close is too close, and if you beef up the fire resistance, how much closer can you get?

"Ultimately, what we're looking at is what does that expose other structures to, or other materials. We have the ADU that we're going to light, and then we have what we call the target building, or the downstream building. As this ignites and as the fire progresses and ignites this structure, what impact does it have on that downstream structure? That's really important for us because what we are trying to understand is what are buildings being exposed to in suburban conflagration events. As fires get into the community and homes start to ignite, what are the neighbors experiencing?

At the end of the day, we want to understand what we have to design against, what we have to resist. Because if we understand what we have to resist, we can do a better job at protecting structures and ultimately reducing the losses from these wildfires and reducing the amount of destruction these conflagrations can have."

We have a massive housing problem in the United States, and a lot of people, particularly in California, are adding ADUs wherever they can. Recent disasters such as the Palisades fire clearly illustrated how wildfire can overwhelm neighborhoods. If people are going to add more density, how can we do it in a way that reduces the potential for property loss and, more importantly, fire spread?

Test variables: wind speed, construction type, code compliance

"We're looking at a lot of different variables. We're looking at different wind speeds, anywhere between 30 and 50 mph. We're looking at different types of construction of the accessory dwelling unit, different types of windows, different types of wall siding. Is it compliant with the California Building Code Chapter 7, or is it not? And how does that affect the exposure the target building experiences? We're looking at the spacing between the buildings and even the orientation. Is the building straight on like you see here, or is it rotated 45 degrees? We're looking at all these different possible scenarios, ultimately to see what we have to design to and how we can make homes more resilient to wildfires."

Fire can jump from one building to the next through embers, direct flame contact, and radiant heat, a powerful heat flow mechanism that can cause spontaneous combustion.

What makes the ADU code-compliant

Like Spinal Tap, the ADU they're going to set fire to goes all the way to eleven. It is compliant with the California Building Code's Chapter 7, which covers fire-resistance-related construction.

"This accessory dwelling unit is compliant with the California Building Code Chapter 7. What makes it compliant is really the wall. In order for this wall to meet those requirements, it has a layer of gypsum between the siding and the sheathing. That allows it to comply. So while it's combustible siding, it does comply with the California building code.

The second aspect is we have dual-pane tempered glass for these specific windows. And the last is we do have an open eave design. It is not an enclosed eave, but what you can't see, hidden behind the wood, is another layer of gypsum board between the roof sheathing and the wood you're seeing. That layer allows it to comply."

Despite meeting the building code's fire-resistance standard, this is the building they're going to set fire to in order to test fire spread to other buildings. We'll see that the gypsum sheathing is the last to burn, but the siding and studs kind of make it a fire sandwich. Similarly, the open roof overhang catches quickly, rendering the gypsum sheathing layer something of an afterthought. The tempered glass windows fall out of melting vinyl frames, another partial measure illustrating that something isn't always better than nothing.

Instrumenting the target structure

"From a scientific perspective, our goal is to have a consistent ignition of the structures. I mentioned all the different variables we're changing: wind speed, distance, orientation. The research is: this has ignited, and what impact does it have on the surrounding structures? The surrounding structure in this case is a little building with all our test equipment in it. About half a million dollars worth of data-logging boxes.

Our goal is to harden that as much as possible so it doesn't ignite at all. It's not built as if it were a real structure in the real world. It's part of the experiment so we can measure everything. We have about half a million dollars worth of instrumentation inside that structure, and our primary goal is to not burn up a half million dollars of instrumentation. It's hard to do science when your instrumentation melts."

"We really have three types of instrumentation on this building. We have heat flux gauges, which are measuring heat transfer and how much heat is actually being applied to this wall. We have material temperature sensors. We have a thermocouple here that's measuring the wall temperature, so how hot is the wall getting? And the third gauge is what we call a radiometer. It's still measuring heat transfer, but only the radiative component, the kind of heat you feel from the sun."

They place several wall panels at different distances around the sacrificial ADU to observe how heat exchange can cause combustion. The instrument shack is most likely to see flame contact and ember swarms.

"What we have done is we have these wall panels here. They are susceptible. We do have different types of combustible wall materials that could potentially ignite during these experiments. And should they ignite, we have kind of a rip cord here where we can pull it and the panel just falls to the ground without posing a threat to the structure. So we're able to get the best of both worlds. We're able to measure everything this building is experiencing, and we're also looking at different materials and whether they ignite, without putting anything at risk."

Inside, the ADU looks a lot like a real home: laundry in the hamper, potato chips on the counter.

"This is really designed to mimic all the types of combustible materials you would have in a real home, so that this is as representative as possible of the real world if this house were to ignite."

Monitoring ignition and fire spread in real time

Sacrificial cameras and sensors also beam data and photos into the cloud until they're engulfed in flame.

"From here we can monitor from our command center in real time when the fire actually finally penetrates the wall and ignites inside the structure."

The post has thermocouples every couple of feet from floor to ceiling.

 "Those are measuring the temperatures at different heights above the ground, just to make sure the basic laws of physics are still on the books. We're able to look at the temperature profile at different heights within this ADU as part of our data set."

"Eventually the fire inside is going to get large enough, and it's going to break the glass. The wind is going to be coming toward us. Once that happens, it'll get into the roof, and you'll really have the entire house fully engulfed. At that point, we're looking at whether the flames are able to reach the target and make direct flame contact. You'll have radiation coming off the structure, and you'll have what we call convective heating, where air being pushed this way gets heated up by the flames and that hot air impacts the target structure."

Interview: IBHS chief product officer on windows as a weak point

I also spoke with Alistair Watt, the chief product officer at IBHS.

"I'm responsible for the teams that take all this content, all this research, and get it into action through codes, through mitigation programs, through education, trying to influence people to have more resilient homes across America."

Alistair pointed out one of the weakest links: windows. Wildfire is a lot like a hurricane. If you can keep it out of the structure, you can seriously minimize the damage. As soon as it gets inside, all bets are off. Windows are the easiest way for fire, sideways wind, and water to get in.

"Intuitively, you would think that the glass is the really vulnerable part, but it's really not. You'll see when the temperatures get high enough, the window frames melt."

Metal entry doors are similarly vulnerable. One surprising thing is that radiant heat can cause a fire just by proximity.

"Where we're standing right now, you will not be able to stand here once that catches."

The controlled burn

Speaking of it catching, the first step in a controlled burn like this is to ignite the wood cribs in front of the building.

"Those will catch pretty quickly. We won't have the fans all the way up. And then we'll turn the fans up just a little bit, and you'll see the flames come across. When the roof goes, then it's really going to get hot."

The wall panels are 20 and 30 feet away from the ADU. The target structure is 30 feet away, directly downwind of the wall of wind, which runs variably from a low rumble of 12 mph up to 55 mph.

The sacrificial ADU is engulfed within 10 minutes.

"The 15-foot siding sample on the left caught, but the other window system with the two panes was unharmed, which was pretty impressive. At 15 feet, that glass and frame survived."

Within 30 minutes, the house has collapsed. The walls are falling. After 45 minutes, all we see is roof and wall fragments burning on the floor, appliances melting in the corner, and embers. There's an ember storm as well.

"That ember storm was probably swirling earlier too, but we were all staring at the big fire, and they weren't as visible in the daylight."

When you see embers attacking combustible material like mulch and vegetation, it's pretty compelling.

"That's what you would see in the real world. And it's no surprise that when one house catches and the houses around it are not prepared, they catch too, and you end up with a conflagration."

Key takeaways on fire-resistant design

So what did we learn today? At least a few things. Tempered windows, gypsum sheathing, metal doors, and other fire-resistant features do not stop fire on their own.

"We saw with this Chapter 7 construction, with gypsum behind the combustible siding and 2x4 framing and gypsum on the inside, that the siding failed pretty quickly. The gypsum, being coated in combustible paper, started to disintegrate on the outside. The windows gave, which let the flames in. Now the gypsum is being attacked from two sides. It didn't take long for the walls to fail. It took a little longer than I expected to get up into the eaves and into the attic, but that still failed fairly quickly. It was around the 14-minute mark when we had pretty much complete failure."

Fire is relentless. It's like water, wind, and gas combined. It attacks a house the way water does, by probing for the weakest opening, but it travels and behaves like wind. A systemic fire-resistance design is the best hope, and in suburban neighborhoods, it begins at the property line.

We've got a whole lot of that on ProTradeCraft, so I'm not going to rehash it here. Instead, I'm going to thank the Insurance Institute for Business and Home Safety for inviting me down to their wildfire show, and also for the pizza. That really hit the spot.