Fire Science Show

230 - Wind driven conflagration experiments with Faraz Hedayati

• Wojciech Węgrzyński

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A facility with 105 synchronized fans pushing hurricane-class wind across a full-size house while a live fire... This is not science fiction - this is a real research capacity that helps us re-shape our knowledge on the full scale building ignition, fire spread, and failure. That’s the stage at IBHS, where we dig into how wind-driven fire behave differently to small-scale and how tiny choices around a building can decide its fate. Together with my guest - dr Faraz Hedayati, we go from embers generation and fire spread studies, to urban conflagration research.

We start with embers, the quiet culprits behind so many structure losses in the WUI. Embers aren’t a single threat but a spectrum of sizes, temperatures, and lifetimes that ride shifting eddies and stall in stagnation zones. We talk through what full-scale tests reveal: glowing ember lines at the base of walls, roof reattachment zones where deposits spike, and the hard truth that counting particles matters less than controlling where they land. The guidance is clear and actionable—noncombustible vertical clearance, hardened vents, defensible space within the first five feet—because under wind, any component can become the first domino.

Then we tackle conflagration: how a spot fire becomes a neighborhood problem. IBHS’s shed-to-structure and fully furnished burns show exposure arriving in pulses, not a smooth curve. Collapse chokes flames and then reinvigorates them, creating multiple peaks where materials succeed or fail on a timer. We compare 30 mph to 60 mph winds and see how plumes lose buoyancy, flatten into the target, soften vinyl frames, and push glazing inward. Separation distance emerges as a decisive lever: around 10 feet, continuous flame contact dominates; at 20 feet and beyond, exposure becomes intermittent and materials can win—unless “connected fuels” like vehicles, fences, and decks bridge the gap.

The takeaway isn’t a silver bullet. It’s a layered defense: control embers, clean the near-wall zone, harden openings, choose noncombustible claddings, and increase spacing where possible. Small-scale testing and modeling still matter, but wind-driven fire demands validation at full scale to catch the peaks, the collapses, and the failure modes no bench setup can mimic. If you care about wildfire resilience, urban design, or building safety, this conversation offers a rare, data-rich look at how communities ignite—and how we can change the odds.

Learn more about IBHS research at https://ibhs.org/risk-research/wildfire/

Cover picture courtesy of dr Faraz Hedayati.

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Wojciech Wegrzynski:

Hello everybody, welcome to the fire science show. Being a fire researcher myself working in the laboratory, I learned one thing while doing those experiments. There are phenomena or things in fire science in general for which the scale really matters. You can learn a lot from experiments on material scales, and I wonder how many PHDs over the world were done just on the cone calorimetry, and that's that's perfectly fine. That's some absolutely beautiful fire science there where you can use those aparatures to come up with clever things. But in reality, some phenomena, some things really show up in a grander scale. And sometimes it's not even about phenomenon, it's about how they interact with each other or which becomes the dominant one. And now if you want to do large scale fire science, it's really hard. It's really hard to get those experiments planned, it's really hard to get them approved, it's really hard to get them conducted. They usually end up expensive, and if you do them in the field, you often lose control over your variables. To have a laboratory that allows you to run full scale fire experiments, I'm talking building scale fire experiments with perfectly controlled pundit conditions, that's the dream. And well, there are people living this dream in the world of fire science, and there is one facility in which something unbelievable is possible. Full scale wind driven fire experiments. Well I mean full scale wind, full scale fire, full scale buildings, just as they are in reality. The laboratory is the one of the Insurance Institute for Business and Home Safety, IBHS. And I've invited a colleague from there, Dr. Faraz Hedayati, to discuss the amazing experiments carried out in the IBHS and what more can you learn in those large scale experiments versus running smaller scale data. And importantly, there's space for everything, and small scale is absolutely necessary to move the science forward. And at the same time, if we had a bigger facility than IBHS, perhaps we could even observe something more. Anyway, this is highly relevant to uh wildfires, this is highly relevant to wildland urban interactions, but it's also important to any type of wind-driven fires. And as we know in the building fire safety, we have plenty of those. Uh let's learn the insights from the large-scale experiments from Faraz Hadeyati. Uh let's spin the intro and jump into the episode. Welcome to the Fire Science Show. My name is Wojciech Wegrzynski, and I will be your host. The Fire Science Show is into its third year of continued support from its sponsor OFR consultants, who are an independent multi-award-winning fire engineering consultancy with a reputation for delivering innovative safety-driven solutions. As the UK leading independent fire consultancy, OFR's globally established team have developed a reputation for preeminent fire engineering expertise with colleagues working across the world to help protect people, property, and the planet. Established in the UK in 2016 as a startup business by two highly experienced fire engineering consultants, the business continues to grow at a phenomenal rate with offices across the country in eight locations from Edinburgh to Bath and plans for future expansions. If you're keen to find out more or join OFR consultants during this exciting period of growth, visit their website at OFRConsultants.com. And now back to the episode. Hello everybody. I am joined today by Faraz Hedayati from Insurance Institute for Business and Home Safety. Hey Faraz, good to have you in the podcast.

Faraz Hedayati:

Yep, good to be with you.

Wojciech Wegrzynski:

Wow man, I am jealous. I am jealous of the facilities at IBHS, and uh let's start the interview with that so people understand what I am jealous about. Can you introduce me to IBHS, please?

Speaker 1:

Absolutely. Yeah. Uh the Insurance Institute for Business and Home Safety, or IBHS. We are a nonprofit located in Richburg, South Carolina. It's the rural side of uh Carolinas. Uh the closest gas station is like a 20-minute drive. Oh my god. As the audience uh Google and look at the facility, you see why we have uh a very large test chamber. We are known for our uh the capabilities that we have to study wind-driven fire. And uh so IBHS uh is a non-profit organization, uh again uh funded by the insurance industry. And uh we have 105 fans, and each of them uh the size of them is about six feet. Apologies for the unit.

Wojciech Wegrzynski:

We'll we'll manage, no worries.

Speaker 1:

So 105 of those, and they can generate we speed from 10 miles an hour to 120 miles an hour.

Wojciech Wegrzynski:

Oh shit.

Speaker 1:

So a cat three hurricane.

Wojciech Wegrzynski:

Wow, I did not know that. I just knew they were like large and plenty, but I didn't know that they can go that far.

Speaker 1:

Yeah, it's been about 10 years that I've been working at IVHS, and each time still, as of today, I step into the test chamber and look up and I'm like, wow.

Wojciech Wegrzynski:

How how big is the is the test chamber itself? Because the I'm not sure if if pictures give it justice. Is it like 100 feet by 100 feet larger?

Speaker 1:

No, it's like four uh basketball feeds uh you can put inside so the wind jet is about 80 feet by 30 feet, so 80 feet wide, 30 feet tall.

Wojciech Wegrzynski:

Okay, yeah. And when you put a building in there, can you cover you you cannot cover the entire focus field of the wind, right? So what was the maximum building size you can put in there?

Speaker 1:

So it depends on the blockage ratio that we want to test. We typically can put a two-story building in the chamber safely. Wow.

Wojciech Wegrzynski:

So this like brings uh a complete different level to fire research because I I do not know a single facility that that can do it, yeah, other than you.

Speaker 1:

So the wind side of the equation, we have a lot of wind engineers at IBHS that help us to study wind-driven fire. And to your point, the interaction between wind and fire. I'm one of those scientists that stay on the site that it cannot be scaled down properly.

Wojciech Wegrzynski:

Yeah.

Speaker 1:

Uh so fully scale testing is always needed to understand the fire dynamics.

Wojciech Wegrzynski:

Absolutely, absolutely. Well, uh, I wrote that in my reviews about wind and fire. It's it's simply not possible, they do not scale in the same way. Uh and also the best way of scrolling fires where we use the fruit number, it's kind of good for buoyancy driven. But once you have forced wind-driven fires, there's I don't think there's a good capacity of of scaling that.

Speaker 1:

Yeah, I I completely agree with that. And if there is, to some extent it's possible. You cannot go from like a full scale to 10 centimeter fire and expect that that would happen.

Wojciech Wegrzynski:

But do you also do a smaller scale? Because of course you do the giant scale, the the biggest scale uh experiments. But you'd also have like a materials department, a small scale department, or not really?

Speaker 1:

Yeah, we do. We have quant calorimetry testing at IBHS and smaller scale tests. So uh we don't want everything to be fully scale until it's needed. So because it's very pricey, as you know, to run these experiments. So we do a lot of due diligence on the modeling side, on the smaller scale side, and build up to these full scale experiments.

Wojciech Wegrzynski:

Uh okay, like this is probably the most boring uh episode because I'm asking for technical details. How much electricity does it take? Like, I'm really curious.

Speaker 1:

That's a good question. So, uh, when the fans are at full power, electricity use can power up 9,000 homes.

Wojciech Wegrzynski:

Wow, I can I can imagine it's a lot. So sorry, listen. I'm like I it's like when I go to sleep, I dream of that wind tunnel. Yeah, you're invited. Fantastic, fantastic. Um, when you do uh fire experiments at that facility, what kind of fire diagnostics you catch? Because I imagine the wind is like perfect, but in terms of of fire, it must be quite difficult. You are you able even to capture like oxygen. I you you you cannot do oxygen chlorometer with that, right?

Speaker 1:

No, we cannot, yeah. It's uh force conviction that all the smoke goes out. Uh, we actually thought about that to sample from the smoke at some point, and WPI uh have some study on that topic. Uh it's on a research agenda to do it, but at the moment we don't want it to research it.

Wojciech Wegrzynski:

And mass loss rates uh uh and our dynamic scales. Do you use that?

Speaker 1:

We do we do have uh like large scale mass loss platforms. Actually, in a study in 2020 that is published uh with NEST, we put a 10,000 pounds, something like that, structures on a mass loss platform and ignited gold and uh got the mass loss uh rate.

Wojciech Wegrzynski:

And I promise this is the last one. I'm like, we're gonna go into fun fire engineering things. I just need to satisfy my curiosity. Are you like limited by the size of the fire? At what point do you start getting worried about the fire size?

Speaker 1:

That's a good question. It depends when you ask that question. Four years ago, running the experiments that we are running right now was a dream. Uh but now we run fully scale structured residential buildings. Uh, I would say there are two factors. One is the size of the jet, right? It's about 80 feet by 10 feet, uh by 30 feet, right? So we can't go larger than that. Other one is the safety side of the equation. Yeah, we we make sure that everything is as safe as possible before running these experiments. So we get a lot of supports from our local fire departments. Uh, Richford, kudos to them. Richford South Carolina Fire Department.

Wojciech Wegrzynski:

Something tells me they love it. Uh okay, faras. Um, there's a lot of interesting research from IB just to fulfill multiple podcast episodes. I would uh love to discuss the stuff that that's probably only possible in your laboratory. And those are two things. Those are the the fire brands research and how uh how firebrands spread fires, and uh the other ways that that fire spread themselves uh in in larger like configuration style configurations. I will like we've agreed to start with the firebrands. So maybe maybe we could start with when did you when did you start looking into fire brands cost ignition? Like something tells me there's a background story to why this topic was raised and and and basically how how do you do the the firebrand experiments?

Speaker 1:

So most of the audience uh of your wonderful podcast know that some of the history about this. So when back in the 70s, some Spanish researchers from Europe moved to Missoula Faiwa and worked with uh scientists in uh Missoula and studied fire rands, right? So that was one peak in in firebrand study. And it was somewhat that was the available knowledge until like 2000 when NIST picked this topic up. And when they generated the first generation of ember generators as what we know today, there are other versions of it existed before 2000 when the Missoula Finland was working on it. And uh when starting in 2010, uh when the embryo generator uh that NIST uh basically built uh was out there, IBHS realizes that the interaction between these embers and wind is something that cannot be stuck. And uh when the uh dragon was shipped into uh Japan and there were some wind driven ember studied there, but it was limited to the size of the facility, right? And at IBHS back then, Dr. Steve Colbs, uh so my PhD advisor, when he retired, it took two people to fit his shoes at IBHS, took the lead on uh studying embers at IBHS, and we generate or we built eight, ten or more emberators, put them in the wind tunnel, and then looked at the interaction in full scale of how embers interact with that solid body, with the blast body, which is this charge. And a lot of those information about how embers penetrate into the buildings or where do they accumulate around buildings, all of those are basically the product of that era of uh research at RDHS.

Wojciech Wegrzynski:

That Spanish research was that Tarifa or Yeah, one of them was Tarifa, yeah. Okay, that is the name I I I recognize. And about the the firebrand generation, you used the name dragon. I I guess the listeners may not be familiar with that uh equipment. So can you tell me something about the generator itself?

Speaker 1:

Yeah, so uh the previous generation of these ember generators were just like a burning body that was like moving around, or like a fire that with a fan that creates a convection convective column that naturally deposits embers and they attack different building components. What was the previous version of it? This was based on the natural development of fire. And then uh what NIST scientists did back in 2000, they uh basically feed uh with uh with an auger system, ignite it in a burrow, and then as the material or as the wood particle lose mass, there is a fan at the base that creates that convective column and they shoot uh embers out.

Wojciech Wegrzynski:

So basically, the upgrade is that it's suddenly the stream is controlled and repeatable, and the experiments become much more like uh scientific variable control rather than an outcome of a natural fire?

Speaker 1:

Uh yeah, yeah, that's it. That's a great way to put it.

Wojciech Wegrzynski:

Uh so now we move uh some years ahead, we're 2025. Some years have passed since this was discovered. What's the importance of this research today? Like, why do you keep researching that?

Speaker 1:

Yeah, that's a good question. So when we go in the field after these disasters and tragedies that we keep seeing disproportionately higher in the US compared to other countries, we see that embers are the leading cause of ignition. When we look at direct and indirect ignition, so as these embers accumulate around the structures, shrubs, decks, or fences, whatever you want to call it, when those ignite, the fire can rapidly go from a few inches or a few centimeters fires all the way up, engulf the whole structure. And that's where the chain of configuration starts, when those spot fires have the opportunity to become very large. So when we see those in the field, we come back to IVHS and then we design experiments to understand how that dynamic takes place. And uh we see that like uh we can on the engineering radius, we can have different wind flows or wind speeds, uh, different size of structures in front of it. And as ignite uh or as they uh attack the building, one of the key moments in my uh career was I'm I'm deep into image processing side of the equation, and I try to track these embers in a controlled environment of 10 miles an hour wind, not heavily turbulent. And it is impossible to track these embers and count how many going through the vents, how many bounce back when they hit the target, how many of them not just fall at the base of the beginning. And the conclusion of this was if it's this chaotic in a controlled environment, how can we take opinions or stretch this into the real fee? So those concepts were like in through years uh shaped into the development of uh Wi Fi Prepared Home, a program that we have, that we actually, instead of targeting a specific structure uh components of a building, we address a holistic uh structure. So defensively space around the building, vents, the vertical cure is on the base of the wall, uh requirements for decks, all of them need to be in place because it's such a chaotic system that we cannot single out a specific component and say decks are more vulnerable than vents, or vents are more vulnerable than base of the wall. This whole thing needs to be together.

Wojciech Wegrzynski:

I I think I found it on the on the webpage, wildland, fire embers and flames, or mitigation that matter. I will link the the materials in the show notes because they must contain a ton of useful uh indications. How do you how do you judge that in that case? So if you if you're not like so you're not interested in saying how much protection a different strategy gives, you're interested in the complete outcome, like overall as a combination of those. How do you judge it then what's better, what's worse?

Speaker 1:

In my opinion, for specifically for embers, it is nearly impossible to quantify how many embers attack different components of a bead. The way that I look at embers is uh like like a smoke, it's a particle with zero mass, it's just at the mercy of the flow, right? Embers do have mass, so they have some inertia to fight back with the wind patterns, right? So because embers have different distribution of size, then uh some eddy sizes can move a specific size of embers, and larger ed sizes can move other size of the embers, right? So it's impossible to put all of them together and say, like again, like decks are less important than vets and cannot quantify it. For frames it's different, but for embers, uh in my opinion, it's nearly impossible.

Wojciech Wegrzynski:

This is super interesting. Uh I think I for the first time I'm having such a conversation. So in the end, when the building is subject to distribution of embers, like uh different sizes of embers will be dangerous for the ventilations for HVAC where they can penetrate, and different will be important for the porch, let's say, fire safety, and different will be important for the gutter.

Speaker 1:

Yeah, absolutely. So uh it's such a chaotic system. Uh so uh the interaction between the wind eddy size and the embered ones always dominates red lumber accumulates. And both of these are variables, none of them are constant.

Wojciech Wegrzynski:

Yeah, but but how about the building aerodynamics? Because you said the eddies matter. In that case, the the building aerodynamics, the way how eddies shed at the edges of the of the building, uh, how the flow occurs around the building, that must be hell of important as well, right?

Speaker 1:

Yeah, absolutely. And uh that's one of the important reasons that IBHS and other research organizations talk about zero to five foot clearance around the around the building. We come up with the stagnation of the flow, embers accumulate in that area. And when it comes to the roof, we can actually clearly see the uh reattachment lengths on the roof. So embers jump over that and then you see uh where they land on the on the roof.

Wojciech Wegrzynski:

A hypothesis I once had is that you probably could make a link if you if you had a constant flow of wind on a building and it just drawn the static pressure around it, you could pretty much see how big the area of stagnation of air around the building is. And I made a hypothesis that a lot of embers would be captured quite well by those pressures. I wonder if we've done ever such a simply simplified uh analysis.

Speaker 1:

Yeah, like I'm not a wind engineer. Uh what I learned from my colleagues who are wind engineers is half of the building height. So that that's basically stagnation flow, right? So at one point that embers accumulated in that length and also at the base of the wall, a lot of these embers hit the wall and just fall. So we see a glowing line at the base of the wall, and that's why a six-inch vertical clearance, non-combustible clearance at the base of the wall is very important. We actually have ignited countless buildings unwantedly because of that vulnerability. So uh seeing that in at in the facility, then we developed requirements involved for a prepared home to harden that area of the building.

Wojciech Wegrzynski:

Okay, so tell me a bit more what drives this research. You're in in a non-profit insurance uh institution. What's the point of that? To to drive the consumers into safer solutions, to uh give uh insurers the ability to uh reject more claims? Like I hope not. No, I I assume you're looking for how to make the structure safer, right? But uh what's the ultimate goals?

Speaker 1:

I I'm not sure that there is one ultimate goal or panel competing goals here. Uh I'm not necessarily competing, maybe. So one of them is to inform the insurance. Industry that what are the vulnerabilities and what they should affect for their loss, right? We have no say into pricing or their profit, we we are not involved in that process at all. Uh, so that's how we serve the insurance industry. When it comes to the scientific dynamic, we work closely with academic entities in the US, uh, Australia, European uh universities. We are after the same goal. How can we make buildings safer or our build environment safer? Right. So, to some extent, the need of the insurance industry and the need and the scientific needs go up together, and then it uh diverges from a lot of different purposes. So during that, the first 80% of it, uh, there is no difference in my view. And then as we get to the end, we actually instead of having a deeper dive into fundamental science, we try to hand it over to universities. As you know, I we work with UC Berkeley, University of Maryland, University of Melbourne, all of those are uh we work closely with.

Wojciech Wegrzynski:

When you design those experiments, are the experiments very like US-centric or they are generalizable to, I don't know, fireband shows in Portugal, in Greece, in in Poland, if we ever have those. I hope not, but most likely we will at some point.

Speaker 1:

Uh yeah, so the building materials are North America focused for sure. So when it comes to the exposure side, which is like the ember side of the equation and the interaction with wind, I don't have a reason to say that there is a difference between European embers. I guess like the fuel is different, right? But to to some extent there is a good overlap between the two.

Wojciech Wegrzynski:

And is the quantification of the ember size distribution or amber life distribution? I I don't even know what would be the critical variables to describe the ember from your perspective. What's important for you? Mass, size?

Speaker 1:

Ultimately the thermal energy that they can carry, right? Okay, which is a function of mass, size, traveling, distance, temperature, what is the material that they depart from? So all of those come to equation and uh define that thermal inertia.

Wojciech Wegrzynski:

Okay, now going back to the uh this pathway from firebrands to conflagration, there's a spot ignition in between. How how do you quantify the ignition in this case? Like how do you investigate it in that case?

Speaker 1:

So when we are in the field, uh we see first responders jumping on the spot fires and suppressing them. A lot of examples of those. And the next or stretching that observation is that it's very likely that when uh first responders were not there to suppress the fire, the fire grew and involved like larger fuels like cars or sheds, and then ultimately structures, right? That if they can if they have some food. And we clearly see the roll-up embers, both flying and rolling embers, as they just get stuck behind the vertical surface like a wire fence, overwhelm that, ignite that, and then the chain of conflagration gets going from there.

Wojciech Wegrzynski:

Are they also like uh do they play a role in any like heat transfer? Is there like uh radiation heat flux from uh cloud of embers, or are they too small to cause those phenomena?

Speaker 1:

I'm sure some level of heat transfer happens because we see the ultimate product, which is uh ignition, right? There are some lab experiments on the pine of embers or like singular ears of like things of that nature.

Wojciech Wegrzynski:

Yeah, I think. Yeah, I am Do you need one ember or two embers to ignite something, five embers? Yeah, yeah.

Speaker 1:

Yeah, yeah. I am not aware of a an experiment that was done based on this. Michael Golner and Wuchan at UC Breakley worked with IBHS during the past year, and they actually took the lead on collecting embers during our full extra full scale experiments. And to my knowledge, that is the only study that looks at a full scale ember exposure, wind-driven ember exposure, uh, like what we did. And that study is under development yet, it's not published.

Wojciech Wegrzynski:

One more thing about embers. Do you also try to quantify how many embers are produced in uh house fires in your experiments?

Speaker 1:

Mostly on the accumulation side of the equation. So where do they accumulate? And uh, I don't want to give too much information because uh UC Baker is working on it. Of course, estimated how many embers can depart from a burning structure.

Wojciech Wegrzynski:

Okay, that's very interesting. Because I also I I think I saw experiments are at IBHS where there would be multiple structures, like one next to another.

Speaker 1:

Yeah, yeah, yeah. We do study those, and uh big part of the equation that we are working on right now, or like research agenda that we have, is how configuration takes place. So when the first building ignites for whatever reason, what is the influence of wind speed, separation distance, fuel load, and like the relative location of these two buildings together that defines junk from the first to the second one?

Wojciech Wegrzynski:

Can you define like so so configuration would be for you any significant fire spread within a community, or how would you define it?

Speaker 1:

Yeah, when we see uncontrollable flame-driven fire spread, we call that configuration.

Wojciech Wegrzynski:

Flame-driven, okay.

Speaker 1:

Yeah, yeah. There are a few criteria for that. In my view, structure separation is the driving factor because when you have dense neighborhoods, it's likely that those homes are connected to each other by cars, by sheds, by fences, things of that nature. And uh when there is more fuel, there is rain, and then unlimited fuel, unlimited oxygen.

Wojciech Wegrzynski:

That's a recipe for disaster. Well, no, not fantastic, but uh that's that's that's very interesting. Okay, uh, let's discuss the conflagrations. So uh maybe again give me give me a story of how did you start working on large-scale building fires and and uh how does uh a fire turn from uh just a spot ignition into uh something that can create a conflagration effect?

Speaker 1:

Yeah, back in 2019-2018, uh when we were looking at ender accumulation around buildings, and then we we put different fuels in the zero to five foot uh building, as that ignites and the flame starts to touch the cladding, typically uh combustible cladding, it's in the order of 45 seconds to one minute and a half for the fire from a few inches tall to like tens of feet tall uh flame. So when we saw that fast transition, those were those were the first questions that uh were generated back then that when this takes place uh fast, how does the fire jump from this building to another? Right. And then in 2020, we worked with NIST, uh US Forest Service Service, NIST, IBHS. If you I hope I'm not missing any any other organization, it was a conversive effort, right? So we started looking at fire, uh, how fire jumps from a burning shed to another structure, and we divide and conquer uh this task by uh NIST taking care of smaller sheds and no wind, IBHS did the larger sheds with wind. And during those experiments, we realized that uh interestingly, as the fire develops in the source compartment, which is just a shed, the fire gets hotter and hotter. Before it impacts the target building, it actually impacts the source structure itself. Okay, and collapse happens. So the roof collapse on the on the fire limits oxygen, and at the peak of the fire, we see an 80% or like 50% drop in the fire intensity or the heat flux that we measure down with. Right. So those were like interesting observations that we start seeing how fire departs from the classical growth steady decay fire curve. And interestingly, a lot of the damage modes actually happen beyond the growth part. So similar to the cone calorimeter test that we see, two peaks, we see multiple peaks in these experiments. In some of them, ignition or damage happens on the first peak, but that's not a must. In many cases, ignition and damage happens on the secondary or third uh peaks decay phase.

Wojciech Wegrzynski:

So in this case, it's like two parallel timelines: one when the fire devours the initial structure and and how that damage interacts with the fires itself, and and how when it will jump to another structure. That's how I understand it.

Speaker 1:

Yeah, absolutely. So there are two knobs, the exposure side, which is driven by how hot the fire burns, and then the resistance side, which is basically the material on the target page. The first one overpowers the second one when fire spreads. And if the other one has a fighting chance, then the fire does not spread, right? And that's how we put these two together. When the exposure is so high, meaning the structure separation of three meters or ten feet, then under those cases, ignition happens on the first peak because there is no finding chance. But when the structure separation is more of like 20 feet or uh larger than that, then the exposure is reduced to some extent that the building material have a fighting chance. So they don't respond necessarily to the first peak. They might respond to the second one, the third one, or even in the decay.

Wojciech Wegrzynski:

By the exposure, like I guess it's all modes of transfer plus firebrands in this case all together at one time.

Speaker 1:

So uh you didn't look at firebrands in this exposure side. What makes it correct? Like in real world, that's how it takes place. We are only looking at the heat flux measurement uh by convective and uh convective heating and frame contact.

Wojciech Wegrzynski:

How big is the convective heating in this case?

Speaker 1:

Uh we saw 140 kilowatts per meter square fire. Okay. That did not ignite wood. Okay. Yeah. So uh when later on, uh maybe like three minutes later we see ignition happening, or like 10 minutes later we saw ignition happening at much lower heat fluxes, the more like like 40, 50 uh kilowatts per meter square.

Wojciech Wegrzynski:

But this 140 kilowatt was the convective heat transfer or combined?

Speaker 1:

Combined. Like the total uh uh heat flux meter, uh, we measure 140, 140 kilowatts, yeah.

Wojciech Wegrzynski:

Mm-hmm. And uh by flame contact, in that case you distinguish it by observation, or there's a like a uh value of heat flux where you just consider it was a flame contact.

Speaker 1:

So uh if we see a sudden drop in the measurement, then we look at the videos and we say, okay, that's what the direct flame contact, right? So most of the flame direct flame contact in a continuous way happened when the structure separation was at 10 foot. At 20 feet separation, about six meters, then it becomes an intermittent flame contact and convectivity.

Wojciech Wegrzynski:

Okay, well, now you're bringing the time component. Thank you very much. Uh very, very interesting. So, okay, uh we'll we'll take the time component in a second. Uh let's let's look at the resistance side. How do you define the properties of the materials in that case? What are you looking into?

Speaker 1:

Yeah, so uh we tested common in North America, like vinyl windows, double pane temper. They have about 25%, 20% market share. Different species in combustible, non-combustible, all of them representative of what we have uh in this country. And then we realize that uh they have a response time, right? So when we consider these as uh semi-infinite solids, and then we solve the equation for it, and then uh based on different heating ramps, we can see that the time that it takes for different building components to go from say ambient temperature 20 degrees Celsius all the way to the failure temperature for wood, about 300 degrees Celsius for glass, maybe 180 or something like that.

Wojciech Wegrzynski:

How do you define failure temperature?

Speaker 1:

So for for wood is ignition for glasses, breakage, for uh vinyl is melting.

Wojciech Wegrzynski:

It depends on the the significant significant event with uh given material.

Speaker 1:

Well, absolutely, yeah. Okay, so that time frame is in the order of five seconds to a minute. It is not instantaneous heat and it's not total heat flows in the order of 10 minutes or something like that. So when we chose that, and then you look at the statistical distribution of how much energy is received during that time, we started seeing some statistical patterns that match exposure and uh resistance of the building materials.

Wojciech Wegrzynski:

Did you have the chance to connect them with some uh modeling of heat transfer within the structures? I wonder how the one the thermal bulk will definitely be in a factor of here and then perhaps the capacity to to like convectively cool on the other side of the of the element, perhaps.

Speaker 1:

Yeah, we are working on some modeling side of this right now. Uh Michael Wilder, his team is doing that right now, so that will come out hopefully sometime next year.

Wojciech Wegrzynski:

Okay, fantastic. And now uh for the exposures. So when you are exposing those building materials to those uh what what's your initial source of fire? Do you set up a big fire in front of the building and then see the flames? How do you carry the experiment actually?

Speaker 1:

Yeah, so the first phase of these experiments, which were sheds, we tested a UL standard or nominally UL standard uh 6A wood cribs. Each of them is about 300 pounds, 140 kilograms, something like that. And we put like 15 of those in each building inside inside the shed, yeah, and then ignite that from like an internal ignition, and then look at the fire go. In the second phase of the experiment, which just finished a few weeks ago, we tested auxiliary dueling unit, uh residential structure. And uh the size of those were 25 feet by 25 feet by 17, so a one bedroom with an office fully furnished with furniture.

Wojciech Wegrzynski:

Okay.

Speaker 1:

And then in these experiments, we ignited those by external ignition. So we put wood crepes uh five feet away from them, ignited those. So we created a configuration condition that the fire impulse or impacts the building from an external.

Wojciech Wegrzynski:

So in this case, you see how the fire enters the building and uh what happens later on.

Speaker 1:

Yeah, absolutely. And like the very clear observation that we had is uh the building is as as strong as its weakest link. You can have a non-combustible siding on your own, but the window and door can fade much earlier than the other components, and that triggers fire spread and how fires enters into the living area. And with wind, flashover is once the fire enters about two minutes after that, and then the train of configuration keeps going.

Wojciech Wegrzynski:

Uh like you're going into wild places with uh with my false process right now. But no, no, I have to stop uh and ask some clarification questions. Did you have a chance to look into a completely non-combustible structure but with the combustible uh materials inside?

Speaker 1:

So non-combustible gliding, yes. We tested stock oil, fiber cement, like those materials. But all of those do have vinyl windows, which is a combustible component. Okay, or doors, which is fiber. These buildings are built to requirements of California's WUI building codes, but you do have uh combustible uh components on there.

Wojciech Wegrzynski:

Okay, and and and still through those weakest links you see spread. But I assume there must have been a difference in the timelines between those.

Speaker 1:

Well, not necessarily when the building itself fails, and that's a very great uh question. The difference kicks in when we look at the response of the source fire on the target building. When we have a combustible uh building with combustible cladding, the flame length is significantly higher compared to other buildings with non-combustible, sorry. So of course the flame length and flame surface is much higher than another wood structure. But when you have a building like a uh fiber cement structure non-combustible, the flame surface area is limited to windows and doors, just openings. So radiation is significantly lower, and then flame length is also lower, and that's how resistance and exposure are merged. So we talked about resistance on one side of evacuation, exposure on the other side, but through this dynamic, they are connected. So resistance actually plays a role in exposure.

Wojciech Wegrzynski:

Fantastic. How does a fleshover in the wind-driven fire look like as a phenomenon?

Speaker 1:

We have like a standard thermocouples of every one foot, uh like a thermocouple tree. Uh, we see the first one as the smoke comes into or like accumulates against the row, it's about a minute. And then if the another window or another door fails during this down because the hot gas is pushed in right, when there is low pressure on one side of the building, so that causes ventilation going out, that prolongs flashover. But for the most part, as we turned on the fans and generated a 30 miles an hour wind, within a minute we saw flashover in the uh in the building.

Wojciech Wegrzynski:

And can you compare that uh to the same experiment without wind or with weaker wind? Does it it I guess it significantly enhances the process?

Speaker 1:

It does, it certainly does, yeah.

Wojciech Wegrzynski:

Okay, uh, so I understand correctly in the the the listeners. In this case, you have to lose multiple openings on multiple facades of the building, I ass I assume, to create some wind path, right?

Speaker 1:

Yeah, that that naturally happens. We don't plan for it. Sometimes it's the window in the bedroom, the other time it's the window in the kitchen.

Wojciech Wegrzynski:

I'm I'm kind of intrigued because I uh like I'm doing some experiments with the real building elements. I'm also having vinyl windows. I don't find them that weak in terms of the response to how did how do your windows fail? Because my windows today mostly fail by losing the gaskets, by damage to the frame. You know, I like when I was uh studying as a fire safety engineer, I was like told that the glass breaks, right? The glass break, glass shatters. I don't know, 250 degrees, the glass will shatter. Well, no, not really, it does not. I like that's my observation. I wonder if if this mode of damage is different in wind, and I wonder how big an element of that damage is wind itself.

Speaker 1:

That's a good question. Uh so we tested these buildings at two different winds, ther about nominally 30 miles an hour and 60 miles an hour.

Wojciech Wegrzynski:

So so for me, 30 miles an hour would be already like 95th, probably 97th percentile of winds in Poland. So that's already extreme wind for me, 30. And 60 probably is a national disaster.

Speaker 1:

So in the recent uh tragedies we had in LA and uh Lahaina, when the speed was in the order of 60 miles an hour. Uh it can get to uh those extreme levels. Yeah. So when we tested at the lower end, which is 30 miles an hour wind, we saw uh non-twopered glass breaking or tempered glass just falling out of its frame. And uh when we tested these windows, which is one uh frame, at 60 miles an hour, the fire source lost its buoyant portion. So the flame was like jetting horizontally to the target building, right? So all that heat that was supposed to get dissipated in the atmosphere is now pushed to the target building that softened the vinyl, and then the wind pressure actually pushed the window in with the glaze. Wow, and that's not so uh some damage modes that you can only see in wind-driven fires or like extreme wind-driven fires. And we did not see that with fiberglass frame windows. So that was something that we this is uh like specifically saw in vinyl. And I know like my colleagues at FSRI are studying windows, and they have been on your podcast before, so uh there's a demographic information in their publication.

Wojciech Wegrzynski:

Yeah, that was very good. Two podcast episodes, you're very welcome to listen. I'll link those in the show notes. I'll write myself down to link to FSRI. That was a good study. I I appreciate good research, I appreciate good fire science. Let's let's get some more uh information uh out of you. In terms of intensity of those fires, were you able to like quantify the the heat release rates or at least the fire durations until a burnout in uh wind, non wind? Does the wind exposure in those fires create more severe conditions in terms of more megawatts or not necessarily?

Speaker 1:

I think so. We put some heat flux pages uh further away from the fire source and with cameras so we can Go back to police source model and estimate heat release rates, right? I find it nearly impossible to estimate heat release rate under these kind of fires, wind-driven fire this size. Because as the structure collapses, the oxygen access and the flame dynamic, everything changes to a level that the uncertainty of these estimation or measurements become so large. But that doesn't necessarily mean that we cannot quantify their impact, right? So we can look at flame lengths to your point. We can look at uh measurements of heat flux radiation on the target building. And during the second phase of these experiments, we saw that we can get around 20% of the total measure key is actually convective cooling, which was something that is very interesting to me. Measured for the first time.

Wojciech Wegrzynski:

And in those experiments, well, I'm pushing it. Do you also uh put external objects around the buildings or like to estimate likelihood of igniting a vehicle parked in front of the building, etc.? You said that they that those elements link the buildings together, so I guess it's important.

Speaker 1:

Yeah, it did is absolutely uh so uh let me take a step back here. Yeah, after tragedy that we saw in the hind up, we visited the island in Hawaii and uh we studied or collected data for a week and then we came back to the lab uh analyzed those. And so we built a machine learning model, like the predictor in perturbs, and then we saw that fire specifically was the textbook condition for exposure overwhelming the resistance side. Because the structure separation was so low, and as people try to evacuate, they jump on one one vehicle, drive away, and then there were a lot of burnt vehicles left behind. That created a connective fuel tank that we saw many cars burned during this tragedy that created the connective fuel. So, in the last experiment that we ran at IBHS, uh with these like large scale fires, we added a hot tub filled with water on a deck and a vehicle and this UV in front of it, right? To see what is the impact. So at the 30-foot separation, uh 30 miles an hour wind, with no connective fuel, resilient material can break the chain of configuration. But when we add those elements, the fire exposure gets to a level that the building materials stop having a finite chance. Meaning the fire overwhelmed all those fancy windows, non-combustible cycling, asphalt shingles, all of those were overwhelmed uh by the fire.

Wojciech Wegrzynski:

So you get the chance to see those uh the damage by real wildfires in a real world, and then you recreate those experimentally in the lab. Do you see a lot of uh reassemblance in the damage patterns? Like, how well are you able to represent those real wildfires in the lab, actually?

Speaker 1:

So, because fire is typically a binary response, so when the building phase, if you get a chance, if an observer or first responder or like a homework evacuated recording something, you get to see the initial phases of admission or like failure of the building. But by the time that we get there, it's just a shed, an empty of the standing walls. So the connection is not easily doable, basically.

Wojciech Wegrzynski:

Okay, yeah, yeah. Well, that that that makes uh makes a lot of sense. It's just uh I I I resonate with what you said at the very beginning of the interview that uh it's so complex, there's so many aspects of it, so many like little imperfection, chaotic elements that that change the outcome that one you have to study it in the full scale, but then I start to wonder like uh how much the full scale is really the real scale with you know shrubs and meadows in front of the buildings and you know your neighbors around and perhaps a tall tree in the neighborhood.

Speaker 1:

Yeah, absolutely. So uh the way that I look at this, or maybe rephrase that I like to look at this. What we test is close enough to what we see in the field, but the conditions might be different, right? So we have a 30-foot separation, the source building of a type, the target building of another. If any of these elements changes, the wind speed, connective fuels, it's structure characteristics, then that adds some uncertainty on it. But it's identical conditions, I would I don't have a reason to say that it's not gonna be the same.

Wojciech Wegrzynski:

And one final one. If if he didn't have the access to the giant wind tunnel, if you only had access to, let's say, small, moderate scale, like normal human resources, how much of your findings would you miss? How how much of your findings do you think are are strictly related to the scale at which is is attested? And do we need a larger scale? That's a good question.

Speaker 1:

I I would love to have a larger uh larger scale uh fire uh that we all can contribute and learn, right? So let me go back to your first question. That's a good one. I would say the small scale knowledge, fantastic knowledge that is like academics, I'm part of it, we can generate that. Only captures a smaller range of the fullest spectrum. Not that they are not valid, they are valid within the boundaries of what they they are designed, right? And when we come to full scales, again, they are valid to the spectrum that we tested, and when we go from full scale to real world, then that spectrum gets wider and wider, right? So all of them are valid, all of them are useful, but they are designed to answer specific questions.

Wojciech Wegrzynski:

I wonder what's closer to an uh like in in terms of answers given, what's closer you to real scale or small scale to you? That's like an intriguing uh concept. Like I I ask because I wonder how much we can learn with small scale and and how much is just you know limited by the scale itself. Like then you cannot work further.

Speaker 1:

I would say none of them are invalid, in my opinion. They are just everything that we as scientists do are valid if they are used in the context that they are designed for, right? Yeah, so um if the small scale condition fire goes up through like the standard fire curve, happens in the field, well, all of those criteria for a small scale is valid. If we are seeing deviation from that, then the full scale uh becomes more important. And I strongly believe that we cannot drop between these scales, even at IBHS, that each of these experiments costs $200,000. Wow. About 25 of my colleagues show up on the test date to run these experiments. So even these, we do not drop into full escape. The first experiment that we ran four years ago was a single wood crate. Then we built on that, then we got to the sheds, then we got to the ADUs, and we will uh we'll uh push it forward. We need to go through that journey, in my opinion.

Wojciech Wegrzynski:

That's very reassuring because it also allows every everyone in the community to uh to eventually contribute to to be able to make a meaningful uh jump into the largest scales, and then we'll see what what what happens. Um Faraz, uh, we're at the end, but perhaps it's a good moment to advertise for the awesomeness of content that IBHS website provides. So I see that your institution not only does uh amazing fire science but also uh releases a lot of useful resources out there. So maybe let's talk about that.

Speaker 1:

So we are funded by the insurance industry. So some of those uh scientific reports are behind the paywall, but a lot of the information about the fire, because it's life safety, they are not behind the paywall. They people can easily access those. And if not, same thing at email. I'm sure I can get the permission to share the uh the information with you.

Wojciech Wegrzynski:

Yeah, fantastic. I will uh link the website itself in the show notes. I'll provide you the direct links to the wildfire uh reports that appear to be open access, uh, so you can immediately uh jump to those. And I I will be looking out for uh more for papers and and uh reports. Faras, uh well, thank you so much for for joining me in the Fire Science show, and thank you for for telling me all about this uh beautiful, beautiful facility and and research that you're doing there. I've like I've been reading and and and observing it a lot. I'm really glad that I finally got to discuss this.

Speaker 1:

Same here. Yeah, I've been following your uh podcast for some time now, and it's great to be a part of it now.

Wojciech Wegrzynski:

Thank you. Thank you so much. And that's it. Thank you for listening. One thing that comes out of this episode there's no silver bullet for fire safety in the world wild and urban interface. This is uh very interesting. Different spectrum of amber sizes will attack different parts of the building. There are pathways for the fire spread. There are different pathways from which the building can ignite. And actually, when there is no fire responders, when there is no one to take down those little fire starters, they will spread into the building and eventually devour it. This is the horrible lesson from the large urban conflagrations. And uh yeah, we can reduce it perhaps, we can improve the situation, but it seems very difficult to prevent. From the full-scale research, like the one carried at IBHS, we learn those mechanisms, and as we understand the mechanisms, as we understand, you know, the criteria at which things fail, as we understand when the things are better, when they are worse, which makes them and which variables are the most important, most influential in the process. That's when we can discover and design new safety tools that actually improve our chances against those horrible, horrible natural disasters, which wildfires and wildland urban interface fires certainly are. Um, regarding the facility, I'm very thankful for uh for us to give me an insights about this magnificent facility. It really is like if you if you are anywhere interested about wind and fire, you must have known the facility because it's it's just a landmark, landmark piece of scientific infrastructure. When I am in the US, I have to go there. I've just been invited, so I have a chance, we'll see. But I would love to see that facility in in real life. I mean, even if you don't do fires just doing this facility is absolutely legendary, and and uh props to IBHS for investing so much money and building it at such a grand scale. I'm sure it uh will pay off for years for giving insights in the skills that we did not had access to with the variable control that that is good for experiments, that is representative for experiments, because that's that's a chance, not a chance to do a very large fire experiment outdoors in whatever wind you find. But if you want to do this experiment in three different winds, well you may have some waiting for you be if you want to have them occur naturally in here, you can control that. That is the major difference, that's the biggest change that that actually changes everything. Anyway, enough of rambling already. Thank you very much for being here with us in this fire science episode. And next Wednesday I will bring you some more fire science. So see you there, same place, same time. Cheers, bye.