Cob Wall Fire Testing (ASTM E119 test), and Thoughts on Fire Resilient Natural Building
Recap on the ASTM E119 fire test we did several years back. There have been so many questions in the last few days, and of course a big upsurge of interest in rebuilding with less flammable materials. Many people are asking the very pertinent question right now: How do we rebuild in a way that is more fire resistant and less likely to burn? What are the ways we can do this that are not so extractive in the building process, and can we build buildings that if they do burn, do not emit so many toxins in the process?
Context:
I think it is important to contextualize these ideas with the knowing that what we have come to think of as normal, building homes out of kindling and chemicals, has been ‘normal’ for a very short period of time in history. Not that many generations ago, everyone built houses primarily from materials found on the site they were building, or very closeby. The industrial revolution brought transportation of materials all over the world, as well as the manufacturing and processing of materials of all kinds. Many of these developments and manufactured products are incorporated into the buildings we build, but it’s important to remember that buildings can be constructed primarily from local materials that have been very minimally processed. Even today, earth remains the most common building material on the planet. There are many ways of using earthen materials to create wall systems that are safe from fires and earthquakes, and a variety of methods can be found in most countries.
In the past few decades, there has been a revival of cob building (also known as monolithic adobe). It is one of that many options we have of building walls out of earth. It is similar to adobe in materials but differs in the building process. Rather than making bricks and then stacking them as in adobe, you build with the material in its wet form, creating one monolithic structure. (for an overview of different types of natural building methods see blog post titled What is Natural Building?)
An example of a open fireplace built out of cob.
We know that earth does not burn.
It has been used for centuries to build ovens and fireplaces, as well as the fact that common sense tells us that earth does not burn. There are examples all over the world of using earthen building techniques in fire prone areas to reduce risk from fire. In Japan there is a very old traditional building style called a Kura, It is a non-combustable storehouse that is built usually built out of earth, and used to store ones valuables. These structures are built in areas where the houses are primarily built out of wood, and they store valuables and food crops. This concept could also serve an emergency fire bunker if someone was unable to escape the fire.
This photo depicts ‘Kuras’, Japenese store houses built of earth. They are the only buildings left standing after the Great Kita Fire, in Osaka in 1909
Official ASTM E119 Fire Test
Why go to the trouble to official testing when we know that earth is non-combustable? There are particular situations in our built environment that codes require a technical ASTM E119 fire rating for the material to be used. But I would say almost more importantly, doing the official test also gives the industry accepted language with which to talk about these materials. It makes it official.
After the seismic testing we did in 2019, we used one of the walls that was still standing to do our own R and D and performed an unofficial fire test on one of the cob walls to explore how cob will do in an official fire test. After four hours of burning a 2000 F degree fire against one side of a 12” thick wall, the opposite side never exceeded 72 F degrees!
We were ready to go for it.
In May of 2021, my partner John and I went to Texas and built 2 cob walls inside of a fire testing facility. (With A LOT of help, collaboration, design and engineering from many individuals, all listed at the bottom) The walls needed to dry for a full 6 months before we were able to perform the test. The process of the test is to blast the wall with high heat on one side, then rotate the wall with a crane, and blast it with a fire hose on the same side that it was exposed to the heat. Although we had confidence they would do well with the heat exposure aspect of the test, we were more nervous about how they would do with the 2.5 minutes of fire hose stream, which can’t create a penetration in the wall in order for the test to be considered a success. The first wall didn't even come close to the hose stream penetrating the wall, and the second wall, which was a lot thinner, passed by a tighter margin.
This is one of the two walls we built at a facility in Texas that was then tested for an ASTM E119 fire rating.
While one side of the wall was heated to 1800 degrees for 2 hours, the cool side of the wall didn't change temperature at all. This test was done 'loaded' to mimic a real building, meaning it had a load baring down on it, mimicking the weight of a roof. The weight on these walls had the weight of 2 SUV's on top of it. While most residential building scenarios do not need this rating, there are some that do, and more importantly, we now have official data of how well these materials do in fire situations.
This is the same wall after being exposed to fire on the back side for 2 hours. You can see that this side of the wall showed no sign of damage, and only changed temperature by a few degrees on the front side!
In an actual wildfire situation, the wall would not actually be on fire to warrant a fire hose blasting it for several minutes, there would be nothing to be burning. This is another example of how the modern testing does not always fit, work, or make sense with these materials. In our unofficial test, we found that the hotter the fire had been against a section of the wall, the better it held up to the fire hose, which of course is opposite of almost any other material.
These materials are inherently free or very cheap, locally accessible, and these walls can be built by people without a huge amount of expertise. This also makes testing them quite difficult. How do you fund testing a material that no one will profit off of? How you do streamline a material when the objective is to make it with materials found locally? Most testing of building materials is performed and paid for by the industry that hopes to make a profit.
What happens when an earthen house goes through a fire?
I will start by saying this is not my specific area of expertise. There are a lot of people who can speak to this more accurately and eloquently that I can, but there are a few points that feel important to include here. I have spent a lot of time touring and looking at earthen buildings that survive firestorms, and ones that did not.
There are many cob, earthbag and strawbale houses that have been ruined and made unusable from fire. There are also plenty examples of these buildings that have withstood fire when everything else around burned. It is easy to over simplify things and simply say earth does not burn. But how any house performs in a fire is dependent on so many factors and so many details. Using non-combustable wall systems is a really great start, and I wish it were just that simple. Some other factors that we need to look closely at are the windows, eves, overhangs, crawl spaces, and any other potential penetrations that heat, sparks or fire can enter through, not to mention the vegetation surrounding the structure. Some of these details might be more important than the wall system itself.
What happens when other buildings burn?
One of the things we dont talk about very much is the level of toxicity of buildings when they burn. The smoke from urban fires is very different than the smoke from wild fires. When we were building the walls at the facility in Texas there were several times we had to leave the premises due to the walls they were testing being so toxic that even with our N95 masks on, it was too toxic to breath the air. The health effects from building materials burning, both short and long term, are not taken into consideration at all. The way that buildings are currently built are also creating a massive amount of kindling to help our neighbors house catch on fire.
What is the best Natural Building Option for Rebuilding in LA?
ALL OF THEM! There is no ‘best’ way. There are pros and cons for all of the options, and like all good design, is based on a variety of factors that are generally site and project specific. If we have any hope of rebuilding even the smallest percentage of the buidlings that have been lost with builidngs that are less toxic and more natural we will need to employ ALL the techniques, all the builders, and all the knowledge that is out there. Cob, rammed earth, Adobe, strawbale, hempcrete, compressed earth block, super adobe, to name of few, are all great options. Everyone of these wall systems can be built in ways the structure would be totally destroyed after a fire, and built to survive most fires. We have such a small labor pool for any of these systems, its crazy to think about promoting only one, or promoting one to the be the ‘best’ option. Yes to all of them.
We hope one day that housing can once again be recognized as a basic human right, and that building a home for oneself and one’s family will be accessible to all.
Resources/ further reading:
Article looking at houses that survived the north bay fires”
Article from Arkin Tilt Architects evaluating some buildings that survived a fire:
https://www.arkintilt.com/strawbale-and-earth-projects-survive-north-bay-wildfires
Webinar with David Arkin:
https://www.finehomebuilding.com/2021/07/29/building-homes-to-survive-wildfires
This video is a great breakdown for understanding details:
https://www.youtube.com/watch?v=YQzJUsUO-5c
Article by Hubble and Hubble looking at Strawbale Buildings that survived wildfires:
https://hubbellandhubbell.com/2023/07/07/fighting-fire-with-earth-and-straw/
Wealth of knowledge and ideas from Art Ludwig about fire:
https://oasisdesign.net/shelter/fire/
ASTM E119 fire testing team:
Thank you to everyone who donated for the building and testing of these walls. And a huge thank you to Daiva Trudeau and Jess Shockley for joining our crew at a moment’s notice. What a dream team of natural builders! We’re all so grateful for our structural engineer Anthony Dente of Verdant Structural Engineers, our fire engineer Nicholas Bartlett and our all around everything else partner Art Ludwig of Oasis Design (without Art, we wouldn’t even be here!).