Prof. Michael Gollner, one of the most prominent researchers in Wildfire Science, and the head of the Fire Research Lab at UC Berkeley, recently wrote this letter:
Thanks for sharing this [the flyer mentioned here]. A lot of information below, hopefully you can use this in discussion with neighbors and other residents.
I haven’t seen this letter, but I’ve been working in this field for 15 years and involved with what IBHS has been promoting to the committee, including organizing several letters of support, and know some committee members and the long history in this process. It’s a little disturbing reading this letter as it reminds me of expert witness work - they’ll selectively pick anything they can to promote their stance out of papers, ignoring those points that don’t support their views. They also completely tried to tear apart our new manuscript without actually understanding the issues. I ran into the same issues when testifying to Congress, this appears to be a very political interpretation of the literature.
First, it’s important to understand the scientific role of defensible space and home hardening. Homes can be exposed to large radiant heating from the wildfire itself or neighboring structures burning, direct flame contact from small flames adjacent to the structure, and flying embers that land and ignite new fires. The concept of protecting structures is to create a defensible space around the home that reduces exposure from the FIRE (both radiant and direct flame contact) to the home, and to make the outside “shell” of the home itself resistant to EMBERS and small flames. Defensible space therefore must accomplish moving flammable materials that can result in flames away from the structure. Hardening incorporates finer mesh over vents to prevent ember intrusion as well as fire resistant materials (and construction methods) to prevent embers from igniting outside materials. The issue is that even small flames, 1 foot tall can penetrate building materials that otherwise are fire resistant if left long enough. Heat fluxes from small sustained flames can far exceed ignition properties of most otherwise fire-resistant materials, therefore there is a need to support hardening on a home by moving any possible flames back some distance, preventing another “pathway” to ignition. The 5-foot zone “0” helps accomplish this.
Defensible space guidelines aren’t to create completely barren landscapes. Within 30 feet there is room for “islands” of vegetation and other materials, however the 5 feet within the home has been highlighted to be clear of any flammable materials to prevent the intrusion of sustained flames to structures. I know it’s a broad stroke, saying to remove everything flammable, but to take an alternative path would be incredibly complicated and have a lot to do with what materials a structure is made with, what wind and direction it might come from, the location of neighbors, and more details than we can possibly specify and understand. Keeping this small area as a “safe zone” around the structure will help give any hardening applied a real chance to actually survive the onslaught and help prevent more destruction. Because many houses are located so close to one another your neighbor’s house is also a potential source so protecting entire communities is critical.
It’s hard to make a better visual than IBHS’ full scale demonstrations: https://www.youtube.com/watch?v=AYvwogREEk4
We’ve been burning houses (7 now) on structure to structure experiments and we’re learning a lot. I do believe the 5-foot zone is an integral part of the solution.
Regarding the criticism of our study (under revision in Nature Comm), what we did is really incorporate the interplay between variables using a machine learning method. Most previous studies did not do this. It becomes apparent when we look at the end that you need BOTH defensible space and hardening to make a substantial difference in losses, but there is SOME reduction from either independently. Of course, home spacing is one of the most important, but we know this can’t be changed. I think whoever wrote this letter has no idea what fire modeling is or what the different techniques used were doing, or purposefully avoided stating the facts. The moisture conditions were set to match weather conditions input from NOAA weather data, which is standard fire modeling practice, and we don’t use the fire modeling to influence the structure loss information, we use the fire modeling as an input of EXPOSURE (fire and embers), missing from all previous studies. Most previous studies also looked at 30-foot defensible space, not 5 feet. We had access to high resolution imagery and Lidar that allowed us to look closer at 5 feet. It’s not perfect, you’d need pre-fire on the ground studies, but there is a strong and clear relationship between zone 0 and reduced destruction, especially combined with home hardening. That also makes physical sense, we expect zone 0 to be most influential in reducing those local exposures and making hardening more effective.
I know Alexandra Syphard and her stance on the trees blocking embers. There is no field or test evidence for that anywhere. There is compelling evidence from investigations that local fuels that are connected leading to a structure help drive the fire to the structure and lead to ignition. I think all scientists agree you need flammable materials out of the 5-foot zone, they are debating on what is “flammable” and that sort of depends on whether you are talking about ignition from embers, a small fire creeping up, or a massive flame from a neighbor’s house that lights your bush on fire that then lights your house on fire.
Sorry there is a lot of information here, but you are welcome to share this with neighbors. I don’t like how clear cut it has to be, but I understand how difficult any regulation in practice will be. A bush honestly is a bad idea - flammability, it’s a lot of fuel if it dries out, litters, how is it maintained, etc. Mulch - we tested it, it is all pretty bad that close to a house. A little potted petunia, probably fine, but how to distinguish how large is not, or if the flower pot is flammable. Do we create a flower pot certification process? It’s really hard. So, I support a straightforward plan that’s going to lower risk in high risk areas.
Michael J. Gollner, Ph.D.
Associate Professor and Deb Faculty Fellow
Vice Chair for Graduate Studies
Department of Mechanical Engineering
University of California, Berkeley