Fighting Fire with Earth and Straw: Drew Hubbell and Anthony Dente

Written by Simone Butler, www.astroalchemy.com

In recent years, California has been ablaze with wildfires. Though this past year’s heavy rains have helped a bit, in 2021 monstrous fires burned almost 2.6 million acres and damaged or destroyed 3,629 structures. The 2018 Camp Fire, the costliest U.S. wildfire of all time at $10 billion, destroyed 18, 804 structures and 153,335 woodland acres.

Clearly, something must be done. Dramatically reducing timber construction comes to mind. And how about building more dwellings out of earth and straw? This solution might seem unlikely to some. But architect and natural builder Drew Hubbell of San Diego-based Hubbell and Hubbell Architects asserts that straw bale construction (using bales as insulation) is incredibly fire resistive.

“It’s actually one of the most sustainable green building methods you can use,” Hubbell said in a talk last February at the Wildfire Resilient Structures (WiReS) Conference hosted by UC Davis in San Diego. “The new way of doing things in our minds is straw bale construction with earthen plaster and metal roofs.”

To illustrate his point, Hubbell showed pictures of a dramatic survival story from the 2017 Tubbs Fire in Mendocino County, in which a fire came right up to a strawbale home and stopped in its tracks; the bale walls survived, almost untouched. A shot from the 2018 Carr fire in Mt. Shasta showed another bale house surrounded by smoke. “They hadn’t cleared the brush very much,” Hubbell explained, “so the fire came right up to the building and burned against the strawbale wall. It did discolor the stucco, but didn’t harm it. One of the barn doors was pretty singed, but the plaster and the strawbales survived.”

Hubbell explained that the two-hour fire rating for strawbale walls is twice that of wood frame construction. This is because strawbale construction creates thicker walls, typically two feet, which provide thermal mass for natural heating and cooling as well as do a good job of keeping flames at bay.  Also, flammable timber content is reduced in strawbale construction. “Our designs use about 50 percent less lumber than a stick-frame house,” said Hubbell.

Strawbale construction had a big surge of interest in the 1990s and early 2000s. But due to a combination of permitting issues, the cumbersome nature of building with bales and the fact that they take up a lot of square footage, progress has been slow. Does Hubbell think that California’s wildfire dilemma will renew interest in building with earth and straw? “Strawbale is a wonderful building material,” he replied, “but it is still a bit of a fringe system. I think the building industry has a lot more power. But we’re slowly chipping away; there’s over a thousand structures in California. There are strawbale buildings in every state, and it has also become popular in Europe and other parts of the world.”

Also at WiReS, Berkeley-based engineer Anthony Dente of Verdant Structural Engineers shared his thoughts on cob building, a traditional method in which clay, sand and straw are mixed by hand into cob loaves, then stacked to create walls. The result is often a whimsical “Hobbit House” dwelling, though more modern versions are evolving. Cob, he explained, is an historic construction technique, also called monolithic adobe, that can be found on all continents except Antarctica.

As VP of the Cob Research Institute for the last ten years, Dente has been trying to facilitate more legally built cob structures, which – like strawbale buildings – are fire resistant. Along with Art Ludwig of Oasis Design and others, Dente demonstrated this on two wall systems weighing 5000 pounds each, by heating the inside to 2000 degrees. And, he reports, the outside temperature changed less than five degrees. The foot-thick walls performed well under the ASTM E119 Fire Rating Tests, and received a two-hour rating. But Dente believes the wall system could do even better. “If anybody has funding for that,” he said, “we’d love to talk to you!” (contact Anthony@verdantstructural.com).

“This type of construction,” he adds, “is very safe in fire regions when you build the right way. Especially for those who have gone through the traumatic event of living through a fire, they are really reaching toward non-combustible materials.” Cob, he believes, can fill the bill. “It absorbs heat from a fire, and slows the heat’s progression down through the wall, since it’s both a mass and an insulating system.”

Dente noted that there are researchers at MIT experimenting with faster mechanization of cob building. “This type of construction needs to be made more convenient,” he said, “and we are working on that.” His latest project is the Verdant Panel, a carbon-storing, straw-based Structural Insulated Panel with a standard size of 4’x8’x6’. This project also needs funding for tests. To learn more about cob, check out Dente’s new book, Cob Construction, due out in 2024 from New Society Publishing, and now available for pre-order.

To join the mailing list at UC Davis Climate Adaptation Research Center and be notified of upcoming events like WiReS, visit: https://climateadaptation.ucdavis.edu/contact-us.