When Nikolai Woolf, founder of the Colorado Cob Company, wanted to build a mini, Earthship-style greenhouse in his backyard, he started by digging a really big hole.
"I told my neighbors that my pet elephant had died," he jokes.
The excavated dirt would later come in handy as a key construction material, even if it was overshadowed by the 110 old tires that accounted for much of the rammed-earth thermal mass of the partially subterranean, 12-by-10-foot structure.
Thermal mass refers to how a structure can naturally store the heat of a day through sun exposure, then slowly release that heat by night as part of an eco-friendly, passive-solar design. In this case it was boosted by the tires and the earthen floor and walls Woolf constructed.
Building with earth is typically called "cob" building, and the word is of apparent unknown origin, but with suspected roots in Old English. Today it's taken to mean something to the effect of "heap" or "lump," as in what an earthen adobe brick looks like — not far from a bread loaf.
And Woolf isn't just a casual enthusiast of the building method; he's the guy with the Westfalia camper van with a bumper sticker that reads "Treehugging Dirt Worshipper."
In building his community cob platform (coloradocob.org), Woolf says, he's aiming to "create opportunities for people to get dirty and learn. It's my purpose." CCC isn't currently monetized, aside from modest workshop donations that go to what he considers his classroom and farm fund.
Having grown up "an Air Force brat" and studied drama at the University of North Carolina School of the Arts, the now-37-year-old father of two (with a third on the way, plus chickens, goats and bees among his flock) had spent seven years as a tile worker and handyman around the Jackson Hole, Wyoming, area. He moved here in 2003, later discovering cob after a naturopath suspected that he might be suffering from heavy metal toxicity after significant exposure to conventional building materials, including cement.
He's now a self-employed editor and investigative analyst by day. But he spends much time in his west side backyard, which hosts not only the earthen greenhouse, but also a custom cob chicken coop and a cob pizza oven. Into the oven's mud mix, he incorporated perlite, a volcanic glass, to hold hundreds of degrees of heat inside, enabling him to "cook your dinner out of the dirt."
Before we get into the basic how-to of cob construction — it's remarkably easy — I should note that unsurprisingly, whole books are devoted to the subject, and Woolf has suggested a reading list below.
For context, keep in mind that to us in the developed world, cob's largely a sustainability darling. Treehugger recently profiled the Kräuterzentrum, Europe's largest rammed-earth building and the new herb-processing building for Ricola (yes, the yodeling one) designed by the internationally renowned Swiss architecture group Herzog & de Meuron.
The rest of the world, however, has built with earth for roughly the last 10,000 years. A recent NPR feature highlighted a Palestinian architecture firm that specializes in packed-earth building in Jericho — reportedly the site of the oldest earthen-structure remains, dating back to 8,000 B.C. A UC-Berkeley professor quoted in the story noted how "Half the world's population lives, works, worships or keeps animals in structures built of earth."
As for compelling reasons to choose cob over concrete, The Earth Institute at Columbia University published an article in 2012 citing that the production of cement (concrete's primary ingredient) alone accounts for 5 percent of global carbon dioxide emissions. Producing 1 ton of the ubiquitous big-city substance requires the equivalent of 400 pounds of coal, and releases a ton of CO2.
With cob, you dig through your topsoil into your substrate for clay. From there, you then nab some straw, sand and, potentially, manure (an option for plastering). Your basic cob assembly (for an adobe-style brick or "poured adobe" solid wall or earthen floor) consists simply of an ideal ratio of roughly 20 percent clay to 80 percent aggregate (sand, silt and gravel to stabilize the clay, which expands and contracts). It gets mixed with a little water and fibrous material — that straw, typically — that absorbs moisture, insulates a bit and acts as "natural rebar," according to Building With Cob: A Step by Step Guide.
A simple "shake test" in a glass jar can measure the clay content of your soil, as can a "sausage test" of vigorously rolling soil between your palms to form a turd-like mass. (No manure needed.)
Once the above is in place, ingredients like lime, olive oil, beeswax and shea butter can be used for weatherproofing external features. And natural pigments can create pretty color finishes.
After touring Woolf's backyard, I try and help fulfill his purpose in life by requesting to get dirty. We settle on an experiment on the south face of my own house, where hail recently chipped paint off my stucco finish and where I'm building an Earthship-inspired greenhouse. I'd been considering cob for the mortar of what's to become a bottle wall, in part, and, after meeting Woolf, began viewing it as a plaster option as well to fix my paint problem.
Over the course of a few hours on three separate weekends, we tested a few dirt mixtures, and created some basic adobe bricks strictly for my learning. The challenge, as Woolf saw it, was "how to make dirt stick to [peeling] paint."
Our first mix consisted of "dung-clay," using a five-gallon bucket each of fresh horse manure, clay that we'd dug and sifted from my soil, and fine-grain, locally purchased mason sand. After letting it "fester" for a week in a mortar-mixing tub filled with water, we applied it about a half-inch thick with an old tile-grout float.
Version two incorporated a wheat-flour paste, and version three a rice-flour paste. Each was meant "to give the earth a bit more stickiness," and was made easily in my kitchen by incorporating cold, then hot, water.
The wheat paste, added directly to our soil blend, "diluted it more than strengthened it," in Woolf's estimation. For the rice, he poured the broth in a paint tray, added some sand, and with a paint roller, applied it to an area of wall that'd we'd chipped more paint away from, as a primer that we let dry before dragging our trowels across with our mud mix.
A week later, through heavy rains, all our tests "remained solidly adhered to the painted stucco," but the rice and basic dung clay outperformed the wheat, which was more easily gouged by a pronged garden tool. That same tool hardly affected the other two plasters, which also resisted our fists pounding against them and our nails scratching at the sides.
"The magical and mysterious enzymatic reaction that took place between the festering manure and clay seems to perform just as well on painted stucco as it does on cob itself," says Woolf. "All in all, the experiment was a tremendous success and the future holds promise for more horse shit to be lathered onto that stucco wall."
And for my planned wall and earthen floor, I took heart in hearing that my soil is what cobbers call "ready mix." I'd kicked off my shoes and stomped around on a tarp with my soil, a little water, and long strands of straw, for 10 minutes or so, until I produced a "burrito" (the equivalent of our mini test "sausage"). That got shaped into test bricks by hand, and a large softball-sized orb that I proudly carried to work to amuse colleagues.
Them: "Did you just put a ball of shit in my hand?"
Me: "No. It's cob!"
All together: proper nods of respect for the utterly simple yet amazingly strong and worldly, natural material.
Nikolai Woolf's recommended readings
The Hand-Sculpted House, by Ianto Evans, Michael Smith and Linda Smiley (Cob Cottage Company)
The Cob Builders Handbook, by Becky Bee
Building With Cob: A Step-by-Step Guide, by Adam Weismann and Katy Bryce
Clay Culture: Plasters, Paints and Preservation, by Carole Crews
Rocket Mass Heaters, by Ianto Evans and Leslie Jackson
Build Your Own Earth Oven, by Kiko Denzer
Build Your Own Barrel Oven, by Max and Eva Edleson