Cob Greenhouse

Cob is magical. 

Dirt plus straw plus water hardens into a mass which can bear significant load, and lasts for years – provided you keep it dry. 

I was first exposed to Cob when I was spending half a year at Pendle Hill, a Quaker retreat center in Pennsylvania. I had taken a bit of time off from college to regroup, find my spiritual center, and awaken my spirituality. As it turns out, I’m an atheist with little patience for sitting around; the self-discoveries of youth can be equal parts profound and confounding. In search of things to keep my mind off my mind, I stumbled upon a nascent project to build a greenhouse out of sustainable and local materials, including trees from the property, large stones, and the aforementioned cob.  The trees were stripped of bark, and sealed with linseed oil (which, as it turns out, isn’t sufficient; they eventually fell victim to termites). The stones were horribly heavy (surprise!) and took amazingly long to assemble into a workable foundation. But cob – oh cob! – was a great discovery. 

To make cob, we did the following:

  • Spread a tarp on the ground, ideally in a slight depression
  • Shovel a few scoops of dirt onto the tarp
  • Throw a few handfuls of straw onto the dirt
  • Add water (start with a bit, add a bit more during the next step)
  • Step and step and step on the mess until the straw is well-mixed in, and it has the consistency of jelly
  • Scootch the tarp over to the area you’re working on
  • Lay out a layer of cob; no more than 1″ thick, and at least 12-18″ wide
  • Push your finger into the wet cob every few inches to leave holes for the next layer to lock into (think: Lego bricks)
  • Wait for the cob to dry (1-3 days, depending on the weather) before applying the next layer. If it rains, be sure to cover your cob!

It’s slow, steady, muddy work. But slowly, walls emerge. Not the perfectly flat grid-walls of modern times; these are curvy, organic, emergent forms.  Surprisingly, cob hardens to an almost concrete-like form; sure, you can rub dirt off the walls, but it’s resilient and solid.

Years later, we had a house. And a boring lawn. But no greenhouse.

It was time to build a cob greenhouse of our own. 

I started my measuring out a 9′ by 14′ area. 

photo 1

I took a photo, and (crudely) used Google SketchUp to make a 3-D rough model of my vision. 

  • Brick foundation, from 3″ (one bricks) below ground level to 12″ above ground level
  • Single sloped roof, from about 2 feet above the ground on the south wall to 7 feet above ground on the north (biggest) wall. The roof is made from clear plastic. 
  • There would be a door on the north wall. This also requires less cob.
  • We would insert bottles into the cob to provide light, color, and take up a bit of room for less cob-work
  • Include windows on the East and West walls (maybe the North Wall too). 

greenhouse-mockup (1)

We were ready to dig out the foundation! This would both put the walls a bit underground, but – as it turns out, more importantly! – gave us a few mounds of dirt to turn into cob. 

We had no idea what we were in for. 

photo 3

photo 3 (1)

Digging is easy. 

Bricks suck.

I mean, they look great. But even the smallest wall has a TON of bricks. And while I could’ve started with new bricks, I wanted to save money and have nice-looking, worn, interesting bricks. So I started bargain-hunting on Craigslist for bricks, hauling load after load to the house. All things told, I certainly didn’t save money compared to just ordering a delivery of new bricks – and it took a lot of time and sore muscles. But finally, we had enough bricks. 

photo 4 (1)

We laid the first layer of bricks out, and pounded rebar in-between, so it would stick up high into the wall. From there, we mortared layer after layer of brick to make a foundation, leaving an opening for the door. 

Sadly, I have hardly any photos of the building process. 

I had completely underestimated just how much dirt we would need; we quickly emptied our yard of its hills, and introduced new depressions. Towards the end, we became perhaps overzealous in our use of recycling materials for construction purposes, in order to take up room. Several hated hawtorne tree stumps made their way into the building, too. 

From the nearby, wonderful re-purposer of materials, Re-Store, I was able to get recycled lumber, and a door. Lisa rescued a stained glass mirror from the trash which was also incorporated. 


We put on the finishing touches, and attached the roof plastic. 


And then… we were done! Not too far from the original mockup, am I right?


Cob also has tremendous thermal mass. As we learned, the greenhouse would get murderously hot in the summer under almost kind of sun. I installed an automatic vent which would lift a big section of the roof when it got hot; the vent was almost always open. (Automatic vents are neat, by the way; a cylinder of stuff (wax?) expands under heat, and pushes open a window just based on the pressure in the cylinder). Even so, it was too warm; the greenhouse became useful for extending the growing season in the spring through late in the fall by moving plants in-and-out.  

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