Straw bale construction makes a novice builder’s greenhouse affordable, eco-friendly and easy.
In a cold climate, the north wall of any greenhouse must be extremely well insulated, which makes straw bale construction an excellent choice. Straw bale walls have a minimum R-value of 38, twice that of an insulated stick-built wall, which averages R-19. Even in winter, Suzy is able to grow spinach, chard, herbs, and Romaine and butter lettuces in the higher beds set against the straw bale walls.
Suzy McCleary is a farmer, not a carpenter. But when her second greenhouse collapsed underneath the heavy snows of Colorado’s southwestern mountains five years ago, Suzy took matters into her own hands. Her first greenhouse had been made of rebar and plastic sheets. The second had been PVC pipe and thicker plastic. She wasn’t going down that road again.
“I read about straw bale construction and loved the whole idea,” she says. “It seemed like a way I could build, plus it’s environmentally friendly.' With a borrowed book, and a little help from her friends, Suzy built her own straw bale greenhouse. That was five years ago. “This time I used the tough stuff,” she says. “This greenhouse will be there for good.”
Before she started construction, Suzy did her homework. Then she asked a carpenter friend to help build a post-and-beam frame. Together they designed her straightforward structure: a lean-to with two side doors; a tall, solid north wall; a steeply pitched windowed roof; and a short south wall made of windows. The greenhouse’s size—sixteen-by-thirty (480 square feet), with a four-foot high front wall and a nine-foot high back wall—was dictated by the site, which includes an irrigation ditch to the east and trees to the west.
For the frame, Suzy bought locally milled rough-sawn pine and fir. She and her friend poured concrete for each corner support and for the middle supports, spaced eight feet apart. They placed vertical pieces of rebar to protrude one foot above the ground to stabilize the straw bale walls.
For a foundation, Suzy used sandbags. “It was cheaper than concrete,” she says. She acknowledges that in wetter climates, this may not be a good idea because the water may seep through the sand, get into the straw, and cause it to rot.
Suzy bought the straw bales—fifty-pound two-string bales for two bucks apiece—just across the border in New Mexico. Peg helped her stack them between the posts and onto the rebar on the long north wall and the two side walls. They pounded more rebar through every fourth bale for reinforcement.
Once the bales were stacked, Suzy attached chicken wire to all sides of the straw walls with U-shaped wire. Then she consulted a book for a plaster recipe. She needed a water-resistant surface because she’s constantly watering the greenhouse plants, as well as using a hose on the walls and ceiling to keep things cool. So she chose a recipe that includes cement.
After mixing the plaster ingredients in a cement mixer, Suzy and a friend tried to apply it to the walls with cement paddles. “That didn’t work for the first coat,” she says. “We ended up pushing it into the straw with our hands.” For the second and third coat, though, the paddles worked fine.
When applying the third and final coat, Suzy added a terra cotta plaster color to the gray cement mix. The result is a warm buckskin adobe hue. Suzy and two friends decorated the plaster with handprints, beads, and spiral designs.
Suzy finished off the non-straw bale wall and the ceiling with double-layer sheets of polycarbonate, available through greenhouse suppliers. This lightweight material is impervious to hail, filters out harmful rays, and has thermal properties perfect for greenhouses. This she attached to the frame with roof screws.
The floor was originally dirt, but ultimately it got too muddy from watering, so Suzy installed flagstones. Beneath the flagstone, she planted a thick layer of Styrofoam peanuts. “I had saved big bags of them,” she says. “I had to get rid of them somehow, and I thought they would be good insulation. They settled, and it seems to work well.”
Inside, Suzy and several friends built rounded raised beds using river rock and cement. The dirt in these beds adds mass and helps keep the room warm. Wire shelves line the north wall and provide maximum ventilation for the plants that sit on them. There’s no electricity and no plumbing, which greatly simplified the building process. Suzy brings in a hose in the summertime. In winter she dips her watering can into a fifty-five-gallon water barrel that she stores inside the greenhouse.
Other practical touches include screen doors that keep out the birds but let wind blow through and a shade cloth for the windowed roof to protect the greenhouse from overheating in the summer. For ventilation, Suzy installed operable windows at the top of the north wall and the bottom of the south wall.
Suzy completed the green house for $1,500, about $3.13 per square foot. It was built in just less than two weeks of five-hour days. “I had to do it in a hurry,” she says, “because spring was coming on fast.”
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