Cob greenhouse warms plants as well as hearts


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As she picks lettuce and other greens, Maureen Kinevey-Gump is bathed in low winter sunlight streaming through the windows of her passive greenhouse.

The building is constructed mostly of recycled materials, and three of the four walls are made of something called cob. Heated only by the sun, the greenhouse never dipped below temperatures of 26 degrees this winter.

Cob is a combination of clay, straw, builder's sand and water. Although it might seem like an innovative new building material, it's been around for hundreds of years. There are cob structures in England dating from the 13th century.

Mrs. Kinevey-Gump, 57, always wanted a greenhouse and discussed possibilities with her husband, Jeff Gump, who does high-end historic restoration and construction. They considered a hoop house or something attached to their Avella farmhouse in Washington County. But the couple has always been interested in sustainability and were looking for something that could grow plants year-round with only solar power.

When their 22-year-old daughter, Elizabeth, and her significant other, Sam Clary, pitched them cob as a sturdy and environmentally sound material for the walls, the two were sold. Mr. Gump, 61, has seen his share of construction over the years and wanted to pioneer something different.

"We've been doing the same housing for the last 150 years, and it's wrong," he said. "I figured if it works from an engineering standpoint in Western Pennsylvania it will work anywhere."

Instead of a concrete footer, he used tamped gravel as a foundation to provide the drainage cob needs. Cinder block was installed over the gravel, and then the process of making the cob began.

Excavated fill from the site was used to build the walls. First it was tested for clay content. The couple found out what all Western Pennsylvania gardeners already know: The soil was heavy with clay, which is a good thing for mixing cob.

Next, test blocks were made combining the four ingredients until they were satisfied with the consistency of the building material.

"Once we got the mixture, it seemed like it wasn't too hard to tell what really worked," Miss Kinevey-Gump said.

Then it was time get their feet dirty -- that's right, feet. Mixing the cob is a barefoot construction job.

"It reminded of when I was really little and running around in the rain and mud. I was shocked at how smooth it was. It felt like you went to spa," Miss Kinevey-Gump said.

The clay and sand are put on a large tarp, then straw and water are added as needed. When the mix is ready, the tarp is used for leverage to roll the mixture into logs that weigh around 120 pounds each; six of them were made at a time and were cut into 5- to 10-pound loaves.

Miss Kinevey-Gump tossed the loaves to Mr. Clary, who slapped them down and then used a tool called a cobber's thumb to push the loaves into each other. This is one of the most important aspects of a cob wall. Instead of stacking each loaf like a brick, the loaves are combined into one solid wall.

Building a cob wall works best if construction continues uninterrupted, with no more than a few days in between. If the walls are left to dry, adding more cob is difficult.

The walls took two months to build. Mr. Clary, 23, says it could have been done in three weeks, but their travel schedule wouldn't allow it.

Mr. Gump and his wife worked on the walls, too, with some help from friends and co-workers. Once the cob dries, it's baked solid by the sun and often coated with a lime clay mixture as a finishing touch for longevity.

Mr. Gump participated and supervised construction of the rest of the greenhouse and installed windows salvaged from a friend in the construction business.

Total cost of the project was less than $4,000 with 300 man-hours, and the greenhouse was completed in the summer of 2009.

"It's been so much better than I ever imagined," Mrs. Kinevey-Gump said, holding a handful of fresh produce. "It's wonderful; it's warm. When it's freezing cold outside and you need that blast of sun, I've got a chair here. You sit here and work on crossword puzzles."

But it's all the winter vegetables that make this place even more special.

"We get something green every day. It's fabulous to be able to come out here and just pick dinner," she said with a smile.


Doug Oster: doster@post-gazette.com or 724-772-9177. First Published April 3, 2010 4:00 AM


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