How To Build A Maginificent Earth Sheltered Greenhouse
12/6/2013 9:19:00 AM
I had a dream once. - I am standing in a vast green summer field looking down into the earth. The ground is a large glass roof in the shape of a cross. I can see tropical plants and seedlings flourish in the warm, light flooded space beneath. There is a sacredness to the space that is breathtaking. - I woke up and remembered both the beauty and impossibility of the vivid dream. Plants growing happily underneath the earth? Can't be.
I met my farmer friend Verena a few weeks later and my dream came up during a conversation. She said, “Yeah, it's called an earth-bermed greenhouse. You can build such a thing and it's been done before.” Really? Well then, I thought, let's make a dream come true.
It took me a while to find the person who was willing to embark on the adventure with me. My friend Jesse, a natural builder who is always on the lookout for new territories to explore, gave in to the calling.
What was supposed to take a couple of months at the most turned into a year long journey, as he ended up not only building a practical and functioning earth-sheltered greenhouse, but a piece of green architecture that feels and looks like the sacred space I dreamed about three years earlier.
We used the basic building plans from Mike Oehler's book The Earth Sheltered Greenhouse for the layout and material list.
An earth-bermed greenhouse is best build into an already existing South facing hill with full sun exposure. We started clearing shrubs and fallen trees from an area close to the house matching these conditions and decided on a 16x16 foot growing space. Oehlers's plans include digging out a 3-4 foot deep “cold sink “ at the South side of the greenhouse. A space that is designed to allow cold air to settle into at night, rather than hovering over your tender seedling that are growing in the work space.
The soil in our area has a lot of clay and does not drain water well. We added French drains around the whole outside perimeter to redirect the water flow around the structure. French drains are trenches with perforated pipes that are wrapped in landscaping fabric. The trench then gets filled up with crushed #4 stone and covered with top soil.
Next we started digging the 3 foot deep post holes. Even with the drainage system in place, the holes immediately filled with water and we decided to add PVC socksto the 6x6 pine posts to make sure they would not be exposed to moisture in the ground, preventing them from rotting. After setting the posts the holes where back filled with cement.
Next, the walls went up. We found a good deal for rough red and white oak boards which we planed. Pine or hemlock would have worked as well, but we found a reasonable source for oak and we knew the walls would look beautiful when oiled later.
Oak does not do well when exposed to moisture, especially red oak, and we had to come up with a solution to protect the outside of our walls from the constant moisture in the ground that would surround them. We used a pond liner to protect the outside of our walls but also had to make sure that no moisture would condensate in between the walls and the rubber liner.
We had just replaced a cedar shingle roof on our house and used a product calledcedar breather to provide airflow in between the cedar shingles and the plywood it gets nailed into. It allows the shingle to dry faster and prevents it from rotting, extending the lifespan of the shingle by many years. We decided to use the same product between the greenhouse walls and the pond liner to ensure airflow and avoid condensation and water damage.