Every time the wind gusts to 50 km/h Leela and I both start to look at each other in a state of worry. Whose idea was it anyways to build greenhouses on an exposed windy hill? The good news is that Leela hasn’t been in the house the two nights that we’ve had 100 km/h winds. And that’s all that I have for scientific support to calm her worries on windy nights. (The photo at left is us racing to repair an end wall after a fall windstorm, before the sun set and the crops under cover froze).
It was my idea to build these greenhouses on a hill. The high spot seemed like a good place for drainage and it was probably the most level place on the property. They have an east-west orientation, which allows the prevailing wind to naturally ventilate the greenhouses. We had to scrape the thin layer of topsoil off the bedrock. We did this to help level the site and get access for drilling into the bedrock. We decided we should cement the greenhouse ground posts into the bedrock for increased support. It was likely a good idea for our location. I’ve even heard since we constructed the greenhouses that the most likely time to have greenhouse support failure is in the spring when the ground is thawing around the ground posts and not offering much support.
Since we removed all the topsoil, we had an opportunity to try several other innovative concepts. Before building the greenhouses, we laid out tile drainage, to drain any water from underneath the greenhouses. Then, we laid out 4” polystyrene insulation on the bedrock to separate our greenhouse from the bedrock. We figured that the bedrock would be thermally conductive and would want to ‘wick away’ any heat that we were storing in our greenhouse soil. Green building design research led me to understand that when insulating a house basement, the perimeter soil conditions should be considered. Sand, clay and rock have differing thermal conductivity and therefore you would want to insulate a house foundation to a greater extent in clay and rock.
So we scraped off the topsoil, exposed the bedrock to drill and cement the ground posts, installed subsurface drainage, and then laid out the greenhouse perimeter insulation. We then installed a layer of used 6 mm greenhouse plastic to create an air barrier on top of the 4” insulation. From here we put down a thin layer of native soil and then a wire grid that holds the radiant floor heating.
Since we first saw radiant floor heating in greenhouses, we knew that we would be starting from the ground up at our farm. We didn’t really have the opportunity to just install a greenhouse on the native soils of the property. By working with the constraints of our shallow loam soil on top of bedrock, we were able to come up with innovative ‘green’ greenhouse design.
By heating the soil instead of the air, an energy savings of 50% can result. This is because air is poor at retaining heat compared to soil. Another advantage is that the limiting factor for plant growth is soil temperature and not necessarily air temperature. By heating the soil instead of the air, we are getting increased biological activity where we want it, at the root zone.
Once the radiant floor heating was installed, and the walls of the insulation were built abutting all the ground posts we were left looking at what almost looked like an empty rectangular swimming pool. Except now we would fill it up with soil instead of water. So a few dump trucks, tractors, skid steers and lots of shoveling later we have 4 greenhouses with 18” of soil.
To help with wind protection, we have built a berm on the western side of the greenhouses. This year we will be increasing the size of the berm and planting quick growing cottonwoods to help reduce wind speeds. This will help to reduce greenhouse heat loss during the winter months, reduce plastic wear and tear, and hopefully help protect the greenhouses on those big windy stormy nights.
It’s now been a year since we first started staking out the greenhouse footprints, and it’s taken me this long to find time to blog about it. After harvesting greens nearly every week since November, we’re finally taking a summer break from greenhouse production. We’re busy with field crops and new infrastructure for our winter CSA, but maybe I’ll find time to write a post about our greenhouse heating system… or maybe I’ll just have a beer and try to relax.