A Community and Volunteer Driven Project
As the month of August was winding down, we headed to beautiful Canmore, Alberta, in the heart of the Rocky Mountains. We’d been working with the folks at Alpine Edibles to plan a greenhouse to be built as a centre piece of their urban farm at the middle school in Canmore. Dedicated to learning, this extensive garden is in partnership with Lawrence Grassi Middle School, and sits in their field. During the school year, the kids come out to learn about building soil, planting & harvesting vegetables, with much of the food grown going to the school. When the kids are away for the summer, Christian Wright, of Alpine Edibles, sells produce from the garden to local restaurants, and Farm Box, which delivers fresh food to individuals and families. The goal for the greenhouse is to provide a learning space and an extended growing season in an obviously short summer in the Bow Valley, and extend opportunities for learning as well. The design for the greenhouse was dreamt up by Christian and his team at Alpine Edibles, and put into a set of stamped and approved plans with the help of a local engineering firm. We worked with them to get the straw bale relevant details right. A local contractor donated his time to do the foundation, framing, and roofing. Much of the building material came by donation from local businesses or through private contributions.
Once the rubble trench foundation was in place, and the framing and roofing done, we came in to work with a rotating group of volunteers. The green house is built with sub grade insulation in the perimeter foundation, but there is no slab or other flooring to allow for planting to happen right in the ground. There is a pipe that was dug into the ground, connecting to the ceiling cavity, the idea being that in the summer hot air can be pumped down below grade, and help carry heat into the winter. Because this is a northern climate green house intended to be used in the winter months, there is insulation on three sides, and glazing on the south, different then a glass house, or a green house that you might find in a warmer climate.
Vapour Barrier vs Vapour Permeable: A High Humidity Environment and Natural Materials
One of the big considerations when building such a structure is moisture and condensation which are risks in an environment with high humidity. Building in a cold climate is a challenge because of the difference in temperature between the warm indoor air and the the cold air outside. Air pressure inside is greater that outside, which causes warm, moisture laden air to want to travel through any route it can to the outside. The risk is that warm air can carry more moisture than cold air, so at some point when the indoor air meets the outdoor air, it will cool off, and moisture that was previously in the form of vapour will turn to liquid water.
Generally, the approach in conventional building is to try and seal out moisture at all costs by wrapping everything in plastic. See this article for a deeper understanding of how a natural, ‘breathable’ wall system makes friends with moisture. This is a deep topic, and it is at the core of current building science and on the leading edge of natural building techniques. The point here is that in a green house, the pressures from hot, humid air are just that much greater than even in a home.
The first question we are almost always asked when the conversation about greenhouses and natural walls systems arise is what about the humidity levels? First off, we are not the first to do this kind of project. Simply put, a natural wall system can handle vapour easily, the trick is to be detail oriented and control the risk of possible air leakage from the interior to the exterior. Imagine you’re a molecule of air. You might be able to slip between the gap at the top of the wall, where the bales meet the ceiling, and the plaster shrunk back just the tiniest bit. Every time two materials met, there was an air barrier placed to protect the walls. This level of detailing is a concept drawn from energy efficient building science and building performance, and is not used just in natural building.
We used this opportunity to build a set of moisture meters that we inserted into the wall at different points, and will allow us to periodically check on the building over the coming years. It is a chance to add to the body of knowledge about how these walls systems work, and also to educate the more general public the viability of thinking differently about the materials in our buildings.
A Lot Can Happen in A Few Days
Over four days we stacked bales, straightened and trimmed our walls, readied the bales for plaster, and mixed and applied plaster, getting the base coat on. Next year we’ll be back with a final coat of clay-lime plaster to finish off the building and give it a final layer of protection.