Thoughts on a Winter Greenhouse
I began thinking that storage in the cellar at high humidity and low temperature (but above freezing) was really about creating a space where the vegetables wouldn't die by passively using energy from the massive heat sink of earth below to provide just enough heat to help out. After the last harvest I realized that I now have the infrastructure in place to grow and store even more than I previously thought. It would be great if I could grow more; more varieties of cold climate vegetables that have good storage characteristics, short season vegetables that lend themselves well to canning, and greens! Lots and lots of greens!
Picture 1B. Unwrapping a 3.5 month old cabbage yeilds surprising results to somebody who has never used a root cellar! It looks and tastes fresh!
So greens are a problem. We have a short growing season. The growing season can be extended using conventional greenhouses and cold frames. However, green vegetables are hard to store for long periods. They wont last in a root cellar. However, plants like radicchio and Belgian endive (chicory family) grow as a root, similar to a carrot, which can be harvested, stored in a cellar, and then forced to grow tightly packed heads months later. Cabbages are easy and versatile and a regular part of traditional Newfoundland cooking. These are great options, and nutritious, but not really that green! To obtain greens you need a longer growing season or some way to extend it. A root cellar effectively extends to 4 seasons by keeping subterreanean vegetables alive but greens need light. So the answer is obvious. A greenhouse is in order.
A greenhouse can have have many forms depending on usage and climate. A simple polyethylene hoop house could extend the season here for a few weeks in spring and fall. However, I want to minimize the gap in our growing seasons significantly. I want our lack of winter daytime hours to be my main concern and not heating. Thinking about the houses we live in...glazings and air leakage are where most heat is lost. So a structure with a decent glazing R-value (for a greenhouse) and good air tightness is a must. The structure needs thermal mass in order to cruise through significant temperature swings in both winter and summer due to a large amount of glazing and some way to capture the Sun's heat when it's present so that we can maintain temperatures that extend life through dormancy. Passive solar greenhouse design is just using a lot of common sense. Insulate where the sun is not able to enter. Picture 2 illustrates a deep winter greenhouse where insulation is key to performance. So unglazed roofs, north, east, and west walls, should all be highly insulated....to passive house standards???? Obviously not. This article (https://university.upstartfarmers.com/blog/decipher-heat-loss-r-value-greenhouse) was compact and informative and can provide some basic design criteria for an extended season greenhouse. The sweet spot appears to be somewhere around R-20 walls and R-2 glazings. R-2 glazings are expensive so I would opt for locally available R-1.5 1.5 twinwall but use a home made type of insulated curtain (R-1) at night to increase energy savings. I am looking at about 220 sq.ft of glazing for the designed building. A couple of quick hand calculations can easily show that going beyond R-20 and decreasing thermal bridging is a waste of time and money. Sinking money into a heat source is probably a better investment. Since this kind of greenhouse really only needs to keep vegetables alive through the cold months, a low grade heat sources such as a climate battery (ground source heat exchanger), or a point source heat source such as a compost pile could produce all the energy necessary to maintain temperatures above 0 C.
This latest set of blog entries will document the progress on my greenhouse. Rather than provide you with the the plot layout, interior plan, and exterior facade, I am opting to leave all of that out and will provide a step by step build which unfolds as the blog progresses! Stay tuned!
Picture 2. An example of a deep winter greenhouse built in Minnesota. https://extension.umn.edu/growing-systems/deep-winter-greenhouses#pageMenu