The theme on the Master Gardeners Fair booth this time was Winter Gardening. Part of the display included details on a solar greenhouse for our climate. I have used one for several years. Frequently I grow all my own transplants for my farmers market booth using only solar energy for heat. This year I used a couple of more flats than I produced but still produced the equivalent of 10 or 12 flats.
Details I consider important.
Orientation toward the south. I haven't put a compass on mine but I bet it is within 10 degrees of true south.
Insulation of north wall. I use salvaged card board and plastic.
At least one gallon of water storage for each square foot of glazing. I use 3 50 gallon drums, 1 30 gallon drum and over 200 2 liter drink bottles.
Very little air infiltration. It is particularly helpful not to create a chimney effect with air openings high in the structure.
Here are a few other details.
The glazing angle for our latitude that maximizes February and March temperatures is 45 degrees. Not sure how critical this is. Mine has a rounded conduit shape that goes from 90 to 0 degrees and it works.
On cold nights there are two covering options. One is a single sheet of newspapers laid on the plants. As the temperature drops the newspaper will get wet. This water will release heat when it freezes. I have had it freeze without damaging the plants underneath. The other option is to totally cover the structure with a nonwoven landscape fabric type blanket.
When I put the greenhouse together several years ago, I used all slavaged materials. Eventually, I purchased plastic greenhouse film. I have seen other gardeners use salvaged windows on a similar structure.
My best night this year was on the Easter Freeze when outside temperatures were 19 degrees by 3 am. Inside it was 40 degrees with no supplemental heat. Not sure what the minimum temeperature would have to be to damage plants. I just know my heating bill is less than most greenhouses.