We are having a frore winter here - not thaws at all and in next few days, the temperature is expected to fall below -30 C (-22 F). Quite a trouble to keep the temperature acceptable in my greenhouse. So far when temperatures have not dropped below -22 C (-8 F), I've managed to keep at least +6 C (+43 F)... If such a weather will continue for a long time, I'll run out of firewood!
Look at how the greenhouse looks like from outside:





Though the air temperature in the greenhouse has been around 10 C (50 F) and only occasionally 12...15 C (54...59 F) for short times, the Ruby Red grapefruit is about to start blooming soon:



I'm a bit afraid of it as the soil is only 9 C (48 F) and warmer it will be, I think, only in March...
Harri.

Harri, I also have a greenhouse situated in a cold climate with lows in the -17F range. The north wall of a greenhouse does not provide light, rather a north wall loses light. By making the north wall an insulated solid wall painted white, doing so will not only reduce your heating cost, the white wall reflects sunlight back into the greenhouse thus providing more light and warmth. . - Millet (1,090-)

My greenhouse is oriented from South to North and the northern wall is insulated. Not painted white, though - but we do not have much sun in winter time anyway that could be reflected back.
The registered absolute minimum has been -43.5 C (-46 F) in Estonia. I do hope this will not happen again!!

"Solar greenhouse" is the term used with such a construction, having only the south facing wall made of transparent material and usually only that side of the roof sloping. Often mass heat storages like water barrels or masonry are also used to retain heat. Floor is also typically below ground level since the ground is warmer in the winter than air. I've never really seen one in Finland, I guess it's not popular in Estonia either, but I'm not quite sure why, since it seems to be quite popular in North America and sounds like a good idea. One would think that it would be good here too. But many people here use the greenhouses only in summer because of high heating expenses in winter usage. I saw just a tv program about some very old "greenhouses" in Sweden, orangeries where the king or other nobles stored their citrus trees over winter - massive stone buildings like little chapels with windows only toward south, if I understood correctly. The trees were brought outside for summer.

Some people have a kind of Solar greenhouses attached to the house, but stand-alone Solar greenhouse seems to be unknown of here.

Yes, I've heard about the solar greenhouses and I even played with a thought to build it. But the best it probably is able to offer here, is some (remarkable?) help maybe in March and April when it is quite sunny here but cold anyway. From October till February, most likely not much use of it. In this January, the winter here has been very cold (today, it was -28 at my place and well below -30 in some other places) and very sunny - the plastic covering of my greenhouse is by snow and hoarfrost so that most of the solar radiation is dissipated and reflected back without getting inside... Thus, if I would like to have a solar greenhouse, it must have additional heating system anyway - in winter time days are short and if sunny, it's very cold. It is warmer only when it's cloudy - and no sunshine.
Harri.

Here in Colorado, winters are cold, but mostly sunny. I always remove any snow covering from the greenhouse, to allow the sun to shine inside on the plants, and to warm the greenhouse. The greenhouse also has a heat retaining double air inflated top glazing , and and a night time close-able insulated wall covering for the south wall. - Millet (1,086-)

I have my own forest and a cheap firewood - but if I had not, I'd probably have thought longer about the solar heating system. My guess is that I could maybe save 10% of firewood or even more. Or maybe not...
Harri.

Actually, I believe the heating cost save would be much more than 10%, but so are the construction costs. I think there should be additional insulation, which can be drawn over the windows for the coldest and darkest times and possibly fluorescent or HPS lighting if necessary. I have given a good few thoughts to one day building one, but I guess I should make it from recycled material to keep the costs low and also the environmental effects as small as possible. And I want to move first, I don't like the current place. At the moment I have a room in the separate garage building, with south facing double (or maybe triple) glazed windows and proper insulation, but I don't like to heat it up very warm, since the only possible heating is by electricity. So most of my frost-tolerant plants are indoors on the porch, which has windows to three directions and floor heating. Even the Helen's hybrid banana, which is only a year and couple of months old, but already quite huge.

I'm a bit suprised to hear that you have had such low temps in Estonia, here in SW Finland the minimum has been around -20C. Actually it was -20C this morning, but now it's already -9 something. It has been very long frost, below zero C constantly for 6-7 weeks already and most time below -10, but not very hard frosts at all, in the past we usually got at least a few days of -30C per year. Last winter was exceptionally warm, so this is just a small step back to the past. In the future these cold and snowy winters will probably be just a faint memory.

Since the growing conditions in Estonia are very similar to the ones in here, I'll be following your posts very closely.

I have not provided any calculations - but it seems to me anyway that a special solar heating wouldn't save much money at our latitude (even if not to take into consideration establishing expenses). Sure, some effect can be obtained - but actually the solar heating effect is present in case of any greenhouses. More than this - my greenhouse would become even too hot in March! The most problematic months are from November till February - but at that time there are too few sunny days... But, of course, this is merely my personal prejudice, not more.
Yes, this winter has been extraordinary cold so far - not because of extremal temperatures but because of the fact that constantly lower than average temperatures have lasted 6 or 7 weeks already. Only last nights have been much colder than usual (luckily, it should start warming up a bit next week).
Harri.

The proposed savings in these latitudes come from better insulation, not so much from the sun. And in March, the day temperature rises very high on sunny day in greenhouse, but the problem is how to retain that heat for the night or cloudy/snowy days and there again good insulation and heat storages are to help.

I've been reading this topic and I would like to know what improvement can be done to add more protection over the plastic on the greenhouse , so that I can reduce the amount of extra heating mostly done with electricity.

The inflated stuff is ok , but I don't have right now the $$ to do it.

Thanks