Effectiveness of a "Chinese" style greenhouse in a cloudy climate?

I've read a bunch of books about greenhouses and how awesome earth-bermed  and insulated north walled (in the northern hemisphere) greenhouses are, but many of those greenhouses seem to be in places where there's lots of sun in the shoulder seasons, and even in winter (between storms at least). Formally, I lived in Ontario, so I know how much snow + sunshine reflects light. However, I'm now living on the "wet coast" of Canada and most people here seem to still be using pretty traditional greenhouses which get too cold in the winter and too hot in the summer. I'm having a lot of trouble convincing the people who's cooperation I would need to get a greenhouse built, how it would do in a cloudy climate.

My understanding is that without clouds, the sun shines in predictable straight lines. Clouds tend to bounce and redirect the light, so you need more glazing to collect it.

It would take a lot of convincing argument before I believed that cloud cover could make the sunlight come in from the north side of a greenhouse, especially in the winter when the sun is much, much lower in the sky to the south.

I just received an email from Greg Schweser of Farm Scale Winter Greenhouses out of University of Minnesota this morning interestingly enough.  They are built with glazing only on the south and insulated on the north side and north side of the roof, in a similar, although I believe more extreme climate.  You have to fill out a short "survey" to get to the building plans, but I am on the mailing list and only get a couple emails a year from them, and only information about updated plans or free classes regarding their greenhouse builds.  It only asks for name, email address, and your county and state.  There may be information there that will help you.

UMN.edu greenhouses

The main problem with cloud cover is that you don't get any heating effect. my greenhouse is a standard glass greenhouse. in summer it can be 20C over the outside temp if I forget to open it, but in winter it is the same temperature as outside, infact it can even get colder than outside on clear nights.
low and weak sunlight combined with cloud cover means no solar gain above what it loses. Obviously insulation would help with losses but it wouldn't help with 3 weeks with no sun.

Trace Oswald wrote:It would take a lot of convincing argument before I believed that cloud cover could make the sunlight come in from the north side of a greenhouse, especially in the winter when the sun is much, much lower in the sky to the south.

One would think! I tried to convince one neighbor that insulating the north side of her little greenhouse would help, but she insisted it needed every bit of light it could get.

It's hard not to think I'm the crazy one, when every green house I've seen on surrounding properties has no insulation. I think that some of it is that we get little snow and only occasional freezing weather, but I don't want to spend my "energy budget" heating a greenhouse if I don't have to.

Skandi Rogers wrote:

low and weak sunlight combined with cloud cover means no solar gain above what it loses. Obviously insulation would help with losses but it wouldn't help with 3 weeks with no sun.

That is definitely similar to what we can experience. My reasoning is that least with insulation, I could hold on to any heat that I decide I need to add. My understanding is that part of what makes the Chinese Greenhouse model different is that they roll insulation down over the plastic in the winter.

I will make a quick attempt to convince you that light does indeed come from the north on a cloudy day.  If you step outside on an overcast day you can see because there is light around.  You will also notice that you cast no shadow.  This is because there is light coming from all directions.

Plants need both warmth and light, a lot of light.  Especially historically it has been easier to warm a place up than to illuminate it. To warm a place you can just burn something.  To illuminate a space bright enough to grow plants requires many high tech lights.  To actively grow plants in the winter you will need to provide one or the other or both.

If you are looking to preserve plants in stasis through the winter, you only need to keep the space from freezing and admit an hour or two of light.  A Chinese style greenhouse would do this well.

My greenhouse plans have some similarities to the "Chinese" greenhouse.  

It is my understanding that there are proven benefits to insulating the North wall and even most of the roof and buffering the whole structure from the elements by going into the Earth, and the more extreme climate you are dealing with the more this is the case.  

Plants need sunlight, for certain, but they really want quality sunlight, as early in the day as possible.  My present garden suffers a bit because I built it on the East side of my meadow and so it takes the rising sun a while to get past the tall trees on that side.  Morning light, and light into the early day are most important for growth during the regular growing season, particularly if your soil is cooled significantly at night, and there is a limit on how much photosyntheis can be done in a summer day, and so the plants just shut down after they've had enough of it.  In the Northern Hemisphere, South glazing and perhaps some sunlight coming from the East is all that is needed for a year round greenhouse.  I would say, keep it in the South though, unless your mornings tend to be clear and then get cloudy as the day progresses (as can be the case where I live).  In the winter however, the low angle sun comes in a short arc from the South, and that is why I understand that this is where the glazing should be concentrated if winter growing is intended as the main purpose of the greenhouse.  While there is light coming from the North (if it is glazed), the quality of light is poor for growing plants, and the heat losses due to the glazing are not worth it.  This North side could (and in my thinking, should) be an insulated white wall, which reflects light onto the plants, or a black wall of thermal mass which absorbs heat and then slowly releases it back to the greenhouse since it is also backed by insulation.  Finding the balance between insulation, glazing, thermal mass, and square footage is going to be difficult to predict.  

I think though, that basing at least some of your conceptualization on this design is a good start.    

I had some friends who had a peach tree in a greenhouse at 54 degrees north, it did fine producing lots of fruit on most years, but on years that were cloudier, the peaches were almost flavourless.  A couple of years ago, when the sky was dense with smoke from forest fires, our tomatoes didn't even ripen in our regular greenhouse glazed on all sides (which in any other regular growing season is unheard of-we had a bumper crop the year before the fire-years).  Definitely, the amount, or lack of, sunlight in these examples is playing a huge role, and, in my opinion, having glazing on the North in both of these cases did nothing to improve the greenhouses ability to provide the needed light, but that is for science to prove.  

I have seen a guy in Alberta of Canada having built a double layer such a greenhouse with the rolled blanket mechanism inside the cavity. His temperature data showed clearly that it has the similar problem as all the other types of greenhouses out there: too hot in summer and during winter day (need ventilation to reduce inside temperature) and too cold in winter and summer night. Filling the double layer with air is not the solution.

The only working solution that solves all the problems is to fill the cavity with soap liquid bubbles. As simple as that!

Check this new video post I prepared with all the proofs and a way to retrofit your existing greenhouse: https://pyrapod.com/problems-with-traditional-greenhouses-how-to-retrofit-with-solaroof

I am at 47.25 North so the 7 foot tall glass on south and west sides penetrates all the way to the far side of the 18 foot square structure. Morning sun is scarce but winter evenings tend to be clear. so I left the east side insulated. I would like to reverse the shed roof to slope north and dig the current south slope of the floor down and insulate the roof.  I find supplementing light is less expensive than electric heat.  During the summer only 4 feet on the south side gets a lot of light but the setting sun still comes all the way in. The summer usage is for tomatoes and basil.  The north side of the greenhouse is devoted to New Zealand spinach which produces the largest leaves with about 2 hours of direct light.
I fill the greenhouse with barrel planters in the winter and then move them out to appropriate microclimates this time of year.
You probably have the same pattern cloudy nights are no problem but clear nights the radiant cooling can be extreme.  So I should use some other insulation on the north and east and use my rolls of carpet to cover the glass on clear nights.

A good way to insulate and capture solar thermal energy at the same time is to use LIQUID SOAP BUBBLE FOAM.

Aubrey Zhang wrote:
The only working solution that solves all the problems is to fill the cavity with soap liquid bubbles. As simple as that!

That may be a solution, but the only solution?  

Here in the Netherlands we have a wet and often cloudy climate too. I see many people have greenhouses on the allotment plot, the traditional glass greenhouses or polytunnels. But I see they don't really use the greenhouses in winter. Now, since end of February, they start using the greenhouses to sow seeds, so they have plants to transplant into the garden when frost nights are over (that's half May here). And then they keep some (sub-)tropical plants in the greenhouse during summer. Probably during winter those unheated greenhouses are much too cold, because of the clouds and the low sun IF there is any sun visible.

Northern hemisphere
I can see getting light on the north on cloudy days…

And I used to have a sun porch on the south side of my house that, though not as warm as a clear winter day, did warm up on the cloudy days.

I think if I had a greenhouse with this problem to solve, to balance heat loss with light, I would insulate the north side, and seek to increase light and heat gain by focusing reflectors to send more light into the greenhouse.  

Threads on solar ovens have plenty of experimentation being reported!  Cardboard with tinfoil.  A discarded satellite dish with reflective coating are a couple ideas.