North wall would be a zillion times better, as you'll get more winter sun, when you need it most. Good to hear you're going for a big greenhouse. If you can afford it you'll be thankful for going bigger. A door directly into the greenhouse is fantastic as you can go out there and read a book in the middle of winter! You will have 24m^2 space. People have lived comfortably off veggies grown in 50m^2 without any extra food in temperate climates. You might be able to do this, with the bonus of some of the more hardy sub tropical crops (perhaps grow some ginger?). domeman: analysis looks good. Depending on what your goal is, you might even use 93% transmission PC (clear) and 50% shade cloth inside. This will give the lower light, but collect more heat. In summer you would either vent, or move the shade cloth outside. PC is also half the price of colourbond (but only lasts 1/3rd as long). If you put the shade cloth on the inside, two things to note: a) it would be good to have a light clear film of something like PE (say drop sheets) directly under the PC (like double glazing) to protect the PC from hot humid conditions (apparently PC breaks down quickly in hot humid conditions, but not when just hot) b) keep the shade cloth say 20cm from the surface to allow good air movement. A large diameter fan such as a ceiling fan at the top of the greenhouse could then move heated air down when required (thermostat).

Very nice self built GH Paul. What will you grow in it in the spring? Start your own veggies? Perennials? I have a self built glass house, which I keep heated (50* minimum) so haven't any words of wisdom on snow removal off a polycarbonate roof. But I live in a similar zone (in Ont) and can point out that you'll be surprised at how warm the GH will get during the day...especially with sun but even on a cloudy day there is still UV rays that will boost the temps. My GH consistantly gets to 75-85* on sunny days in the winter with outside temps below freezing and to 60*+ even on cloudy days. The inside temps will help warm the roof and the snow will slide off or melt away a good deal of the time. You may just have to wait until the afternoon to see that happen though.

A great alternative would be Polycoolite by Polygal. It only shows a 50% light transmission, but that is an overall light transmission. I have included a section of the chart for Polycoolite light transmission between 450nm and 700nm. This is where photosynthesis takes place. You can see that Polycoolite has between 60% and 70% light transmission in this range. I have used it on my personal greenhouse in Louisiana and loved it. That is my greenhouse in South Louisiana in the snow. That was quite the odd day. Polycoolite has diffused light properties as well. The ice sheets will not give you enough light. If you are choosing between ice and clear the choice should be clear.

Our shade will be ~10’ x 12’ and cost $3200. Pricey, yes, but with Sunbrella fabric and the flexibility of having it open or closed with ease, we felt it was worth it and will definitely increase the usage of our space.