This is mostly conjecture, Tim, altho' I've had experience with the backyard greenhouses, beginning about 20 years ago. Sometime before that, I worked in a commercial greenhouse for several years.
At one time and maybe continuing, the makers of greenhouses seemed to be making some fairly wild claims about their products. The ads talked about the ability to allow light, filter just the
right light thru or, almost,
gather light for the greenhouse interior. I think these crazy notions were mostly a matter of my interpretation and the inability of the manufactures to write clearly about their glazing materials.
There are differences in glazing material and light transmission can be tested. There probably are no standards in this country of what is important to the plants and maybe even what can be said about test results. I finally came to just think of it as a rule of thumb: 1 layer of most any kind of glazing reduces light to the plants at about 10% of the outdoors. Two layers would then be 20%.
Of course, a single layer holds in heat but it does a very poor job of it. Still, there is a loss of not only the light needed for photosynthesis but solar heat gain with additional layers. If the layers are too close together, the air space is inadequate for good insulation. And yet, if you have 4 inches between layers (usually recommended), that means you have 4 inch glazing bars. And, that means you have quite a bit of shade from the bars.
All these things become trade-offs. The span, roof peak to foundation, is about 10' for my current greenhouse and I initially wanted to use double layers of plastic film. So, I used 4" bars. I'm afraid I ended up with a poor choice since I've chosen to never run the greenhouse in the winter (except for a few weeks in late winter for the plant starts). The bars create quite a lot of interior shade. If I tried to save some money on heating during the spring with double layers of film - I'd cut down on what has proven to me to be inadequate light. Yes, I could set up supplemental lighting but then I've got the fixtures blocking the sun . . .
.
I am just suggesting that you look at your limiting factors and decide which are most important. You may not want to go the double glazing route. With glazing bars that are not especially long, you may want them quite narrow and somewhat wide apart. If wind is an important factor, that might be problematic but it would depend on your location. There may be a possibility of "retrofitting" with putting in double glazing on the interior or even adding additional glazing bars if things don't quite turn out the way you'd hoped or planned.
Shade cloth may be available to you to stretch over the structure in the summer. It used to be that greenhouses here were commonly whitewashed if they were used during the summer months. I don't know why that was discontinued.
With a greenhouse as small as you are planning, windows as large and high as you can get them should be adequate for ventilation in all but the hottest weeks. You may want to replace one window with an exhaust fan in the future and you should be able to go entirely with a roof vent instead of windows. That would be true even with a large structure. However, without some automation, you will need to be very attentive during the daylight hours.
Steve
On a much
lighter note . . .
