posted 8 years ago
I fully agree with you with regard to curtains in residential applications. Just in passing, you may note that such curtains are often topped with a valance. This is not just for appearance; the actual efffect is to "seal" the top from convective heat losses. And this is the essence of the problem. Warm air rises. On a vertical surface the major issue is sealing the top of the thermal barrier, to trap the air, (which is the actual insulator in large part). In a greenhouse, one is working with either a sloping or a curved surface, and sealing along the sides becomes more important, while sealing at the top is much more difficult than using a simple valance. Curtains will hang vertically with no further support; in a greenhouse they need support over large areas. In areas where snow is not a problem this is easily done by rolling down some sort of insulating barrier on the outside, but if one may get 3 feet of snow on top of it, (Ladakh?), this becomes less feasible.
This is not a trivial technical challenge. But I have also been wondering about the need. As things stand in my passive solar greenhouse temperatures drop to just above freezing on cold nights, but do not drop below 0 degrees C. at night. During te day the temps go up well into the high 20's C. The growth of our leafy greens is very slow, but my suspicion is that the limiting factor is not the temperature at all, but the light. If this be the case, no amount of fiddling with heat retention at night will make any difference.