When building or buying a greenhouse, it seems to me that the material to use is one of the first concerns. The two options I'm trying to compare right now are glass and plastic films. The characteristics that seemed important to me were transmittance, insulation, maintenance and cost (both purchase- and maintenance-). What I could find on the net was the following.
As Wikipedia shows, plants need mostly light from the visible spectrum:
at which range both glass and most plastics transmit most light. A quick scholar.google search gave me The Radiative Effectiveness of Plastic Films for Greenhouses, with the following info.
The mil used here was strange to me, which I converted using the first converter Google gave me. It seems that there is hardly any difference between glass and plastic here, although there are special glasses designed to transmit >99% of the sunlight, as described in Transmittance of optical glass, which are out of reach for most people due to their cost.
As can again be read on Wikipedia, it is mostly (the lack of) convection that accounts for the temperature, and both glass and plastic stop this. Of course, glass insulates better than plastic, and double glass would do so even better (again due to the lack of convection in between these layers of glass), but I couldn't find any literature on this.
Plastic changes colour over time and needs to be changed every few years (see The Radiative Effectiveness of Plastic Films for Greenhouses again for precise numbers), while glass lasts ages (look at the royal greenhouse of Laeken). I would thus say glass needs less maintenance.
Plastic is cheap to buy, but needs to be changed every few years, as noted at the maintenance point. Glass is more expensive at first, but needs less maintenance. I could not find any thorough comparisons of the costs on the web.
Glass just looks better. It does. It gives a different atmosphere, and you can actually see the rest of the garden when you're for example using it as a hangout spot in the winter.
From this intensive search of the web I can only conclude that glass is a little bit better at some points, but I have no idea if it really differs that much. The only point where I think the glass really wins is the maintenance, but that isn't really such a breaking point (I care more about which of the two is best for the plants). Is glass worth the investment? Did I overlook something? Does any of you have experience with both, or argumented reasons for why one of the two is better?