The answer to what is the best "glass", or glazing, depends on what kind of building you're working with, where and how it is located, and, unfortunately, how deep your project's pockets are.
You have two conditions which work at cross purposes here: You want to allow as much light into the garden space as possible, and you want to lose as little heat as possible. You also don't want it to overheat, which is entirely possible if you have quite clear days in the winter time and a lot of glazing.
In a descending order, the best insulating systems available are:
Vacuum glass windows/ glazing assemblies. I don't know if this is available to any extent yet, but here's and article about it:
http://www.buildinggreen.com/auth/a...olutionary-Vacuum-Glass-Coming-from-Guardian/
I also didn't see any reference here as to how much light passes through the windows. The Guardian company can answer that, if they've made it this far since 2009. But I think these come in at R-20.
The next best choice would be panels containing an aerogel. The draw back with these is that it has a much lower light-transmissivity level than glass.
http://www.kalwall.com/walls.htm
And
http://www.cabot-corp.com/Aerogel/Daylighting
These are apparently upwards to R-13/inch or thereabouts.
The third option is Low-e triple-glazed windows such as offered by the following companies:
http://www.seriouswindows.com/ Claims an R-13 rating.
http://www.floriangreenhouse.com/glass.html Claims an R-10 rating.
Ref:
http://www.greenbuildingadvisor.com/blogs/dept/energy-solutions/making-case-triple-glazed-windows
It should be noted that a lot of glazing can lead to overheating, even in the winter time.
http://www.buildingscience.com/documents/insights/bsi-006-can-fully-glazed-curtainwalls-be-green
An overlooked trick to getting more light farther back into a structure is to provide 'ledges on the outside of the building which bounces light off an angled ceiling into the deeper parts of the structure. If the ledges angles can be adjusted, they can be controlled to maximize the bounce during the winter and, handily, to provide shading in the summer.
Here's a small gif showing what I mean:
http://www.wbdg.org/images/daylighting_6.gif
Found at:
http://www.wbdg.org/resources/daylighting.php
The glass which allows the most amount of light to pass, btw, is a low-iron glass.
One thing to keep in mind is that argon-filled windows will lose a percentage of the argon every year, especially at high altitude, so they become less effective at insulating over the years.
Unfortunately. Planning on replacing them in 15-20 years is part of the price of doing business, I guess.
I am highly in favour of vertical farming, and the idea of skyscraper greenhouses occurred to me at least 15-20 years ago.
