So far, I had the skeletal outline of a small house, but it might have become a tool shed, or perhaps even a quaint little guest house (okay, okay, so the raised beds inside wouldn't have been very comfortable). What would finally distinguish it as a greenhouse was the glazing, so it was exciting to get to this point.

In doing my research, I had very early decided on twinwall polycarbonate as my glazing material. For the sake of appearance, and for durability, I had ruled out vinyl. Glass makes a beautiful, classical greenhouse, but it has a number of drawbacks: it provides virtually no insulation (unless one can afford double-walled glass); it breaks, even the tempered stuff; it's heavy and difficult to install; the building has to be absolutely plumb and level, otherwise the glass will probably crack.

Of the rigid alternatives to glass, multiwalled polycarbonate seemed the best choice to me: it's almost unbreakable, it's lightweight, it offers a reasonable amount of insulation, it allows ample light transmission, and it diffuses the light. Unfortunately, it also is somewhat more expensive than tempered glass, and even though it is treated for UV resistance, eventually it yellows and has to be replaced.

Polycarbonate for greenhouse glazing comes in several versions, depending on thickness and number of layers. I had really, really wanted a greenhouse with curved eaves, like this gorgeous (glass-glazed) number on the left. The problem was that to get a curved-eave model, I would have had to settle for 6mm polycarbonate, and I wanted the heavier 8mm version. Apparently, it's tricky to curve the thicker polycarbonate, although models with gentler curves, such as the Gothic Arch and Therma-Gro greenhouses are available with 8mm glazing.

Here's a close-up of the 8mm twinwall polycarbonate glazing. Both sides are covered with protective plastic film, peeled away here; the side with the blue film is the UV-resistant side, and the panel has to be installed so that this side faces outside.

There's some work to be done before installing the polycarbonate. It is always oriented with the flutes, or tubes, running vertically so that any moisture that condenses inside them can drain. The tops of the tubes have to be sealed tightly with aluminum tape to keep out moisture, dirt, bugs, etc., while the bottoms should be sealed some way that allows air and moisture to escape but keeps out live intruders.

I stored my polycarbonate sheets, lying flat, for about two months before I started using them, and discovered that all kinds of little creatures thought I had provided a marvelous condominium for them; a can of compressed air was useful in evicting these squatters. Had I realized how attractive the polycarbonate flutes are to our six- and eight-legged neighbors, I would have started taping the panels right away, while the foundation was being prepared.

Here's a panel of polycarbonate ready for installation. Its top is sealed with aluminum foil, which comes as a roll, and the bottom is fitted with an aluminum U-cap. A good description of how to install polycarbonate glazing is found on the International Greenhouse Co. site.

The glazing panels are laid from the outside, against shoulders on the bars of the framework, and are held in place by bar caps, shown at the left, which screw to the framework. The black strips are rubber "O" gaskets, supplied in large rolls; these rubber strips become compressed against the plastic glazing as the bar caps are screwed down, to provide a tight seal.   Okay, we're ready to make this aluminum skeleton a greenhouse!