When double glazing first became a popular window choice in the 1960s, most frames were made of aluminum. Aluminum remained the most popular choice for framing double glazing windows through the mid-1980s, when it held over 60% of the market. Since the introduction of PVC window framing, the market share of aluminum framed windows has dropped steadily. As of 2003, less than 17% of windows sold were aluminum framed. There are many reasons for the drop in popularity - and still some good reasons for choosing aluminum over PVC or wood frames.

The early popularity of aluminum was based on price and convenience. Aluminum was far less expensive than wood, the only other choice for window framing in the early years of double glazing. In addition, aluminum is easily extruded in the shapes and lengths needed to frame windows of any shape or size. It's strong, durable and very close to maintenance free.

Aluminum frames do have one significant drawback, however. Aluminum is an excellent conductor of heat and cold. It's such a good conductor, in fact, that in colder temperatures, frost often forms on interior surfaces of the windows close to the aluminum joints. The end result is windows that are significantly less able to conserve heat and energy than those framed in other materials.

PVCu was introduced in the mid-80s as a choice for framing double glazing windows, and immediately began to climb in popularity. When compared with aluminum frames, PVCu was less expensive, and more energy conservative. It can't match the strength of aluminum, however, and there are security concerns with its use. In addition, the introduction of 'thermal breaks' reduces the heat conductivity (measured in U values) of aluminum framed windows significantly. By fitting a less conductive material between the panes of the window as a sort of 'bridge' between the glass, manufacturers can bring the U value of aluminum framed double glazed windows within conservation standards.

The main selling points for aluminum window frames, then, were:

1. Strength - aluminum framed windows are far less prone to warping. The aluminum withstands weather well, needs no painting and forms strong, rigid window frames that will fit for far longer than wood frames.

2. Cost - aluminum frames are far less expensive than wood frames. They are easier to manufacture, and the material is less expensive to begin with. On the other hand, the introduction of PVC has largely negated the advantage of cost. Far lower in price, and with more efficient heating, PVC has become the material of choice for framing double glazing windows.

3. Ease of maintenance - As opposed to wood, which is subject to warping and decay and needs repainting every 3-5 years, aluminum is virtually maintenance free. It never needs painting, doesn