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?t rot or warp, and is rigid and strong enough to bear the
load of window lintels with minimal reinforcement.

4. Security - Because of the tight fit possible with aluminum
framed double glazed windows, they were - and still are - the
choice where security is a paramount concern. It's very
difficult to 'pop' an aluminum framed window from its frame if
it's properly fitted.