Tuesday, 22 October 2013

Mind the gap - A short history of double glazing

Commercially available double-glazed windows were first available in the US as Thermopane, patented by C. D. Haven in the 1930s, although he does not claim to have invented them, and mentions several previous patents, some of which had expired. Apparently the idea came from Victorian Scots, but double glazing did not take off in the UK until much later. In parts of the US through the prosperous 1950s Thermopane windows were the height of luxury. Meanwhile in the UK, double glazed windows were expensive and houses were heated with coal that was simply coming out of the ground, so the financial incentive was not there.

Double glazing hit Europe in the 1970s in the wake of the oil shock. The response to high oil prices was an attempt to increase energy efficiency of buildings. The same energy shock spurred the Japanese to make more efficient electrical goods. The main effects on US policy seem to have been a 55 mile per hour speed limit, and increased interest in Middle Eastern politics, although it undeniably increased awareness of environmental issues and possibilities for saving energy.

The first double glazing in the UK had a quarter-inch, 6 mm air gap. The extra air made a huge difference to the insulation, and people's heating bills, and double glazing salesmen trod the nation's doorsteps and appeared on off-peak TV advertising, no doubt late on those cold evenings of autumn and winter.

The double glazing sales teams were competing against each other, and sooner or later someone brought out panes with an 8 mm air gap. A bigger gap meant more insulation, but more importantly, a bigger number for salesmen to impress their marks with. They could also, perhaps, go back to old customers, and persuade them to upgrade to these new, improved models.  Next came a 12 mm gap, and half an inch became the standard. The insulation was significantly better, although the improvements were not as stark as the jump from single to double panes.

Science and engineering can reach optimum values, but sales and advertising always want more, and the windows continued to get thicker. Unfortunately, beyond 12 mm the increased insulation of the extra air has less and less effect because the air within the enclosure starts to circulate, air heating up on the inside pane, flowing up to the top as hot air rises, then losing that heat to the outside pane as it heads down the other side. 

So they could try bringing out windows with a bigger gap, sounding better, but they wouldn't perform any better.

According to  the window man, the introduction of argon filling was just a way to get thicker windows that could both sound better, and perform better. There is another story though.