Aerogels are materials that are mostly air - usually around 99 per cent by volume - and can be fabricated from silica, metals even rubber. They are extremely light. For example, a cubic metre of silica glass would weigh about 2000 kilograms. A silica aerogel block of the same dimensions would weigh 20 kilograms. Despite this aerogels are relatively strong. They are sometimes called 'frozen smoke' due to their translucent appearance. In the case of the silica aerogel it consists of tiny dense silica particles about 1 nanometre across which link up to form a gel.

Aerogels are excellent insulators, having about one hundredth the thermal conductivity of glass. Double glazing which replaced the gap with an aerogel would improve the insulation value by a factor of three as against the very best current multiple glazing. It would be possible to achieve a 99 per cent vacuum between the panes since they are supported by a solid. However, even with a thin aerogel sandwich the window would have a slightly frosted appearance.

The thermal properties of aerogels also make them ideal for harvesting solar heat. Flat plate solar panels collect heat then radiate it back into space. An aerogel glass sandwich would provide a one-way barrier to the re-radiation of heat from the absorbing surface. This would have an obvious application in active solar panels and also in solar walls. The outer surface of the wall would be coated black to maximise absorption. Faced with a glass aerogel screen the heat would be retained and radiated into the interior of the building. A blind behind a glazed rain screen would minimise an excessive build-up of heat in summer (Figure 6.5). The Freiburg Solar House shows Trombe walls with some blinds lowered (Figure 5.6).

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