Heat can only be transmitted by three distinct physical methods, Conduction, Convection and Radiation. As far as double glazing is concerned the only methods that are applicable are Convection and Conduction.
Conduction may be defined in the classical physical sense as the ability of a material to transmit heat a distance from an area of high temperature to one of a lower one. Due to the Laws of Physics it is impossible to transfer heat up a heat gradient. Therefore it is self defeating in attempting to discuss the situation. In principle all materials can conduct heat to a greater or lesser degree, metal for instance mostly have high rates of conduction, textiles, gases, feathers etc have low rates. To quote a phrase it all a matter of degree!
Transmission of heat by Convection is a completely different method of heat transfer and consists of the heat contacting a mobile fluid, by definition this fluid can move and it takes its heat with it, a good example of this is a radiator in a car, the combustion sequence in a car engine generates heat, this heat is passed on to the water surrounding the cylinders and this water is then pumped to the radiator. A stream of cold air is blown over the tubes carrying the water and the heat is removed. The cooled water is then pumped back to the cylinders and the whole process is repeated.
Radiation cannot be considered since this would entail reflective techniques such as aluminium film mirrors and since this would also reflect light it would be absolutely self defeating in windows!
Air and most gases are very poor at conducting heat therefore if one had an air gap between two surfaces, this in fact would be a deterrent to heat transfer between two surfaces. Examples of such structures would be cavity walls in a building, two pair of curtains covering window cavities, as a matter of fact this was the technique used in the 10th. Century for castles and such like types of property.
It was found that if two pairs of curtains were hung in front of a “ hole in a wall” then they could have vision for shooting arrows but once the curtains were back in place the place would remain relatively warm. They were unable to explain why it worked but it did! Hence they did it.
A more modern method would be to have two sheets of glass with an air gap in between.
As can be seen, the reduction in heat loss by Conduction is very much reduced but the problem of Convection is remaining and this can be tackled by using the following techniques:-
Take a sheet of glass, insert spacing bars around all perimeter sides, put a second sheet of glass of the same dimensions on top of the spacing bars and adhere the top sheet of glass to the bottom sheet. You have now in effect a sandwich with a layer of still air at its core, conduction will be very much reduced and convection will be eliminated as the trapped air cannot escape.
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Derek Pearson