Why does adding a second pane of glass (double glazing) make such a huge difference, even if your walls etc are the same?
In: Physics
*loads* of heat is lost through windows. The whole point of walls is to keep stuff from getting through them. That’s easy to achieve, just make it thick and dense and stuff’s gonna have a hard time getting through it. However, the whole point of windows is to *allow* stuff to go through it. So making a window that only lets light through and not anything else is a hell of a lot harder than making something that either lets nothing through or everything through. To make a window, you need to make the material thin, because it’s difficult for light to pass through thick materials, even if those materials are relatively transparent (like glass is). Thin windows are also a lot cheaper, which is good cos glass is really expensive and hard to make compared to the lumps of stone or clay you might use to make a wall. Before double glazing was invented, windows tended to be very small in colder countries, just large enough to let some light through, but not so large that the inside of the building becomes freezing cold.
Also, double glazing has special insulating properties – two thin sheets of glass are better than one thick sheet. This is because heat struggles to transition between mediums. Once it’s in a particular material it can travel easily, but it takes quite some effort to switch between mediums, because it has to travel in different ways through different mediums. Through vacuums and gases, heat can travel as radiation: invisible light (infrared – it’s this light that’s detected by thermal vision devices). In gas and liquid, heat travels by convection: individual molecules become more energetic and move faster, and they transfer heat when they collide with another molecule and pass that energy on. In solids, heat can only travel via conduction, which is similar to convection but because the molecules aren’t freely able to move they can only pass on energy to adjacent molecules one by one, and slowly. A double glazed window is two solid mediums separated by a gaseous medium (air), with gas (air) on each side of it. So for heat to escape through a double glazed window, it has to convect through the air in the room, then transition into the glass and conduct through it, then transition into the air between the panes and convect through it, then transition into the second pane and conduct through it, then transition into the air and convect away into the night. That’s two additional phase changes compared to a single, thicker piece of glass.
Walls actually utilise a similar principle – a lot of walls have two layers separated by an inner reservoir of air. If you want to be even warmer, you can fill that space with special insulating foam, which is basically hundreds of layers of solid separated by hundreds of thousands of air pockets, which means escaping heat has to go through a whole bunch of extra phase changes.
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