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
Walls are thicc, windows are not
Your wall has drywall, wooden studs with insulation, an outer wooden layer, a heat and vapor barrier, and then the final siding. None of these things are good conductors, and the fiberglass insulation in the middle prevents convection from helping so it is very hard for heat to move through the wall
The single pane window is think and can conduct straight from the warm air in the house to the cold air outside(or vice versa), adding a second pane removes the conduction path as the heat has to go through the first one, hop into the air, go from the air to the second pane, and then conduct through that. Thickness and transitions both help slow down the heat transfer.
A basic single pane window will come in around R-1 (R factor measures heat loss in ft^2 * F * h/BTU, basically higher means less heat lost), a double pane window can be around R-4 meaning it loses heat at a quarter of the rate over the same area, while a wall can be upwards of R-20 so it loses 1/5th the heat of the fancy window over the same area. You do have more wall area but windows lose a crazy amount of heat for their relatively small size. A 2’x3′ R-1 window loses as much heat as a 10’x12′ chunk of wall while a big 5’x5′ picture window could lose as much heat as 10’x50′ of wall, that’s the entire length of most houses!
Windows are the easiest improvement that can be made without radical design changes, and considering how quickly they lose heat through their relatively small area they can have a considerable effect. Houses in northern regions are built with highly insulating walls and require the higher end windows, and have ceiling insulation requirements of upwards of R-40 to keep all the heat inside in the winter.
It’s easy to put insulation inside a wall, since you just make the outside part pretty – no one cares what is inside. This means walls are (usually) pretty good at keeping in the heat.
Windows on the other hand don’t have insulation. As such, this is where heat leaves the house. If it’s just a single glass heat can rather easily go through it. Now, air is actually a great way to insulate, so two pieces of glass with air between works great to keep heat inside the house.
So the little layer of air between the glass is actually the insulation, it has nothing to do with having more glass – that’s just a way for us to trap the air while keeping the window transparent.
Wall insulation actually also work due to air – here we use materials that trap a bunch of small air bubbles.
Basically you are trapping a layer of air which doesn’t move as an insulating layer, air when it isn’t moving is a very poor conductor of heat so trapping air in-between works well.
Because the walls are generally well insulates. Windows are your weak point and that’s where the most heat gain or loss comes from. The 2 layers of glass is filled with argon in between them since it’s an inert gas which is much less thermally conductive than air.
The windows are always the weak spot when it comes to thermal insulation, so the walls staying the same doesn’t matter [in regards to this question, that is – of course, insulation for walls is important as well].
With double glazing, you are trapping a layer of air (and sometimes even vacuum) between the two panes of glass, which helps slow down heat diffusion by quite a bit.
*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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