I don’t know about you, but I’m a bit skeptical when people say that having a small amount of air in between two panes of glass will significantly increase insulation and decrease heating costs. Does it make sense to spend a few hundred dollars on windows that simply have some air in between glass panes?
It turns out that double-paned windows actually make a huge difference. That extra little space of air is great insulation and helps prevent a ton of heat from escaping; but, why?
A guy named Fourier (some of you may know of Fourier transforms… same guy) came up with a law that models conduction. Conduction is simply heat flowing through matter (think of electrons moving through a wire). The law is as follows:
Fourier’s Law: Qconduction=-k*A*dT/dx
Qconduction is the rate of heat transfer
A is the cross sectional area
dT is the temperature gradient
dx is the distance of separation
and k is the thermal conductivity
For matters of explaining why double paned windows actually work we will only be focusing on k. “Thermal conductivity” simply means how well a substance transfers heat. A high number means the substance is great at transferring heat and a low number means that a substance is poor at transferring heat. For example a diamond has a k value of about 2,300 W/(m*°C) whereas water has a k value of 0.61W/(m*°C).
What about air? This is where it gets a little tricky. Moving air is a fairly decent heat conductor because moving air partially transfers heat through convection (ever heard of a convection oven?). However, still air is actually a really poor conductor of heat. In fact, still air has a k value of 0.026 W/(m*°C). That’s really low! So that little bit of air between two panes of glass makes it significantly harder for heat to move through the window.
[Image Credit] Andersen Windows