Heat-reflecting low-e glass
by Chris Woodford. Last updated: August 12, 2015.
When people wave to you through the windows of your home, you probably wave back. If so, you're using a handy property of glass that we all take for granted: it lets light pass through it pretty much unimpeded. If you have a cat, it probably curls up on your window ledges in the sunlight. If so, it's taking advantage of a different property of glass: it lets heat pass through it too. Now glass is brilliant stuff and most of the time we're happy to let light and heat stream through it in both directions without a second thought. But in extremes of summer and winter, that's not such a good idea. On hot summer days, wouldn't it be cool (pun intended) if your windows reflected back the sun's heat automatically? Likewise, in the dark depths of winter, wouldn't it be great if the windows of your home helped to stop all that expensive heat from escaping? If you have heat-reflecting glass in your windows, that's exactly what will happen. Let's take a closer look at how it works!
Photo: This conservatory glass has a distinctly blueish coloring, which tells us it's fitted with heat-reflective glass (it's also self-cleaning).
What's the difference between light and heat?
Before we go any further, it helps if we understand a bit more about light and heat and the difference between them. Both are kinds of energy, though they behave in different ways. Sunlight contains both heat and light: it's a mixture of different kinds of electromagnetic radiation, made up of vibrating waves of electricity and magnetism, that zip along in straight lines at 300,000 km (186,000 miles) per second. Our eyes can see only a very small, very limited band of all the electromagnetic radiation that buzzes around the world—and we call this the spectrum: it's the band of light you see in a rainbow that stretches from red and orange through indigo and violet.
Photo: Left: "Yellow" sunlight is made from a wide range of different light waves, but we only really notice that when we see rainbows. In a rainbow, drops of water split the different colors from sunlight by bending different wavelengths of electromagnetic radiation by different amounts. Blue is bent more than red, which is why it's always on the inside of the curve.
Light exists outside the rainbow. Take some red light and stretch its waves (increase their wavelength) and you get an even redder light our eyes can't see. That's called infrared. Take some blue light and squeeze its waves (decrease their wavelength) and what you'll have is an invisible blue light called ultraviolet. Ultraviolet is the high-energy part of sunlight that causes sunburn, but it makes up only about three percent of the light that streams down from the sky. Infrared (which makes up just over half of sunlight) is the heat you can feel on your face when you stand some way from a camp fire (or any other hot object). Heat-reflecting windows work by treating ultraviolet (short-wavelength), visible, and infrared (long-wavelength) light in different ways. Broadly speaking, they reflect ultraviolet and infrared light but allow visible light to pass straight through: they reflect heat like mirrors but transmit light just like glass. (In practice, they do transmit some infrared and ultraviolet and they do reflect some visible light.)
Chart: Right: Thanks to the ozone layer, which traps ultraviolet light, the sunlight that reaches Earth is split (very roughly half and half) between visible light (yellow slice) and infrared (red slice).
How is heat-reflecting glass made?
Heat-reflecting windows are usually sealed, double-glazed units—ones with two panes of glass separated by a noble (unreactive) gas such as argon that improves insulation (stops heat from escaping in air drafts). The inner surface of one of the panes of glass is given a very thin reflective coating, usually made from two or more layers of metal or metallic oxide (typical metals include titanium, zinc, copper, tin, and silver, and typical alloys include brass and stainless steel). The coating has to be microscopically thin if you still want to see through your windows, but you'll notice that it does give them a slightly brown or gray color. The exact form and thickness of the coating can vary quite widely from manufacturer to manufacturer. In early coatings, it was often a layer of silver sandwiched between two layers of metal oxide. But window makers such as Pilkington discovered they could get better results with a layer of silver, a layer of metal oxide (made from a metal other than silver), and a third metal oxide layer on top. This improved "recipe" seemed to reduce the emissivity while keeping the light transmission high.
Photo: How low-E glass works by treating light and heat in different ways. On hot summer days, light can pass through, but the heat from the Sun will reflect back out again. On cold winter nights, more of the heat generated inside your home will stay there instead of leaking out through your windows. Pilkington K Glass™ is perhaps the best-known type of low-E glass.
Coating the glass
Typically, the heat-reflective coating applied in one of two ways. One way is a process called sputtering, where a thin film is fired onto the surface of the glass to make what's called soft-coated glass. The other process involves heating glass to high temperatures in a chemical vapor, so the coating material condenses onto its surface. That makes what's called pyrolitic or hard-coated glass. The coating in soft-coated glass is relatively delicate (gradually eroded by moisture and air and easier to rub off), so it's used only on the inside face of sealed double-glazed units. Hard-coated glass is slightly less effective at reflecting heat though much more robust, so it can be used in single-pane windows (though that also makes it less effective as heat insulation, since the air gap in double-glazed units plays a major part in retaining heat).