What is Low-E Glass?
Low-E glass is a term that’s commonly thrown around by window professionals, but for the layman, what is Low-E glass and what’s so special about it? Low-E glass is short for Low Emissivity Glass, which is characterized by its ability to reflect heat rather than let it through. Low-E glass uses a certain type of coating that enables it to have such properties, and it’s a great option if you’re concerned for the environment or want to save on your utility bills.
Materials Used in Low-E Glass:
The main material that gives Low-E glass it’s emissivity properties is a microscopically-thin, transparent layer of metal. This metal is typically silver or tin, and is sometimes combined with other additives that enhance the heat reflection properties of the coating. The first thing that might come to mind when hearing about metal coated windows is their transparency, but it’s nothing to worry about, as the metal layer is too thin to reduce a significant amount of visible light, sometimes reaching as low as 1/10,000th of the width of human hair.
How Low-E Glass Works:
To understand how Low-E glass blocks heat and allows light to pass through, you’d first need to know what the solar energy spectrum is. Ultraviolet light, visible light, and infrared light all reside within the solar energy spectrum, and each make up 3%, 44%, and 53% of the solar spectrum, respectively. Below is a short description of infrared and ultraviolet light:
- UV light: a type of high powered electromagnetic radiation that ranks right below X-rays in terms of strength; responsible for sunburns, accelerated skin aging, and the fading of interior materials such as curtains and furniture.
- Infrared light: a lower-powered type of electromagnetic radiation that isn’t visible to the human eye; most heat is emitted in this wavelength range; is harmless.
To understand what all of this has to do with heat without getting into Einstein’s Mass-Energy Equivalence Theory, heat is simply a form of energy, and most bodies (ex: the sun or your body) emit heat in the infrared spectrum. The main playing field of Low-E glass is in the ultraviolet and infrared spectra, and it thus prevents heat escape/entrance by preventing a significant amount of infrared radiation from passing through.
As indicated by the graph above, all types of Low-E glass allow the majority of visible light to pass through while preventing a large amount of infrared from doing the same. There are the two, distinct types of infrared radiation that radiate towards your house though; one is directly from the sun while the other is reflected off of other objects such as your driveway or your neighbor’s house. The infrared radiation that comes directly from the sun has a wavelength of about 0.8-1.2 microns and is stronger than reflected infrared radiation. On the other hand, reflected infrared radiation is typically long-waved, with a wavelength of around 1.5-2.5 microns long. (Longer wavelengths are weaker, while shorter ones are stronger.) All types of Low-E glass do a great job at reflecting short-wave infrared radiation, but different types excel at reflecting different wavelengths. We’ll expand more on the types of Low-E glass later on in this article.
During the winter, any infrared radiation (heat) generated by your heater will almost exclusively be long-waved, meaning that all types of Low-E glass will bounce the infrared back inside and prevent it from escaping, keeping your house warm and cozy. In the summer, the same thing happens. The sun heats the outside environment (air, roads, sidewalks, etc…) and long wave infrared radiates towards your house. Once again, the long-wave infrared will be reflected towards the outside, keeping your house cool and efficient.
Low-E glass also prevents the majority of UV light from passing through. This helps protect you and your family from the harms of UV light that can range from premature skin aging all the way up to skin cancer. UV light also causes fabrics, materials, and décor to fade, so preventing UV light from entering your house will prolong the life of your furniture, wallpapers, and curtains.
Another benefit of Low-E windows is that they prevents condensation on the glass during winter. Due to Low-E glass’ coating, windows will be significantly warmer during the winter than they would if they used regular glass; this decreases or even eliminates condensation, especially if the window uses more double panes. This also prevents mold and mildew growth around your windows, as they need humidity to survive.
Types of Low-E Coatings:
Currently, there are two main types of Low-E glass that are used: the Pyrolytic Hard Coat and the Sputtered Soft coat. These names refer to the manufacturing process that each one is made with and the durability of the Low-E coating that’s on each type of glass.
Pyrolytic (Hard) Coat Low-E:
The pyrolytic process uses metal oxides as the main element in the coating, which is typically tin, zinc, or silver–all three of these materials have nearly identical performance levels. The metal oxide is applied to semi-molten glass sheets through a process called chemical vapor deposition. The end result of this process is “baking” the metal oxide onto the sheets of glass, causing the coating to be extremely durable–hence the name “hard” coat. Once put into windows, this type of coating can withstand being scratched or scuffed off of the glass, which leads many manufacturers to make single layer glass windows out of it. Pyrolytic Low-E glass can also be cleaned with traditional glass-cleaning products and methods without being damaged or having its emissivity properties affected.
Sputtered (Soft) Coat Low-E:
The process in which sputtered coat Low-E glass is made is a bit more complex than the pyrolytic’s, and is referred to as Magnetron Sputtering Vapor Deposition or Cathode Vapor Deposition. During this process, the coating is loaded into a high-voltage electric circuit while process gases are fed into a vacuum chamber in which the glass sheets will pass through. Inside this chamber, an ion discharge takes place and interacts with the to-be-sputtered Low-E coating, which then evenly sputters across the glass sheets. The end result of this process is a layer of deposition that is 1/10,000th of the thickness of a human hair. This process is typically repeated multiple times, with each layer increasing the desired properties of the glass. Due to the nature of this process, the Low-E coating can be applied to plastic sheets as well. In certain cases, these plastic sheets can then be applied to the glass of existing windows to serve as a cost-friendly alternative to replacing the windows altogether.
Sputtered Low-E glass can’t withstand chemical or mechanical stress like pyrolytic Low-E can though. This means it must be installed in at least a double pane window with the coated side inwards alongside an inert gas that fills the space between the two panes, which will prevent anything from damaging the coating. Combining Sputtered Low-E glass with an inert gas such as argon or krypton also increases the windows’ insulating properties, which makes sputtered coat Low-E Glass an even more viable option in terms of efficiency.
Which type of Low-E Glass is best for you?
Answering this question solely depends on your area’s climate, because each type has specific properties that make it better at one thing over the other.
- If you live in an extremely cold climate, then Pyrolytic Low-E Glass is the best option for you due to the selectivity of its coating. It has the ability to let in heat from the sun while preventing heat from inside your home from escaping outside.
The red line on this graph indicates the short-waved infrared that comes directly from the sun, which is allowed to pass through in Pyrolytic Low-E glass. On the other hand, long-waved infrared, which what your heater’s heat manifests itself in, begins to be prevented from passing through, which indicated by the yellow line on the graph. This will allow any heat that comes from the sun to enter your house while keeping any heat inside. For this reason, pyrolytic Low-E glass is also called High-Solar gain.
- If you live in a cold to hot climate–which is the majority of the United States–then Sputtered Low-E glass would be the best option for you, because it prevents heat from both entering and escaping your house.
Both the red (infrared from the sun) and yellow (infrared from your heater) wavelengths are significantly blocked from passing through. This will provide you with all-year round insulation, keeping your house warm and cozy during the winter, and cool and efficient during the summer.
All in all, Low-E glass is an amazing option that is bound to pay off in the future, whether that be through decreased utility bills or decreasing the pressure on Mother Nature. Besides that, it will also prolong the life of the interior of your house and your family’s health through blocking harmful UV rays from entering your house, all while keeping your home’s temperature just where it needs to be no matter what season it is.