Energy-efficient windows are becoming increasingly popular. Consumers are looking for options that allow them to cut their bills and reduce their impact on the environment.
Not all window technologies, however, are the same.
In this post, we explore the meaning behind terms such Low-E, argon gas, and krypton gas you inevitably come across while shopping for windows.
What Are Low-E Windows?
Low-E stands for “low emissivity.” It refers to a special type of treated glass designed to reduce the amount of infrared and ultraviolet light entering your home, without sacrificing on visible light.
The sun emits a vast range of electromagnetic radiation. Some of it is the visible light you can detect with your eyes. But much of it is in the infrared and ultraviolet bands which you can’t see, but still heats your home.
For homeowners in some areas, infrared is a problem. As it penetrates regular glass windows, it heats up surfaces it interacts with, increasing cooling costs. UV is also problematic. The energy it imparts causes heating, plus it also damages occupants’ skin and interior finishes (such as painted surfaces).
Low-E glass, therefore, is helpful. In the summer months, it prevents some of the light outside the visible range from entering. And, in the winter, its high infrared reflectivity helps to keep heat inside your home, lowering your energy bills.
You can measure the quality of low-E glass via the following metrics:
- U-Value: The U-value is a rating that reflects the degree of heat loss the window allows. The unit of measurement is W/m2K which stands for watts per meter squared Kelvin. Also known as “thermal transmittance,” it helps to capture the rate of heat transfer through a structure, divided by the difference in temperature across the structure. So, in this case, the heat difference would apply to the inner and outer layers of the window.
- Solar Heat Gain Coefficient (SHGC): The SHGC measures the fraction of insolation (sunlight) the window allows to penetrate into your rooms (including light in the infrared and UV bands). The lower the SHGC, the better the glass can shade and reflect energy from the sun, reducing cooling costs in the summer. The higher the SHGC rating, the more effective the glass will be at collecting solar radiation, making it better for heating homes in the winter.
- Visible Light Transmittance (VLT): VLT measures the fraction of visible light the glass permits to enter. Defined scientifically, visible light encapsulates all electromagnetic radiation with wavelengths between 390 and 780 nanometers. Typically, clear glass has a VLT of around 0.75 to 0.92, meaning that it allows 75 to 92 percent of regular light to enter the building. Colored glass can have VLTs ranging from 0.15 all the way up to 0.85, depending on the hue and tint.
Good Low-E glass, therefore, should have a relatively high VTL, but low U-value and relatively low SHGC.
What Are Argon Gas Windows?
Glazed windows work by sandwiching gas between two panes of glass. Gases are good insulators, so when incorporated into windows, they help to keep the temperature of your home constant, regardless of the weather outside.
Argon gas windows use argon to fill the gap between panes. It is a top-performing insulating gas because it has molecules that move six times more slowly than regular air, reducing heat transfer. It is also odorless and non-toxic which makes it safe and convenient for home use.
What Is Krypton?
Krypton, like argon, is a noble gas, but with even higher performance. It is much heavier than argon, so its constituent molecules move twelve times slower than regular air, making it one of the best known insulators.
Kryton is also rarer than argon, so krypton windows typically cost around 40 percent more than argon windows. Some windows offer a mixture of krypton and argon gas to balance price with performance.
You typically find argon gas in windows with a space of ½ an inch or more between inner and outer panes, while krypton windows typically have a ⅜ gap (usually found on triple-glazed models).
While krypton windows are more expensive upfront, data from the Efficient Energy Collaborative suggests that they could offer savings over the long-term. Compared to an argon-filled window, each kryton version could save homeowners around $20 per year. Multiply that by ten exterior windows on the average home, and savings could reach $200 annually.
Air leakages can be a problem in argon gas windows. Over time, argon gas can escape and get replaced by air. When this happens, condensation can build up on the interior surface of window panels, especially during spells of cold weather.