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Understanding the “E” in Low-E Windows and Doors

Thermogram of a Passivhaus building, with trad...
Thermogram of a Passivhaus building, with traditional building in background. (Photo credit: Wikipedia)
By Gates Dearen

You’re ready to replace your windows – you’ve determined your needs and your budget – but when you start shopping, you find yourself confused by terms like “Low-e”. You’re not alone! While you may understand that it’s important to look for windows with a Low-e factor, you may be wondering what that means and why it’s important. No explanation of Low-e (short for Low-emissivity) would be complete without the definition of a few other key terms, such as “thermal radiation”, “reflectivity”, and “opacity”.

The “E” is short for emissivity, specifically thermal emissivity.  Low-emissivity windows and doors cause heat to be reflected off, rather than allowing it to collect and “seep through” (be emitted) into the home. Perhaps an easier way to think of it is to consider a surface that has a high emissivity factor: asphalt. If you’ve ever stepped into a parking lot in Florida in August, you’ve felt the effects of high emissivity. The asphalt absorbs and emits heat at a ratio .90 to .10. In other words, 90% of the heat directed at the asphalt is absorbed and emitted, and only 10% is reflected. You certainly wouldn’t want the glass in your home’s windows and doors to have that level of emissivity! But plain, uncoated glass has an even higher level of emissivity than asphalt: .93. This is why Low-e is such an important factor in choosing your new windows and doors. But how does it work? 

Thermal Radiation
Everything with a temperature greater than absolute zero (−273.15° Celsius, −459.67° Fahrenheit) emits thermal radiation. When the body emitting thermal radiation, the sun, for example, has a higher temperature than whatever is in its path, the resulting heat is absorbed – unless, as in the case of a Low-e coating, something reflective is put in place. The Low-e coating keeps the heat from being absorbed and emitted, instead reflecting it away from the glass. This technology is related to the insulation used in spacecraft to keep heat from accumulating on both reflective surfaces, inside and outside, but actually the application of the technology is more complicated in windows and doors because glass has such a high level of emissivity, thus posing the challenge of maintaining low-e while letting in light. 

Through our discussion up to this point, you’ve probably gathered that reflectivity refers to the property of objects to “bounce” heat off, as opposed to absorbing and emitting it, and is inversely related to emissivity. Metals, including aluminum, as in aluminum foil, are among the most reflective substances, but most of us wouldn’t want to coat our windows and door with foil. Instead, manufacturers of high-quality windows and doors apply coatings to reflect radiant energy, which helps to keep heat on the same side of the glass it’s coming from – outside in summer, inside in winter – while allowing light to pass through. This improves thermal efficiency (insulating properties) because radiant heat stays where it belongs, outside or inside depending on the season and the weather.

Unless you want to keep out all light – for example, through light-blocking blinds or shades that are always closed – there isn’t really a way to entirely prevent at least a small fraction of thermal radiation (heat) from coming in, or going out. Opacity refers to the “see-throughness” of objects: for our purposes, glass in windows and doors. The more opaque an object, the less its ability to let light through. It’s fairly obvious that having no windows and doors could potentially allow you to nearly completely insulate your home. Even a fully-sealed house will have some energy loss, if the occupants ever leave, allowing for air transfer from in to out and vice-versa. However, who wants to live like that, never seeing the sun shining, kids playing in the yard, birds flying from branch to branch? For those of us who like to come and go, who like to watch the world when we can’t always be out in it, or choose not to because of less-than-ideal climate conditions, coated glass gives us the best of all worlds. And the low-e coating is the best technology we have to achieve that goal.

Putting it All Together
So what, in practical terms, does this mean to you as you choose new windows and doors? It means you should look for windows and doors with a Low-e coating or glaze, which can increase your home’s thermal efficiency by as much as 100%. Glazed or coated glass is even more efficient when it is combined with the additional technology of multi-paned glass with argon between the panes. The combination of technologies maximizes the insulating properties of your windows and doors. Not only will you conserve energy – and, consequently saving money – but you will also enjoy a more comfortable home.

In this article, we explain what "Low-e" means and define the terms “thermal radiation”, “reflectivity”, and “opacity”, particularly as they relate to glass in windows and doors. Finally, we have tied it all together to show how this information can help you understand what to look for when choosing new windows and
doors. If you found this article helpful, please pass it on.

THE AUTHOR:  Gates Dearen is the co-owner of HomeRite Windows and Doors in Jacksonville, Florida. Owners Dearen and Richard Walden have been serving the building products industry in Florida for over 25 years. They know the products, the industry, the market and what adds great value to a home. Their approach is a somewhat different than others. They strive to match the homeowner with the right windows and doors for their home and budget. They know that home improvements projects can be a hassle. They strive to make the process pleasant with first-rate, energy efficient products; affordable prices; and expert, award-wining installers that employ the best practices and who respect your home as if it were their own. 

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  1. I have this low e glass in my windows and it really works great. Lowered my electric bill by 20%. They have paid for themselves in two years. Now I just save money! :D

  2. I never knew that windows went high tech

  3. Very explanatory. This article really "showed some light" on window technology. ;^D

  4. I saved this article for when my wife and I are ready to upgrade our windows. Thanks for all the technical info.

  5. The way energy efficiency works is that you can pay a little now, or you can pay a lot later.