“Low-E” Windows Maximize Buildings’ Energy Efficiency

by Benjamin Roman •




How does it work? A low-E window uses a microscopically thin layer of metal or metal oxide as a coating on the glass, which prevents heat from being transferred through the window. This coating can either help to keep a building cool in hot weather, or it can prevent heat loss in the cold winter months.

Low-E glass can be customized for different amounts of solar gain, meaning heat from sunlight. For example, a structure in a cold climate would benefit from a window that prevents heat loss, but admits as much heat from sunlight as possible. Low-E glass is typically transparent to visible light, so it shouldn’t be confused with the tinted or mirrored glass commonly seen in commercial buildings.

Low solar gain design for hot climates (photo: EWC)

Low solar gain design for hot climates (photo: EWC)

The performance of a low-E window is usually described as its “U-factor” or “U-value”. Simply put, this is the inverse of the “R-value” that is commonly referred to in describing insulation materials. So, whereas a high R-value (resistance to heat transfer) is a good thing for insulation, the U-value (heat flow) will be a very low number in an energy-efficient window.

Low-E glass is often combined with other design elements to maximize a window’s energy efficiency. Two or more panes of glass create added insulation, and the gaps between panes may be filled with an inert low-conduction gas like argon or krypton. And remember, the installation is just as important as the design of a window. To enjoy the efficiency benefits of low-E windows, the window frames must be properly mounted and sealed to eliminate any drafts or leaks.

(via Efficient Windows Collaborative)

Comments 7

  1. I believe they are probably one of the most efficient windows on the market today. It is my understanding from a recent article that they are replacing 26000 windows in a New York skyscraper. They are going to actually set up a mini manufacturing site on one of the floors and use all of the glass from the existing windows as a way to eliminate waste. I would highly recommend looking at this company.According to the specs I have seen they have commercial windows that have an R value of 20 and a residential window that is R12,which is almost unheard of for a window. I would be glad to send you an article when the President of Serious windows addressed our government. Great article!

  2. I am thinking of putting in windows by Serious Materials (they were one of only two companies that came up in California where I live as qualifying for energy credit). Has anyone used them? How do they like them? Thanks, Louise

  3. Forgive me for pointing out the obvious:
    The smaller the window the more likely the house will remain cool in the summer and warm in the winter. The size will limit the amount of heat entering and leaving the house. Windows that are too large equal a large utility bill. The height of a house will also determine how it will keep energy. A high roof in a hot environment will assist in making the hot air go up while keeping the cool air down. A low ceiling in a cold environment will help keep the heat in.
    It’s very simple to adapt the architecture to the environment and there are multiple examples in countries all over the world – low tech and hundreds of years old – that should be studied a little better.
    I would go as far as studying a way to adapt the wall to the weather, instead of the window. How about creating moving walls to change the size of the window to the needs of each season like a tile puzzle?

  4. For the lowest-e windows look here alpeneg.com
    Not such a sexy web site, but the windows are by far the best, effectively like quad-glazed windows with two heat mirror films between two outer glazed layers. I used them here: netzeroenergy.org and I did a lot of research before purchasing, and very pleased with them. They are about as effective as a 2×4 wall, but let the light in!

  5. We used low-E windows with a U factor of 31 on our new house in NW Arkansas. Tremendous results so far. I’m cataloging on the strategies I used in the construction of my home on my website. After foam insulation and a high efficiency heat pump, windows were the next most important part of our building project.

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