A Guide To Buying Energy Efficient Windows.


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Windows are one of my favorite topics. I love windows. I have lots of them. I like to feel like I am outside. If I had 100 acres, I’d build a glass house right in the middle. Windows today are much more efficient than they were ten years ago. If you have the inclination and the means to replace your old ones, do! When I remodeled two years ago, I replaced about 2/3 of my windows, and it made a huge difference in my heating bill.

Today’s energy efficient windows are made for the different orientations in your home. Some keep heat out, some let it in. Also take into consideration views you want to take in or block, natural lighting and furniture placement.

When you are shopping for efficient windows, this information will tell you about window performance:

  • U-factor rating of the National Fenestration Rating Council (NFRC) – the lower the number, the more efficient the window, based on the glass, frame and spacer material.
  • Solar Heat Gain Coefficient (SHGC) -This figure lets you know how much heat a window transmits. For passive solar gain, this number should be higher than .6. For windows on the north side that let sun in during the summer, this number should be lower to block the heat.
  • Visible Transmittance (VT) – This number, between 0-1, states how much light comes through a window. For passive solar, this number should be high.
  • Low-e / low-emissivity. A thin, invisible coating on the glass controls the amount of heat moving through it in both directions, in and out. This affects the U-factor and the SHGC. Low-e windows can save you 30-50% on your energy bills.

For passive solar applications, you want:

  • A high SHGC
  • A low U-factor
  • A high VT
  • Low-e windows

For northern orientation, you want:

  • A low SHGC
  • A high U-factor
  • A low VT
  • Low-e windows

Talk to a reputable energy efficient window distributor, read the labels on the windows, and check out these websites for more information – www.energysavers.gov and the website of the NFRC, www.nfrc.org.

Guest post by Nan Fischer, a Certified EcoBroker specializing in green real estate in Taos, NM. Follow her on Twitter for a daily green news feed, www.twitter.com/nan_fischer. Nan writes about green building, solar energy and the environment on her blog, www.desertverde.com. .

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  1. “For northern orientation, you want:…A high U-factor”.

    Are you sure? Other things being equal you almost always want a low U-factor.

    Generally speaking a well designed building will have a smaller area of windows on the north (or, to be inclusive of the southern hemisphere, the poleward) face so U-factor may well be less critical than other considerations but still a higher U-factor is not desirable in itself.

  2. Don’t forget about the new energy credit from the government. You can get up to $1,500 in tax credit back!

  3. I want to buy a good quality window. All the companies tell me that what they sell is the best. So how can I make a decision considering quality price warranty and labor?

  4. Ed, that was an error/typo/carelessness on my part! Thanks for the correction for everyone.

    Patricia, do your due diligence. Read the labels on the windows, check with the NFRC, http://www.nfrc.org. for more info. When I remodeled a couple years ago, I put in vinyl windows from Lowe’s due to my budget. They have cut my energy bills enormously and were not expensive. The labels had everything I needed. Ask a lot of questions, and bypass the sales pitch. Hope that helps!

  5. Nan–this is good information, and sorry I didn’t see this post sooner, but amidst the current barrage of window manufacturer advertising, it is also important to note a few things:

    1. Windows, even the most energy efficient on the market today like those from Serious Materials, are still typically the weakest link in the overall building envelope.
    2. Windows are in a state of rapid innovation right now, and many experts agree that their energy efficiency properties will improve dramatically in the next several years.
    3. Quality windows are expensive, often at more than $1000 each.
    4. For bang for the buck, there are almost always more cost effective ways to permanently reduce energy use–like air sealing and insulation.

  6. Thanks for sharing! It is quite interesting as well as informative especially for people planning to build a home in hot places like the south Asia. We discussed the role of windows in our new properties across India and its surprising how these slabs of glass can contribute to a greener way of living and doing business especially since we are a group of eco-hotels. Some of the technicalities should become part of common knowledge among builders so that we see smarter use of windows in the future!

  7. I have a new sliding glass door that I chose for a sunny summer wall. When it is closed, and I am standing inside in the sun coming through it, I don’t feel any heat. The sun lights up the room, but it does not add any heat that would increase my cooling bill. I have an old window near the slider, and I do feel the heat when I stand in the sun it lets in. I will keep a curtain closed on this window until it can be replaced.

    So, yes, you can buy windows that will keep the heat out for a climate like India. Builders need to be educated on green building materials, no doubt!

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