Energy Efficient Windows

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Photo: _Circa_

I love windows. There is very little wall space in my house for art! Not only do I like natural light during the day, I also like to bring the outdoors in.

I installed a few windows about 12 years ago, when I first moved into this old ranch house. There were huge expanses of sunny, blank exterior wall, so I decided to bring in the winter sun as passive solar supplemental heat.

I’d found a local warehouse full of brand new double pane windows that a couple of guys had bought at an auction. The prices were amazing as they tried to sell out and move. I felt very fortunate! I had sun, heat and more views for little expense!

Those windows still do their job today, but there are much more advanced and efficient windows on the market now. When I remodeled in 2007, I replaced a lot of my older windows with vinyl low-e windows. What a difference in my heating bill, and the summer sun does not heat the house up!

Low-e windows

Low-e stands for low emissivity. A thin coating is applied to the glass, and it reduces infrared radiation and the transfer of hot to cold. Think of it as a reflective barrier that bounces heat away from the window and back to its source. The coating is applied on the inside in cold climates to keep heat in, and on the outside in hot climates to keep hot air out. It is also used on the outside for east or west facing windows and unshaded south facing windows.

The cost of low-e windows might be 10-15% more than a traditional window, but they can save you 30-50% on your utility bills. As fuel prices rise, low-e windows will have a higher and faster ROI (return on investment).

Low-e coatings are applied to new windows during manufacture, but you can buy low-e film at a home center and apply it to your existing windows. It has a 10-15 year life span and does not peel. It’s a less expensive alternative to window replacement.

The benefits of energy efficient windows

The benefits of high performance windows are the same as all energy upgrades:

  • Save money
  • Save resources
  • Increase your comfort
  • Reduce your carbon footprint

Efficient windows will reduce glare and fabric fading, and will produce little or no condensation.

How energy efficient windows are rated

The National Fenestration Rating Council (NFRC) is a non-profit organization that has created a window rating system based on whole product performance. Windows are labeled with that information:

U-factor – 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 low U-factor
• A low VT
• Low-e windows

As you can see, you want different ratings for different orientations. Notice how much direct sun you get through your various windows and what you want to the sun to do. Do you want it to heat the house? Do you want the light but not the heat? These things will determine which windows to purchase.

Energy efficient windows: what next?

Talk to a reputable window distributor, read the labels on the windows, and check out these websites for more information – and the website of the NFRC,

Go to the Database of State Incentives for Renewables and Efficiency to find out about federal and state tax credits. Also, check with your local utility for incentives, funding and tax credits. These incentives can bring down the cost of new windows considerably!

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Nan Fischer

Nan Fischer has been living and building green for over 35 years. Nan’s emphasis on the BuildDirect blog is about how to make your dollar stretch further, while also moving toward a more sustainable lifestyle, as well as upcoming and existing technology to help us live in an ecologically-friendly way. Nan also authors posts on the website of her seed business, sweetly seeds.