Windows and Energy Efficiency

Thu, 2012-09-06 08:02

Everybody wants the corner office and the room with a view. Natural daylight and beautiful views are desirable features in any living or workspace. Windows keep us connected to the outside world. Exposure to light is important to our overall health and well-being. But, when it comes to energy efficiency and comfort, windows are often singled out as the cause of a home performance problem and an area needing improvement. What does this mean for energy efficient design and our beloved windows? Can we get the light without the heat in the summer?

In terms of green building and energy efficiency, windows rank fairly high in terms of technological complexity. Windows are a key component of the building envelope, helping to regulate what comes in and out. Invisible glass coatings, spaces filled with inert gas between the panes, improved framing materials, and better weather stripping are all advances that help to reduce undesirable heat gain and loss. Thanks to these advances and proper installation, it is possible to have views, natural light, and an energy efficient building envelope too!

Understanding the technology behind energy efficient windows, and how they are rated, can shed some light on the impact windows have on the effectiveness of a building envelope. The rating system is a good place to start. Building codes, the LEED rating system, and ENERGY STAR rely on energy performance ratings provided by the National Fenestration Rating Council (NFRC) to evaluate the performance of a window.

The NFRC is a non-profit organization that sponsors the rating and labeling to compare the performance of windows. The ENERGY STAR program advances the rating a step further. As part of the program, the U.S. Department of Energy has set specific standards for superior performance, taking climate zone into account. High performance windows are part of the “complete thermal enclosure system” that is required for a home to achieve the EPA’s ENERGY STAR Home certification.

What do the NFRC ratings for windows really mean?

NFRC Sample LabelThe NFRC label lists manufacturer, describes the product, provides a source for more information, and provides performance ratings in 5 categories:

1. U-Factor measures the rate of heat transfer to indicate how well a window insulates. The lower the U-Factor, the better the insulation.

2. Solar Heat Gain Coefficient (SHGC) measures the fraction of solar energy transmitted and indicates how well the window is blocking heat caused by sunlight. The lower the SHGC, the less solar heat the window is allowing through.

3. Visible Transmittance (VT) measures the amount of light the window lets through. The higher the VT, the more light you will see. A window with a high VT will look clear and be easy to see through.

4. Air Leakage (AL) measures the rate at which air passes through window joints. AL is measured in cubic feet of air passing through one square foot of window area per minute. The lower the AL value, the less air will pass through.

5. Condensation Resistance measures how well the window is able to resist moisture build up. The higher the score, the less build-up the window allows.

In addition to reading NFRC ratings carefully, it is worthwhile to understand the technologies related to window framing, glass coatings, spacer materials, gas fills and the best practices for installation. Where and how a window is installed can determine if the product will perform to its full potential. But, those are all topics for another day. If you are interested in learning more about energy efficiency and how various components of a home interact to produce an overall performance rating, check out Everblue’s RESNET HERS Rater Training and Energy Star v3 Training.

By Amy Malloy

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