Ratings play factor in how windows save energy


Windows have been around a long time. The origin of the word is centuries old and the original “wind eye” was nothing more than an unglazed hole in the roof that provided ventilation and day lighting.

Today’s windows have come a long way since then. Good windows add much to the enjoyment, comfort and value of a home, but bad windows … well, let’s just say they can be more like a “wind eye.”

In past columns, I’ve covered the importance of window placement when building or selecting a home. Most of us have to deal with what’s already in place, but that’s OK. There are a lot of options.

When thinking about energy efficiency, many homeowners naturally think about upgrading their windows. It’s a natural assumption but not always the best choice. For serious retrofits, a quality home energy audit is usually the best place to start since changing out all the windows in a typical home can be expensive. If replacement is deemed to be the best option, it helps to understand some of the terminology of window-speak.

Windows are rated based on several aspects of their performance. Most people have heard of low-e windows. The “e” stands for emissivity which is the amount of heat that can pass through. Insulating value is expressed as the U-factor. The lower the number, the better the window will block the transfer of heat.

Solar Heat Gain Coefficient (SHGC) measures the amount of solar heat that can pass through a window. Visible transmittance (VT) indicates how much light comes through.

These ratings affect each other and there is no such thing as one-size-fits-all. A finely tuned window strategy might use differently rated windows in different areas of the same building, depending on exposure to direct sun, orientation, building design, microclimate, etc.

For example, south-facing windows can provide warmth on sunny winter days, so a higher SHGC can make sense, especially if the building design provides some shading of the window during the hotter months. East- and west-facing windows with low SHGC can reduce the effects of direct morning and afternoon sun. Northern windows that get little direct sunlight can utilize a lower U-factor for better insulation properties.

There are options for existing windows too. The basic rules of thumb are: Reduce air infiltration and whenever possible, reduce the amount of direct sunlight on windows during the hot months. Infiltration can be improved be replacing seals or caulking around frames and glass. Sunlight can be controlled with awnings, vegetation or, my least favorite option, solar screens (they permanently block too much light for my taste).

Transparent window films are available that can reduce solar heat gain and some offer low-e features as well that can reduce heat loss. Interior shades and blinds can provide insulation and shading, but, remember, once the sun hits a window, heat happens inside the home. To get the most out of your window coverings, adjust them throughout the day depending on the position of the sun and other outdoor conditions.

This is the time of year when we can take advantage of the changing weather. As days get longer and begin to warm, opening windows at night can cool down the house naturally. Openings on each side of the home can set up a nice flow of cool, fresh air. As the day warms, close things up and adjust awnings or blinds to prevent or insulate against the heat gain of direct sunlight.

I think of the windows in my home as a system, tools that when used properly can provide comfort and enjoyment while reducing energy costs. A sailor raises and trims his sails to make the best use of nature’s energy. We can do the same with our homes.

Steve Rypka is a green living consultant and president of GreenDream Enterprises, a company committed to helping people live lighter on the planet. For more information and links to additional resources relating to this column, or to reach Rypka, please visit www.greendream.biz.

 

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