Green Window Voodoo

During December’s holidays I got behind on reading the paper, choosing instead to grab just enough news headlines to stay “aware”. Consequently, I missed an article on a local green building project featuring “special windows.” When people ask about the cost of green building, windows are a good example of where the difference in cost could be great or small. Today somebody called me about it.

“It said they’re using special windows in affordable houses,” my friend relayed enthusiastically. Special windows? Several features contribute to a window qualifying as “green”. What’s the voodoo that makes them special and what makes them affordable?

I retrieved the article from the online archives and found the feature my friend was referring to described “special windows that filter the sun’s rays.” That’s a clue, but not an answer unless you know why the sun’s rays are a factor. Control of the sun’s radiant heat for energy efficiency is the issue.

Heat moves toward cold so whether you want the heat to be inside or outside, the windows must do a job. Reflection of the sun’s heat during the cooling season is an attribute that makes a window green, while retaining heat inside when temperatures are cold outside is the heating season feature. These aspects are measured as solar heat gain and heat transference. You want windows with good rating in these aspects.

Coatings added to glass reflect heat and are more effective than just tinting to keep out the sun’s radiant heat. Tinted glass may or may not be coated, but even tinting alone is better than clear. In Florida, radiant heat from the summer sun coming in through windows is the greater energy efficiency concern. The measurement of the Solar Heat Gain Coefficient (SHGC ) is the specification that matters most in south Florida. Coated and tinted windows cost a little more than clear glass, but they are affordable and they will make a difference in your energy bill.

Where climate turns very cold, measurement of heat transference is key and the measurement is called the U-factor. Windows with double or triple panes of glass that have inert gas between the panes are highly efficient in maintaining indoor temperature against freezing outdoor temperatures. But, if you’re adding double paned gas-filled windows to your home in south Florida, you’re paying a lot of money for a feature that will take a long time to recoup the added expense.

Whether you are building a new home or looking to replace windows in your current home, understanding the key elements of solar heat gain and heat transference will help you make the right decision. The Florida Energy Efficiency Code sets minimum requirements for SHGC and U-value. Even so, you should review the window specifications and confirm your product by looking at the label on the windows you buy. “Special windows” will help you toward owning a green home.