Dear Umbra,

I just bought an old house and need to replace some of the windows. Are there alternatives to vinyl windows that will still cut down on heat loss?

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Noah Winer
Philadelphia, Pa.

Dearest Noah,

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I hope I’ve caught you before you’ve placed your window order, because you are at a moment of opportunity. Window replacement will make a huge difference in your house’s energy efficiency. You could be losing up to a third of your heat through those old windows, and I’m sure they’re not helping with cooling either.

What a pane.

Here are the basic window-shopping questions: Does your climate require keeping heat out, or in, or both, and which side of the house needs new windows? In light of these concerns, a window’s efficiency actually has less to do with its vinylness than with other factors such as glazing, design, and installation. (But don’t buy vinyl windows — ick. Try wood instead.)

First, there’s glazing. This refers to whatever material fills the window frame. Usually for us this is treated, coated, or doubled-up glass. Modern glazing does amazing tricks, like letting some types of light rays through and blocking others, or letting all light into the house but only some heat-type rays back out. Double-paned windows have space between the panes, often filled with argon gas, which is a better insulator than plain old air. You may find the potential choices dizzying, but don’t despair.

The basic type of window also plays a role. Fixed-pane (non-opening) windows obviously give less opportunity for wind to whistle through than casement (side-hinged) windows. Casement windows, in turn, usually have a tighter compression seal than double-hung windows (the kind with one or two sashes that you lift up and down, found in most old houses).

How to differentiate between all these options? Fortunately, the National Fenestration Rating Council has developed a ratings system. You’ll learn all about the numbers relating to energy efficiency: the U-factor or U-value, which measures thermal transmission; the Solar Heat Gain Coefficient, which measures how much sun slips in to roast you; and rankings for condensation resistance, air leakage, and visible transmittance.

It’s truly too much to describe in this space. Read about choosing windows on the DOE’s Energy Savers website or in the American Council for an Energy-Efficient Economy’s Consumer Guide to Home Energy Savings. You are about to spend a chunk of change on excellent new windows, so get the best bang for your buck. And remember to get a good contractor. If your window is clumsily installed, all your hard work choosing the right one is shot.