Air leakage siphons about half of a typical home’s heating and cooling energy to the outside. Air leakage through windows is responsible for much of the loss. Well-designed windows have durable weatherstripping and high quality closing devices that efficiently block air leakage. Hinged windows such as casements and awnings clamp more closely against weatherstripping than do double-hung windows. However, the difference is slight; well-made double hungs are okay. How well the individual parts of the window unit are joined together also affects air leakage. Lowest values are greatest.

Letting in the right amount of sunlight

In a cold climate we welcome the sun’s warmth and light the majority of the time. And once we catch the warmth, we don’t need to give this up. In a warm climate, we do not need the heat, but we do need the light. Advances in window technology let’s have it both ways.

Longer wavelengths–beyond the red portion of the visible spectrum–are infrared, which can be felt as heat.

And in cold climates, they signify long-wave radiant heat back into the home, again while admitting visible light. This shorter wavelength visible light is absorbed by walls, floors and furniture. It reradiates out of them as long-wave heat energy which the reflective, low-E coating retains indoors. Low-E coatings work best in heating climates when applied to the inner, or interpane, surface of the inside pane.

And mixing low-E coatings with low-conductance gas fittings, such as argon or krypton, boosts energy efficiency by almost 100 percent over clear glass. Argon and krypton are safe, inert gases, and they will leak in the window with time. Studies indicate a 10% reduction over the course of 20 years, but that will cut the U-value of the device by just a couple of percent. The extra cost for low-E coatings and low-conductance gas fittings is just about 5 percent of the window’s overall price. It is a no-brainer.

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