If you have old, single-pane windows, you experience substantial heat loss through your windows every winter. For this reason, you might be considering a replacement just in time for cold weather to arrive.

But you have also heard that single-pane windows increase heat gain in the summer. This has you wondering: how much heat is lost through windows in the summer compared to the cold months of winter? Depending on the answer, you may want to either replace your windows in the autumn or the spring.

Heat Loss Through a Window During the Winter
When you heat your home, warmth comes from air registers or radiators. Unfortunately, a great deal of this heat is wasted if you have inefficient, single-pane windows. Even if they’re sealed tightly around the edges, a one-quarter-inch sheet of glass only has an R-value of about R1. This low heat resistance value causes heated air to cool off as it passes by the frigid glass surface separating your home from the outdoors.

As a result, even if your walls are insulated to an adequate level of R13 to R19, the heat that transfers through the glass window defeats the purpose of having a well-insulated wall.

Heat Gain Through a Window During the Summer
The heat transfer that occurs through a single-pane window in the winter also takes place in the summer, just in reverse. As your air conditioner runs, it cools each room by transferring heat outside, but your home readily heats back up as hot outdoor air passes by your single-pane windows. The air inside picks up this energy and heats your home, making your AC work harder.

Solar heat gain is also a summertime concern. When direct sunlight hits an inefficient window, heat radiates inside, warming the space excessively, increasing your cooling bills, and causing uneven temperatures from room to room based on sun exposure.

How Efficient Windows Reduce Energy Loss
If you install new, energy efficient windows, you can expect the investment to pay for itself. In fact, when you upgrade from single-pane to double-pane windows with other energy-saving features, it’s typical for heating costs to decrease by about 40 percent. Likewise, cooling costs go down by about 30 percent.

Here are the features that make energy efficient windows so superior to single-pane windows:

IGUs: Insulated glass units describe windows with two or three panes of glass. The air pocket between the panes is the key to improving insulation. This slows heat transfer to more than double the R-value of a single-pane window to about R2.
Gas infill: Heat transfer through a window slows down even more if the space between the two panes of glass is filled with argon or krypton. These invisible, inert gases are denser than air, so heat passes more slowly through them. This upgrade increases the R-value of a window to about R3. Low-E coatings: This glass treatment is comprised of a microscopically thin metallic layer that reflects UV light to keep solar heat gain at bay during the summer. You’ll benefit most from a low-E coating on your south- and west-facing windows.

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