Energy Efficient Windows: What to Know Before You Buy

With so many options to choose from, buying windows can be a tough job. Learn how to buy windows, with special attention to energy efficient windows.

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Meet the Expert: Steve Urich is the Senior Director of Programs at the National Fenestration Ratings Council.

When you see all the options and add-ons available for your windows, you’ll realize they provide more than a nice view and some fresh air. In this article I’ll outline the important terms and numbers you need to know to make informed window buying decisions. For guidance I spoke with Steve Urich, a windows expert and senior director of programs at the National Fenestration Ratings Council.

Choose windows that suit your climate and location

When selecting windows for our Getaway, we favored a modern look. Beyond style, we needed windows to work in our climate, and for that we learned the language of window shopping.

For in-depth, location-specific information for your house, use the window-choosing tool from the National Fenestration Rating Council at efficientwindows.org. What’s a fenestration, you ask? Well, according to Merriam-Webster, it can mean the arrangement, proportion and design of windows and doors in a building, or it can simply refer to any opening in a surface, such as a wall. 

Fh21sep 613 50 06 M01Courtesy of National Fenestration Rating Council

Coatings

A low-E coating is standard on most windows. It’s transparent to allow in all the light, but it reflects heat. In warm climates, it’s applied to the outer glass to keep heat out. In cold climates, it’s applied to the inside glass to retain heat.

Insulating glass

Insulating glass adds another layer of energy efficiency and comfort to your living space. Most new windows have two panes of glass with gas – typically argon – between the panes.

U-factor

U-factor tells you a window’s rate of heat loss. The lower the number, the better the window will keep heat in. Because our Getaway is in a cold climate, our windows have a U-factor of .27, which meets Energy Star ratings for Northern zones. A higher U-factor number requires an adjustment to a window’s solar heat gain coefficient (see below). 

Solar heat gain coefficient

Typically a bigger concern in warm climates, solar heat gain is also worth considering here in Wisconsin where we have seasonal heat. The lower the number, the better the window is at blocking heat from the sun.

Most people buy windows with the same ratings for the whole house, but it’s possible to fine-tune your windows to better suit your needs. For example, south-facing windows would block more heat from the sun than north-facing windows. Some people even go as far as buying two sets of sashes for their windows and change them seasonally.

Visible transmittance

Expressed as a number between 0 and 1, visible transmittance signifies the fraction of the visible sunlight spectrum allowed through a pane of glass. The higher the number, the more light the window lets in. Generally, most people prefer to let in all the light they can, so be aware that a lower solar heat gain coefficient typically means a slight reduction in visible transmittance.

Air leakage rating

There’s nothing to be concerned about here. Most new windows will have good air leakage ratings.

Condensation resistance

The higher this number, the better your windows are at resisting condensation. Bear in mind this is greatly affected by interior relative humidity.

What should you pay for?

You can choose plenty of add-ons for your windows, but you’ll need to decide what’s worth the price.

  • With windows, price is not always an indicator of performance. Shop around, using the selection tool noted above. You can find quality windows below the top price tier.
  • Stock sizes or custom? You might think you can save a little money buying in-stock sizes from a home center. Most manufacturers, however, build windows to order, so the idea of “stock sizes” isn’t always relevant. I’d just order the sizes I want instead of just grabbing what’s available at the store.
  • If you’re thinking about upgrading to triple glazing to save on energy bills, consider it carefully. It’ll take a long time to recoup the expense. In most cases, double-glazed, insulating windows are sufficient. If you enjoy sitting by the windows in the winter, however, compare the price of double-glazed to triple-glazed and decide if the price difference — sometimes as much as 30 percent — is worth it.
  • Grilles, high-end hardware and exotic wood species are decorative only, and cost extra. You’ll need to weigh the look you want against the added cost. Be aware that grilles might raise the U-factor number slightly.
  • Having windows pre-painted does cost extra, but it’s a huge time saver. If I had to choose one thing from this list to splurge on, that would be it.