You should find a window’s U-factor and SHGC measures on a label developed by the National Fenestration Rating Council (NFRC). Don’t buy a window that lacks this label. Window manufacturers aren’t required to test their windows for, or display on the label, scores on air leakage. Testing and reporting on this measure is voluntary.

If you want to minimize your home’s energy usage regardless of cost, then (as the figure below shows) your best options are windows with non-metal frames and triple panes. But most homeowners will want to do a bit of cost-benefit analysis: Among windows you’re considering, it makes sense to compare the energy efficiency of each option. Calculate how much money you’ll save by installing more efficient windows and compare that with the extra cost of buying upgraded windows.

You can use several resources to do this. Many window suppliers use computer programs to estimate the energy cost consequences of windows with different efficiency ratings.

On the website of the Efficient Windows Collaborative, the Window Selection Tool tells you how most window types will affect your utility bill. Using information provided by this tool, on the figure below we illustrate the effect of several window options on how much you’ll save or how much more you’ll pay to heat and cool your home. As you can see, some window types and features produce more energy savings than others.

Despite what window suppliers might tell you, even if your current windows are extremely inefficient, you’re unlikely to save enough money from lower utility bills to offset the cost of the project. And there are other issues to consider. For example, the extra energy savings yielded by triple glass compared to double glass might require you to accept lower light transmittance, greater visual distortion, heavier and harder-to-move sashes, increased risk of breakdown of seals and the resulting condensation between panes (because there are two sets of seals rather than one), and less attractive grids (because grids placed between panes must generally fit into a smaller space in the triple-glazed windows). But many homeowners will want to minimize their home’s energy usage, regardless of these downsides or a cost-benefit analysis.

In addition to thinking about energy savings consider—

  • How much you value the increased comfort and improved appearance the windows will provide
  • How much the window improvements are likely to increase your home’s resale value
  • How many years you expect to live in your house

For more information on window-related energy savings and many other aspects of window purchases, check the Efficient Windows Collaborative website.