If you are building a new home or looking to make your home more energy-efficient, you may be thinking about windows. Up to 30% of household energy is lost through windows, and depending on age and maintenance, losses can amount to much more.

Energy-efficient windows can be a costly investment, so understanding which windows are suitable for your home is essential and not relatively as easy as one, two, three. Here’s the rundown on energy-efficient window basics.

Getting to know NFRC and ENERGY STAR Labels

NFRC is a non-profit organization dedicated to educating consumers based on four categories of information provided in their labels. They don’t make recommendations about a product one way or the other – they simply rate and provide information about the performance of windows, doors, and skylights.

ENERGY STAR makes it easy for consumers to identify NFRC-products with superior energy performance. However, choosing a window isn’t as easy as high or low performance. Different climates or exposures call for different conditions. For example, a gas-filled window with low e-coatings would be appropriate for a colder climate but not for hotter conditions. That being stated, if you find ENERGY STAR labeled windows appropriate to your climate (i.e., part of the country) you are doing quite well. Now, here’s  where things become a little more scientific.


U-Factor is the measure of heat gain or loss through a window – in other words, how well a window can keep heat from escaping in the winter or entering during the summer, with values ranging from 0.15 to 1.20. The lower the number, the better the window’s insulating abilities, including its frame as well. .

Solar Heat Gain Coefficient (SGHC)

SGHC measures how much of the sun’s heat comes in through a window. The lower the number, the less heat that will enter the home. Typicially a lower number iss better, to keep more heat out during the summer but if you have a home with windows facing south, with a good overhang to shade those windows in the summer, you’ll want to consider a higher SHGC on those south windows to bring more free heating in on sunny winter days.

Visible Transmittance

VT measures how well a product does at letting light into your home during the daylight hours. The higher the number, the more natural light that gets through the window.

Air Leakage

AL measures how much air can enter a room through a window or other glass product. The lower the number, the fewer drafts will enter a room.

Understanding NFRC labels is a great place to start when deciding which windows are right for you. For example, if you live in a hotter climate and have a south-facing window, a low U-factor might be appropriate for keeping the heat outside and the cool air in. Considering where your windows will be placed and how you want them to function is key for energy efficiency.

Determining the right windows for your situation will depend on whether you live in a hot or cold climate and whether they face north, south, east, or west. Check out the table on page 2 of the Department of Energy Guide to Energy Efficient Windows for a helpful visual.

There are many other things to consider when choosing windows, doors, or skylights. For example:


The term glazing refers to the type of glass in the window. A single pane of glass is a single glaze, two panes are double glaze, and three are triple. The more glazing the window has, the more insulation it offers.


The R-value (or thermal resistance rating) is simply the inverse of U-factor and a more common way of quantifying the insulating value of window, door or insulation. Specifically, it measures the ability of heat to transfer to cold. It determines the overall insulating value of a window. Windows with an R-value of 5 or higher are the most energy-efficient windows available.

Energy-Efficient Glass

The right glass in a window for your climate can help to reduce energy costs and improving comfort by slowing thermal transfer. For example, a dual or triple-pane glass may contain a gas, such as Argon, between the panes to improve efficiency and function in a very cold climate. Glass can also be tempered, tinted, laminated, or UV protected.

Energy-efficient windows aren’t cheap, so thought and planning are necessary to avoid expensive mistakes. This is where home energy auditors can be of help. Especially, if your local utility offers free or discounted energy audits contact them to schedule an energy audit and tell the energy auditor you want to know from her/him what they know about replacement windows. Still, knowledge is power when it comes to energy-efficient planning, and no one knows your needs quite like you.

Appropriate energy-efficient windows can provide up to 12% energy savings on annual energy bills to offset the cost. Additionally, the American Recovery and Reinvestment Act gives homeowners a 30% tax credit when investing in energy-efficient home upgrades. And, remember doing your homework will pay off when it comes to replacement windows and when getting advice from a 3rd party energy auditor you will learn how urgently you need new windows or if there are other, lower cost items (i.e., insulation and air-sealing) that could be better first steps to improve your comfort, lower your energy costs and reduce your carbon footprint.