• Category:
    Energy Efficiency
  • Published:
    March 10, 2017

Energy Efficiency Series: Benefits of Energy Efficient Windows

Windows can provide natural light, warmth from the sun, and be decorative additions to a room. Windows can also negatively impact a home’s energy efficiency, however; they’re a prime spot for air leaks, for instance.

The good news is that energy efficient fixes exist. Adding storm windows can improve window energy performance ratings. So, too, can properly weatherizing your windows. But the best way to guarantee your windows’ energy efficiency is to have the most energy efficient windows in the first place. Here’s some information on various window types to help you decide if different windows might help lower your energy costs.

What Are Window Performance Ratings?

Before comparing specific window types, it’s important to understand how window performance ratings work so you can buy the most energy efficient window, no matter what type of window you ultimately think best fits your home and your needs. offers some pointers on window performance ratings:

  • The NFRC label. The National Fenestration Rating Council has a voluntary program that tests, certifies and labels windows (as well as doors and skylights) based on energy performance ratings. It’s one way to compare the most energy efficient windows. Making sure any window you choose has this label is a good start toward ensuring it’s energy efficient.


  • ENERGY STAR-certified windows. ENERGY STAR is a government program that tests a variety of appliances and home goods, including windows, and goes a few steps beyond the NFRC. While all ENERGY STAR-certified windows include the NFRC label, those with certification have had additional screenings to receive their window energy performance ratings. ENERGY STAR bases its qualification only on U-factor and solar heat gain coefficient ratings.

What Is a U-factor? defines U-factor as the rate at which a window conducts non-solar heat flow. Sometimes a U-factor only refers to the glass or glazing. NFRC U-factor ratings include the glass as well as the window frame and spacer material. Here’s what you need to know when reading the U-factor on a window: The lower the U-factor rating, the more energy efficient the window.


What Is Solar Heat Gain Coefficient?

ENERGY STAR also includes the Solar Heat Gain Coefficient (SHGC) as part of its window energy performance ratings. SHGC refers to the fraction of solar radiation transmitted by a window into your home. The SHGC rating you want depends, in part, on where you live. The lower the SHGC, the less solar heat a window transmits, which makes it a better window for its shading ability; it can help reduce cooling needs by blocking the sun’s rays. A window with a high SHGC rating is better at collecting the sun’s heat during the winter. So if you live someplace that’s cooler more of the year, you would look for a SHGC rating that’s higher overall.

Benefits of Energy Efficient Windows

Now that you know a bit about how windows are rated, we’ll compare different window types to see which are the most energy efficient windows. A window, after all, also needs to fit the room you have and the style of decor, as well as the house itself.

Casement vs. double-hung windows


A casement window is a window that opens more like a door. A crank on the inside of the house opens and closes the window. A double-hung window has sashes that move both up and down and slide vertically. gives a higher energy efficiency score to casement vs. double hung windows. Casement windows typically allow less air to leak into your home because the sash usually presses equally on all four sides of the window frame and therefore seals more like a door than a window. By contrast, double-hung windows typically only fit snugly in the side tracks, which can allow air seepage along the top. You might be able to offset this with weatherstripping.


Sliding Windows vs. Casement Windows

Sliding windows, which come in single and double-sliding versions, do exactly as their name implies: The sash slides horizontally to open the window. In a single sliding window, only one sash slides; in a double sliding window, both sashes slide.

Casement windows are the most energy efficient windows in the battle of sliding windows vs. casement windows. The reason, says, is similar to the reason casement windows beat double-hung windows: They typically have lower air leakage rates than sliding windows because the sash presses consistently against the frame. By contrast, sliding windows usually don’t grip the frame as well, allowing for more air leakage.


Single Hung vs. Slider Windows

A single-hung window only has one sash — the bottom one — that slides vertically. In the energy efficiency test of single-hung vs. slider windows, gives single-hung windows the top energy efficient rating. Sliding doors have a higher probability of air leakage due to how they operate and typically fit in the frame.


The benefits of energy efficient windows go beyond potentially lower energy costs and a cozier home. Window replacements might be eligible for tax credits and rebates. That’s a win-win all around!

Click to see the full “Choosing the Most Energy Efficient Window” Infographic:


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Comments ( 4 )

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Rev. Terry L. Pergl - 3/17/2017

We are looking to replace windows in our church auditorium and would appreciate info on who we could talk to concerning energy efficient casement windows. Could you make any recommendation for a company that could make us a bid on commercial style casement type windows.

Thank you,

    Constellation Community Team - 3/20/2017

    Hello, Reverend. It’s great that you are considering energy-efficient church windows. The companies and windows available to you will likely depend on where you’re located. It might be a good start to reach out to a trusted commercial contractor with which you’ve worked in the past to see if you can find recommendations. Thank you for making energy efficiency a priority!

Judy Vest - 3/14/2017

Energy efficient windows are wonderful – if they work. Unfortunately we have a 3 year-old house and the “custom” mahogany energy windows have almost all lost the gas between the panes, are fogged and need replacement… will be for the all-time bargain price of $100,000–plus tax, plus the expense of repairing the walls around the windows and re-painting.

It’s great to be efficient but at what cost to families that have children, etc., etc. That $100,000 would pay for a couple of years of college!
Before energy companies and city/government entities start requiring builders to use things like low-e double paned windows they need to think down the road as to how long they will last and the expense that will be required to replace them. We have been told by multiple window makers that none of the products on the market today last more than four or five years.

Thanks so much!!!!!

    Constellation Community Team - 11/20/2017

    Hi Judy, we’re certainly hoping that energy-efficient window technology will get better with time, as most new energy-efficient technologies do! We’re sorry to hear that your gas pane windows have fogged. If using energy-efficient windows isn’t for you, we suggest weatherstripping your windows to create a very similar effect, and can still save you energy and money! You can find more about weatherstripping here.

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