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Understanding Insulating Glass Windows

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You walk in a window showroom, and start talking to a salesperson about energy efficiency. He explains all the advantages and intricacies of the IG unit to you. If you are like most of us, he has lost you already with that acronym. IG stands for insulating glass and it’s what keeps the heat from coming in through your window in the summer and the cold out in the winter.

What is Insulating Glass?

Insulating glass refers to glass that is made to prevent significant heat transfer into or out of a home or building. It consists of multiple pieces of glass separated by spacers made of either metal, such as aluminum, or structural foam. The space between the glass is sometimes filled with a noble gas, such as argon or krypton. Insulating glass is often abbreviated IG and is sometimes called double-glazed or double-pane glass. As technology evolves, triple-paned or quadruple-paned glass is now available as well.

Insulating Glass Units

Insulating glass is comprised of several components: multiple pieces of glass, materials that create and maintain space between the glass and any gas added to the space between the glass. All of these pieces are assembled into a single, sealed unit that holds the entire system together and helps prevent changes, especially to the air (or gas) between the glass. These complete units are called insulating glass units, or IGUs.

Low-Emissivity Glass

Your IG unit windows will typically be made with low-E glass, which has a coating that reduces the ultraviolet and infrared light that passes through a window. It helps regulate temperature (and energy consumption) within a home by redirecting heat back in the direction from which it is coming. Low-E coatings are made of microscopically thin coatings of materials that reflect heat much better than it absorbs or emits it. (For more definitions of windows and glass lingo, visit the dictionary.)

Where You Live Affects the Windows You Buy

When looking for new windows and an efficient IG unit, where you live plays a major role. For example, windows in Michigan will need to focus on keeping that cold weather out in winter while in Florida the focus will be on keeping the heat out. What plays a major role in both of those scenarios is the solar heat gain coefficient and the U-factor.

U factor, in essence, is the rate of heat transfer and a measure of how well the window insulates. They generally range from .25 to 1.25. The SHGC is how much solar radiation is admitted through a window, and ranges between 0 and 1. The lower the number the less heat it transmits.

If there is a certain area of your house that gets a great deal of sun for example, you could even choose different window types for the various units in your home based on these factors.

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Role of the Window Frame

If purchasing an energy-efficient window is your main priority then you will want to choose a window frame that will aid in that goal. There are many variables to consider when choosing a window frame, but here we will just look at how well it helps insulate contributing to the overall efficiency of the window.

  • Aluminum window frames conduct heat rapidly so it typically falls to the bottom of the list when comparing energy efficient window frames.
  • Fiberglass window frames have air cavities that may be filled with insulation so they offer high thermal performance.
  • Vinyl window frames have hollow cavities that can be filled with insulation as well so these are also a good insulator.
  • Wood window frames also insulate well but keep in mind climate as this material expands and contracts in extreme temperatures.

Window Cost Versus Energy Savings

It is difficult to determine the payback associated with an energy-efficient window purchase but there are several items that go into this evaluation:

  • It’s not necessarily the higher priced window that is a better insulator. With thousands of window options make sure if you are compare apples to apples when price shopping. And just because one window has a $500 price tag doesn’t mean it is a better insulator (although it could be) than on with a $350 price tag. That is where the research you did on U factors and IG units will come be useful so keep that readily available when meeting with salespeople.
  • Reducing the load on heating and cooling systems. Remember to factor this into the equation. Newer insulating windows will no doubt reduce the load placed on your heating and cooling systems and could be another cost saver.
  • Check with utility companies. Some utility companies offer rebates for new window purchases so check with your local provider.
  • Upgrading windows will save money in the long run. While it may be difficult to put a specific savings on it. Research from one window company says older, single-pane windows are often the source of up to 30 percent of your home’s heat loss. So those new windows will reduce your utility bills.
  • Choose Energy Star Windows. The Department of Energy says that by installing Energy Star windows you will lower your energy bills and save money as compared to single-pane or double-paned, clear-glass. The DOE estimates these savings:
    • $126–$465 a year when replacing single-pane windows.
    • $27–$111 a year over double-pane, clear glass replacement windows.

The site also states that you can reduce your energy bills at an average of 12 percent through a purchase of Energy Star windows.

Final Reminders

Now that you are armed with what factors go into an energy-efficient window purchase don’t get confused by multiple sales pitches. Know what you want, listen to the information you are given and then re-evaluate once more before making that final purchase. Don’t get swayed by pitches for triple pane units with krypton gas if these are not needed for your home. Focus on the IG basics, look for that low-E coating and consider your climate.

More article on energy efficient windows can be found on as well as how to find a window dealer near you. Good luck shopping!

Please note, this article may contain links to Amazon products. As an Amazon Associate, earns from qualifying purchases.



Tara Taffera

Tara Taffera was the editorial director for USGlass magazine, AGRR Magazine , and Window Film Magazine. Her skills and more than 20 years of experience have helped her earn numerous journalism awards, including coveted Jesse Neal Awards. Tara enjoys spending time with her family and staying active with her husband by competing in races together, including triathlons. She also spends time volunteering in her community and with her church. Find out more about Tara on Linkedin.

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4 Responses

  1. A great read on a much-needed topic! Insulated glass needs to be on top of every homeowner’s mind if he/she does not want to be burdened with high energy costs in the future. As I read in a blog by Grand View Research titled ‘Insulated Glass Market: For The Energy Conservator In You’, insulated glass goes a long way in positively impacting fuel poverty, something which residents and organizations pay clear attention to.

  2. Hi Tara,
    we are working on a project that will include 1/4″ thick decorative tempered triple glazing with 1/4″ gap between each pane. There will also be a set of therma-glass panes on the outside of the triple glazing. Will it be necessary to inject argon gas between the decorative triple glazing to avoid condensation. The triple glazing will only serve for decorative reasons, not for insulating purpose.

    I will appreciate a prompt response.

    1. David,
      The argon would not have much if any impact on condensation. The absorbent put into the spacers of hermetically sealed insulating units is what deals with the condensation issues. If the three panes referenced are separated with spacers, filled with adsorbent, and properly sealed, they should perform well for many years.

  3. hi tara,
    we have a frameless IGU project for facade application, can you give us an installation tips.? some do’s and don’ts. looking forward for your help.

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