How to Choose Energy Efficient Window Glass

Windows are responsible for up to 30% of heating and cooling loss in a home. And since they last for over 20 years, finding energy-efficient windows for your climate is crucial.

Energy-efficient windows can save you money on your energy bills and keep the temperatures in your home better regulated. But, with all the options available, choosing the best windows can be a real challenge.

Energy Efficient Window Glass

The truth is, there’s no one standard for energy-efficient windows. Different climates require different features. Here’s what you should know about the types of energy-efficient windows and how to decode an Energy Star rating.

Types of Energy Efficient Glass

Energy efficient glass is also known as insulated glass units, or IGUs for short. There are two types of IGUs – double pane and triple pane glass.


Double Pane Windows

Double Pane WindowsView in gallery

Average cost: $600 for a standard-sized window

Double pane windows feature two sheets of glass separated by a spacer, filled with air or gas, and given a tight seal. These windows are today’s standard and what most top window brands offer.

The efficiency of a double-pane window varies depending on special coatings and the type of insulator between the glass panes. 

Double-pane windows can be filled with air or a dense gas like Argon or Krypton to increase insulating properties. Manufacturers can treat double-pane windows with a low-E coating to keep heat inside or reflect solar heat away from the house.


Triple Pane Windows

Triple pane windows

Average cost: $660-$720 for a standard-sized window

Triple pane windows feature three sheets of glass, separated by spaces, filled with an insulator, and sealed tight. These windows are not nearly as standard as double-pane and are usually 10-20% more expensive.

Like double-pane windows, the efficiency of triple-pane windows depends on special coatings and whether they have gas or air fillings. Triple pane windows can be helpful in extreme climates but aren’t worth the extra cost in areas with mild temperatures.


Thermal Windows

Thermal Windows

Average cost: $600-$720 for a standard size window

A thermal window is another name for a double or triple-pane window. If a window is labeled “thermal,” the glass stops heat transference to a certain degree.

The value of a thermal window depends on the number of glass panes, the filling between the panes, and if the window has a Low-E coating.


Look for an Energy Star Rating (And Decode the Label)

Look for an Energy Star Rating (And Decode the Label)

The easiest way to find an energy-efficient window is to look for the Energy Star label. The label will feature several numbers and a map with blue shading. The blue shading indicates if the window is Energy Star Certified in that area.

The two most important numbers on an Energy Star label are the U-Factor and the SHGC.

  • U-Factor measures how well a glass insulates. Most windows have a u-factor that ranges from .20 to 1.20. The lower the U-Factor, the better a window insulates. Unless you live in a southern climate, your U-Factor should be .30 or below. If you live in a southern state, your U-Factor should be .40 or below.
  • SHGC (solar heat gain coefficient) measures how much solar heat a window lets in and ranges from 0 to 1—the lower the number, the lower the solar heat gain. If you live in a year-round hot area, you’ll want an SHGC of less than .25. But if you live in a cold climate, you’ll want a higher SHGC, up to .42.

Here are other numbers you’ll see on an Energy Star label:

  • AL (air leakage) measuring how much air a window leaks at a specific pressure
  • VT (visible transmittance) how much sunlight comes through the window
  • Condensation Resistance is the rate at which a window experiences condensation

Factors that Make Window Glass Efficient

Not all glass is equal. Several factors can help, or hurt, a window’s energy efficiency. 

  • The number of glass panes – Single pane glass is not energy efficient. Double pane glass is more efficient and the standard for residential windows. Triple-pane increases energy efficiency but isn’t necessary for all climates.
  • Gas fills – A window with a gas filling between the panes is more efficient than one with air. Argon is the most common type of gas found in windows. It’s non-toxic and six times denser than air. Krypton gas is denser than Argon but less common and more expensive.
  • Low-E Coatings – Low-E coatings help homes retain warmth. And a special type of Low-E called solar control can reflect solar heat away from the house. Different climates benefit from varying levels of this coating.

Frequently Asked Questions (FAQ)FAQ

Is thick window glass more energy efficient?

Thicker glass is not more energy efficient, but it is more durable. So if you’re worried about broken windows, thick glass is a good option.

Is thick glass better than double-glazing?

Double-glazing is better for insulating or regulating temperatures than thick glass. Thick glass is less likely to break, though, so if you want durability, thickness is something to consider.

Does installation affect a window’s energy efficiency?

Even the most high-rated energy efficiency glass won’t help your home if the window installation isn’t correct. Bad window installation can lead to drafty windows and allow moisture in the house. Choose professional window installation to ensure your windows are leakproof and efficient.

Final Thoughts

Since windows are expensive and long-lasting, choosing energy efficiency should be a priority. At the least, choose a double-pane window. To increase energy efficiency, look for an Argon or Krypton filling and a Low-E coating.

You can also refer to the Energy-Star rating to ensure your chosen window is certified for your area. Different climates require different ratings, and selecting windows that will benefit you in the long run, is important.