Energy Efficient Windows: Your Questions, Answered

A window’s energy efficiency is important. It makes your home more comfortable, saves you money, and is better for the environment.

If you’re in the market for new windows, you’ve probably heard something about the benefits of energy-efficient windows, but a vast array of options and confusing terminology can make choosing a window difficult.

We’re here to simplify the process by answering the most common questions we get from prospective window buyers. Keep reading for our energy efficient window FAQs.

What, exactly, makes a window energy efficient?

Windows let in more than light. Heat and solar energy can come in through the transparent glass, and air can flow in through cracks—especially if the window is old or improperly installed. If you have a lot of heat transfer and air flow, you’ll have high energy bills (especially in the summer and winter). This is because the window lets outside air in, and you’ll have to crank up the heating or air conditioning to compensate.

Decades ago, windows were simply made with just a single pane of glass with no coating. Single-pane, uncoated windows are not very energy efficient.

Today’s windows, in contrast, can be made with multiple panes of glass, each separated by insulating gas and coated with a glazing that reduces heat and solar energy transfer. Windows that have these features insulate better and let in less heat and fewer cancer-causing UV rays.

How do I know if the window I’m looking to purchase is energy efficient?

There are hundreds of different windows on the market, but you can narrow down your options to only the energy efficient variety by looking for the following labels:

  • NFRC (National Fenestration Rating Council)

ENERGY STAR is an EPA (Environmental Protection Agency) program that identifies products that meet basic standards of energy efficiency. If you see this label, you know that you’re looking at an energy efficient window.

The NFRC label will provide you with ratings for multiple energy performance categories. That means if you’re looking for a specific feature—such as a window with a low U-factor—you can see how different windows compare.

What is an insulated window?

An insulated window is made up of two or more panes of glass, insulation at the edges, and air (or gas) in between the panes. A gas-filled window has argon gas (or another inert gas) in between the panes, because gas is more insulating than air.

You can have double or triple pane windows. Learn about the differences between double and triple pane windows here.

What does “low-E” glass mean?

Low-E is short for “low emittance.” This means that the window glass is coated with a microscopic, virtually invisible material that cuts down on the amount of UV rays transmitted through the glass. Low-E coating makes the window more efficient, reduces glare, and can prevent UV damage to fine furniture and artwork.

What is U-factor?

The U-factor of a window measures how well it can keep heat inside a room. The U-factor can range from 0.20 to 1.20. Look at the U-factor rating on window labels: the lower the number, the more efficient the window is.

What is SHGC (solar heat gain coefficient)?

The SHGC measures the amount of solar radiation transmitted through the glass into your home. This measurement can range from 0 to 1. A window with a low SHGC rating is better at blocking heat from the sun, making it easier and more efficient to cool your home in the summer. A window with a high SHGC rating is better at collecting heat, making it more efficient to heat your home in the winter.

The SHGC rating you want depends on your climate and even the orientation of your home’s windows (whether they face north, south, east, or west). Since we get both extremes—cold winters and hot summers—here in the Midwest, it’s a good idea to talk to a window installation expert about the rating you should look for.

What is Visible Transmittance?

Visible transmittance is a measurement of how much natural light the window allows into your home. The number ranges from 0 to 1. The higher the rating on the label, the more light let in. A window with a high number will help you reduce the need for interior lighting (saving you money on energy bills). Still, you may want a window with a lower number in certain cases: for example, you may to cut down on glare in the room where you watch TV.

What is Air Leakage?

Air leakage (like the name suggests) measures how much air the window lets into your home. The rating ranges from 0.1 to 0.3. For this rating, you always want to look for a lower number.

Other than Energy Efficient Windows, What Can I Do to Make My Home More Efficient?

Great question! From minor fixes like closing your blinds to major alterations like swapping out appliances, there’s a lot you can do to make your home more energy efficient. For inspiration, read How to Make Your Home More Energy Efficient.

Still want to learn more about energy efficient windows? Be sure to read Through the (Energy Efficient) Looking Glass: What You Should Know, or contact us at one of our three convenient locations: Elsmere, Covington, or Dry Ridge, Kentucky.