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7 Window Energy Ratings To Understand Before Buying Home Replacement Windows

March 17th, 2022 | 7 min. read

7 Window Energy Ratings To Understand Before Buying Home Replacement Windows

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If you’re looking for a window that will help improve your home's energy efficiency, it’s important to understand what to look for. 

Whether you’ve met with a contractor already or are continuing your research online, chances are you’ve seen different metrics for window energy ratings. You’re presented with a number, percentage, or range of either for different energy rating categories.

Each category may give you a brief description of what that energy rating means, but you want to dive deeper. You want to know how this will affect your home and if it will provide you with the energy efficiency you need. 

Southwest Exteriors has been a window replacement contractor in San Antonio since 1989. We offer the highest quality Marvin, ProVia, and Anlin replacement windows based on durability and energy efficiency. 

One of the things that we know is confusing when trying to find the best replacement windows for you is these window energy ratings. 

We want you to truly understand what these energy ratings mean so that if you meet with any contractor, you’ll know what you need in terms of energy efficiency. 

This article will define the key replacement window energy ratings, provide a quality range for the ratings, and explain how it affects your home.

After reading, you will better understand how to read window energy ratings and choose the right replacement window that will meet your needs.    

7 window energy ratings you should know

For replacement windows, energy ratings essentially tell us how the glass performs. The National Fenestration Rating Council (NFRC) creates the standards for window energy ratings in five major categories: solar heat gain coefficient, U-factor, visible transmittance, air leakage, and condensation resistance. Other energy factors measured are UV percentage and infrared percentage. 

ENERGY STAR is another household name you may have heard when looking for energy-efficient windows. Backed by the U.S. Environmental Protection Agency (EPA), ENERGY STAR certifies products, like windows, when they meet specific energy efficiency standards. 

The NFRC creates the measures and standards for energy efficiency, and ENERGY STAR is an added certification. This is why while you’re shopping for replacement windows, you've probably seen ENERGY STAR mentioned on a window manufacturer’s website or an emblem on the actual window. 

So, let’s dive into each of the measures of energy ratings to better understand what they mean for your home. 

1. Solar Heat Gain Coefficient (SHGC)

This measures the amount of solar heat that comes through a window, ranging from 0 – 1. The U.S. Department of Energy’s consumer resource, Energy Saver, says the lower the SGHC, the less heat from the sun comes through a window. 

So, simply put, the lower the number, the better. 

What does this mean for my home?

Think of the SHGC as how much heat is absorbed and transmitted through your window into your home. This measure helps you understand how well your home will stay cool during the hot summers. 

  • Solar Heat Gain Coefficient = Heat

2. U-Factor

This measures how well a window can insulate. According to the U.S. Department of Energy, the U-Factor for windows generally ranges from 0.20 – 1.20.

This NFRC measure considers the window glass and all other parts of the window, like the frame and any spacers. Some other ratings not from the NFRC may only take the glass or glass glazings into account. An NFRC rating will give you a wider measure of the energy efficiency of the entire window, not just the glass.

With this measure, the lower the number, the better insulator it is. 

What does this mean for my home?

The more your window can insulate, the better your house will keep heat in the winter and air conditioning in the summer. This is an important factor to consider, especially for homes in areas that experience extreme heat and cold. 

  • U-Factor = Insulation

3. Visible Transmittance 

Visible transmittance (VT) refers to the amount of visible light that penetrates through the window. This can be affected by glass glazings and tints. VT is measured 0-1.

The higher the number, the more visible light will shine into your home. 

What does this mean for my home? 

Look at the VT rating to know how much natural light will come into your home during daylight hours. A window with high VT and low SHGC tells you that the window will allow as much natural light in without heating your home. 

  • Visible Transmittance = Light

4. Air Leakage

Also known as the air infiltration rating, this measures “how many cubic feet of air passes through a window per minute in relation to the size of the window,” according to a blog post by window manufacturer Marvin

This rating is measured from 0.0 - 0.3 (cfm)/ft2, which is the rate at which air passes through the window. 

The lower the air leakage rating, the less air passes through the window. 

What does this mean for my home?

The air leakage rating indicates how well your new windows will insulate, just like the U-factor. The better your window is insulated, the less air will come and go as it pleases into your home.

A window with a low air leakage rating means you won’t be experiencing as many drafts, and less air can pass in and out of the window. 

  • Air Leakage = Drafts

5. Condensation Resistance

The condensation resistance rating measures exactly how well the window is resistant to condensation. Condensation is when excess moisture in the air accumulates into water and forms on the window.

This is measured from 0 - 100. The higher the rating, the more resistant the window is to condensation. 

What does this mean for my home? 

While condensation doesn’t necessarily affect your window's energy efficiency, it is a performance rating. Condensation depends on the temperature outside and inside your home. 

Typically in cooler months, you’ll notice condensation around your windows because the moisture in the warm indoor air meets the colder temperature of the window. When looking at the condensation resistance, it depends on your personal preferences for your windows and how much potential condensation matters to you. 

  • Condensation Resistance = Water Droplets

6. UV Percentage

This measures the percentage of UV rays that penetrate through the window. UV rays are classified into three categories: UV-A, UV-B, and UV-C. 

According to the International Ultraviolet Association, “normal glass (as used in windows) is transparent to UV radiation to a wavelength of about 330 nanometers (nm) (UV-A),” allowing almost 100% of UV-A rays through the window. Below 330 nm will block UV-B and UV-C rays, which are more harmful.

The lower the UV percentage, the fewer UV rays come through your window. So, a window with a low UV percentage will block more UV rays. 

What does this mean for my home? 

UV rays can not only bring excess heat into your home, but they can potentially damage your furniture inside and cause fading. Tints and glazings are most popular to raise the UV percentage and block more UV rays from penetrating through the window.  

  • UV Percentage = UV Rays

7. Infrared Percentage

This is the percentage of infrared rays that penetrate through a window. Britannica defines infrared rays to be the “portion of the electromagnetic spectrum that extends from the long wavelength, or red, end of the visible-light range to the microwave range.” 

These light rays are invisible to the eye but produce heat just like any other ray on the electromagnetic spectrum. 

Similar to UV percentage, the lower the infrared percentage, the fewer infrared rays penetrate through the window. 

What does this mean for my home? 

Infrared rays are another form of heat. You want a window with a low infrared percentage to help keep your home cooler and protect your furniture and items from damage and fading. 

  • Infrared Percentage = Infrared Rays

Finding the right replacement window to meet your energy efficient needs

Now that you know how to understand all window energy ratings and how they affect your home, you can choose the right window that will perform as you need it to. 

Whether energy efficiency is one of your top priorities for your replacement windows or not, understanding window energy ratings will also help you to choose the right contractor for your project and avoid choosing a contractor that may take advantage of your lack of knowledge. 

Overall, the energy efficiency of a replacement window is important to protecting your home. If you are looking for the best replacement window with high-quality energy ratings, looking for ENERGY STAR certified windows is a clear indicator of an energy-efficient window. 

With Southwest Exteriors, we want you to be as educated as possible on all things replacement windows. We believe that education and information are your best defense mechanisms against choosing the wrong products for you and receiving a faulty installation. 

Also, we want to help make complicated terms and definitions easily digestible so you can choose the right products and contractors for your window replacement project. 

If you want to learn more about energy efficiency in windows, learning about different glass types and options is essential. 

This article will outline six types of replacement window glass, like Low-E and gas-filled glass, how they work, and what they look like. Then, you can better understand how glass types factor into the efficiency of the window and can better customize your replacement windows to fit your needs.