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A Beginner’s Guide to Energy Efficient Windows – Part 1: What Makes a Window Energy Efficient?

Energy efficiency is gaining favor all over the country so many homeowners are turning to home improvement projects to take advantage of the many energy efficient options available today. There are different ways to achieve energy efficiency but one of the easiest is to replace old, faulty windows with newer, more energy-efficient models available today.

What makes a window energy-efficient in the first place?

To understand what makes energy-efficient windows tick, you have to be aware of the factors that determine how well they perform. These include:

  • U-Value/U-Factor. The U-value or U-factor determines how much heat can flow through a window, including those passing through the glass pane, frame, seal and spacers. Measurements range from from 0 to 1, with the most energy-efficient windows having the lowest ratings. The lower a window’s U-value, the easier it will be for it to help a a home to stay cool during summer and warm during winter because outdoor heat is kept out and indoor heat stays in.
  • Solar Heat Gain Coefficient. The Solar Heat Gain Coefficient or SHGC specifically measures how capable a window is in reducing the amount of heat it lets through from the sun, making it a crucial factor when choosing windows for areas with hotter climates. Like the U-value/U-factor, the SHGC scales from 0 to 1. The lower the score, the less heat a window lets through, making it the more energy-efficient option.
  • Visible Transmittance. Visible Transmittance or VT indicates the amount of light that passes through a window. A high VT rating is great for daylighting but keep in mind that sunlight brings both natural light and heat. Like the two other factors, VT scales from 0 to 1. The typical double-glazed window can have VT scores ranging from 0.7 to 0.8. Adding tinted or reflective coatings can reduce values to as low as 0.1.
  • Air Leakage. A huge part of a window’s energy efficiency comes from its ability to stay airtight. This is where the Air Leakage rating comes in. Also scaled from 0 to 1, a rating must be as low as possible to be considered energy-efficient, with scores 0.3 and below being most ideal.

To get good ratings, energy-efficient windows rely on certain features. In Part 2, we’ll be talking about glazing, one of these features. Read on for more!


"We love our new windows. The install was painless even with moving around 3 dogs as the crew worked through the house. So glad we went with Anderson windows."

Apr 11, 2019

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