Understand Window Energy Ratings: U-Value, Low-E Glass & SHGC

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When looking for new windows, among the most important factors to consider is; where they stand on energy-efficient window ratings. If you are someone just starting out, it can sometimes be challenging to understand the technical terms. To make things simple for you, we will discuss the two most important window energy performance ratings scales.

Yes, we are talking about U-value along with SHGC. These are the two most well-known energy-efficient window ratings.

The goal of this post is to clarify the two energy ratings that are currently assigned to modern energy-efficient windows. These ratings are essential for making educated choices when shopping for energy-efficient windows; for a new house or when replacing older, less efficient windows.

If you’re in the market for new windows, we’ve collected a summary of the two key performance indicators that go into determining a window’s energy efficiency rating. After reading this article, you will be able to make a wise choice for your property!


The U-value of a product is represented as a figure that can range from zero to one. The rate of heat loss through the object is represented by this value. Windows and doors that have a U-value lower than average have a higher resistance to the flow of heat. In simple words, it means they are better able to maintain the temperature inside the house. 

U-values are essential in colder climates. This is because people living in those climates spend a greater amount of time and money on keeping their homes warm. 

Technical Terms

The U-value is the reciprocal of the R-value. Generally speaking, this is the word that will be heard most often when discussing the insulation business. Simply dividing 1 by the specified U-value yields the corresponding R-value. 

The R-value increases as the U-value decreases. Due to the widespread adoption of the International Residential Code 2006, low U-values are increasingly in demand. All exterior door and window units with IG are required to have a U-Value of 40, which automatically makes to an R-value of 2.5, as per this regulation.

The insulation value of most windows and doors ranges from R-13-R-19. This is for the standard 2 x 4 framing and 2 x 6 framing, respectively. As a matter of fact, even the best energy-efficient windows are approximately three to four times less effective than the wall they are installed in.

The energy efficiency of a house can be significantly improved by reducing U-values, despite the wide gap between the insulating factors of walls and windows. For homes in these colder climates, a U-value that is smaller is preferable so that residents can enjoy a more comfortable indoor temperature throughout the heating season.


The solar heat gain coefficient (SHGC) reveals how much of the sun’s heat is able to penetrate a building. It simply gauges a window or door’s ability to control the amount of solar heat gain that occurs through its glass. 

This window energy performance ratings scale is expressed as a ratio between 0 and 1, with 0 being the best and 1 being the worst. A rating of zero indicates that no solar heat enters the room through the window or door. On the other hand, a rating of one indicates that all of the solar heat that is potentially available enters the room through the window or door. 

SHGC is most important in places with warm climates. This is because people living in those places expend a greater amount of time and money trying to keep their homes cool. A lower SHGC is desirable in those warmer regions because it reflects more of the sun’s energy and helps to keep the house at a cooler temperature.

Some Other Window Energy Performance Ratings Scales

Below are some other energy-efficient window ratings that are used by experts in the window energy rating:

1.Leakage Of Air

The quantity of air lost through a window’s gaps is known as its air leakage. If the air escape rate is high, heat loss and gain through the window will be greater. It will make the interior of the structure less delightful for the people who are there. Also, poor fitting can also cause air leakage, particularly around the frame.

2.Energy Rating (ER)

This rating represents a window’s general effectiveness rating in terms of energy use. It incorporates air leakage, SHGC, and the U-factor to provide customers with a more comprehensive metric by which to evaluate products. Window energy performance ratings scale regulations change from one temperature zone to the next. 


Calculating the quantity of visible light that is let in or let out through a given window or door is known as the visible light transmittance (VLT). In terms of numbers, VLT is simple shown as a percentage that falls between 0 and 1. A zero grade indicates that no visible light penetrates the glass; this level of tinting is very dark. 

With a VLT of 1, all visible light would enter through the window, creating a very brightly lit interior. Too much light can enter a house through windows and doors with a rating close to one, potentially damaging furniture and textiles. 

Most modern windows and doorways let in plenty of visible light to illuminate a room without bleaching out furnishings or flooring.

What is Energy Star?

For consumers in North America, ENERGY STAR is the seal of approval on windows and doors that have been certified as effective energy-efficient window ratings. In order to earn the ENERGY STAR label, a product must first be evaluated by a third-party testing facility. Not only this but it should also be found to be in compliance with or to have exceeded all applicable requirements.

When goods are all held to the same standards, it’s much simpler for customers to make informed purchasing decisions. You should always make sure you’re using the most up-to-date numbers when purchasing windows and doors. However, those norms are subject to change to accommodate new technologies or environmental policies.

It’s also worth noting that not every product with the ENERGY STAR label is equally energy efficient when it comes to conserving power. Examining the different metrics of success will help you to see the distinctions clearly.



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