NFRC Cartoon Window Choices

When evaluating window energy ratings, the most common figure is known as the U-value. Three other main window energy ratings include the SHGc created by the National Fenestration Rating Council (NFRC) the VT rating, which stands for Visible Transmission, and the air infiltration rating. Understanding a bit about these ratings and how they indicate a windows energy efficiency can help you make better buying decisions.



Much like the R-value that applies to various types of insulation products, U-value (sometimes called the transfer coefficient) is another measurement of a product’s effectiveness as an insulator. R-value defines the measure of a product’s thermal resistance. Whereas, U-values measure a product’s thermal transmission (the amount of transference through solid material.)

U-value is a much better window energy rating identifier. This is because glass is an ineffective insulator and cannot resist heat. However, coatings applied to glass (such as low-E) reduce its thermal transmission, which is important for keeping a barrier between the inside and the outdoors.

The lower a material’s U-value, the longer it takes for heat to pass through it. And as a result, the better it performs as an insulator. As such, the lower a window’s U-value, the less energy it takes to keep the inside comfortable. U-values are expressed as numbers between 0 and 1, with the lower end meaning better energy efficiency.

Solar Heat Gain Coefficient (SHGc)

Like U-values, SHGc ratings are also expressed as numbers between 0 and 1. The SHGc (Solar Heat Gain Coefficient) rating is the direct measurement of how well a material blocks solar heat from the sun.

The lower this number, the more efficiently that product blocks heat gain caused by the sun. This is vitally important in places like Florida, Southern California, and deeper south where this heat can vastly increase cooling costs. However in colder climates, one might opt for a higher SHGc to allow the suns heat in. Thus saving on heating in the colder months.

Visible Transmission (VT Rating)

Finally, visible transmission ratings are important for understanding how much light comes through a product. VT is a very important piece of information to have when buying windows. Though VT has little to do with thermal ratings, it does affect energy consumption. The higher this number, the better the window lets in light. This means that a window with a high VT rating is more likely to allow enough natural daylight into your home that you can reduce the amount of artificial lighting – and therefore energy – you consume.

It is a common misconception that a high VT rating will automatically result in a warmer room. Before today’s window glazing techniques, this was absolutely the case – a brighter room was usually a hotter one. Today’s innovative technologies have changed this. It is now possible to have a very bright room, lit only by the sun, that is just as cool as a dark room elsewhere in your home. Like the other two numbers, VT is expressed as a number between 0 and 1, with the higher number meaning more light coming through.

Air Infiltration (or Air Leakage) Rating

The air infiltration rating is exactly as it sounds. This rating measures the air that leaks in through openings in the frame, or around seals. The AI rating measures how much air leaks through. For more in-depth coverage on this rating: visit our What is Air Infiltration post.

Overall Window Energy Ratings

Knowing what each of the three different numbers means gives you the info you need to decide what works best for you. For example, if you live in a cold climate and your goal is to heat your home and trap as much of that heat inside your home as possible, consider each of these three numbers. A high VT rating means you can enjoy plenty of natural sunlight.  A moderate SHGc value means that some of that solar heat is allowed in. A low U-value indicates heated air stays in and cold air stays out.

Harvey’s SunGain insulated glass package is perfect in this situation. The VT rating of 0.58 allows plenty of sunlight, and the SHGc rating of 0.48 shows that a moderate amount of solar heat is likely, but the U-value of just 0.29 shows that air heated indoors will not readily escape.

Pro Tip

You don’t need to put the same type of energy rating windows everywhere.  If a window gets very little direct sunlight, the SHGc won’t be as important a factor for a window that does.

Another example, if you live in a hot climate, and your goal is to let in natural light, but block as much solar heat and heat transmission through your windows as possible. In this case, you would want to find a window with a moderate VT rating, but a very low U-value and SHGc.

Optional features on the Tribute double-hung window change the ratings significantly. This packages includes triple Low-E glazing, Krypton gas insulating between the glass, and foam insulation added to the frame. The SHGc value sinks to just 0.24, blocking the vast majority of the sun’s heat, and the U-value dips to an incredible 0.19, stopping almost all heating/cooling loss. You can still enjoy a VT of 0.44, which allows a great deal of sunlight in.

It’s Now Easier to Compare

Now that you have a better understanding of window energy ratings, it becomes easier for you to compare windows side-by-side and make the best possible choices. This is also where we truly shine here at Harvey; we take fenestration very seriously, and we know that energy efficiency is important to all. That is precisely why we have developed entire lines of windows, including our Tribute and Classic lines, that offer up low U-values and SHGc ratings alongside high VT ratings.

Learn More

To learn more about our window thermal performance data, and to compare our different glass package options.

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