When evaluating window thermal performance, the most common figure is known as the U-value. Other figures commonly used to help consumers better understand window thermal performance include the SHGc rating created by the National Fenestration Rating Council (NFRC) and the VT rating, which stands for Visible Transmission. Understanding a bit about these ratings and how they indicate window thermal performance can help you make better buying decisions.
What You Need to Know about U-Values
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. Whereas the R-value defines the measure of a product’s thermal resistance, U-values measure a product’s thermal transmission (the amount of transference through solid material.)
In other words, U-value is a much better window thermal performance identifier because glass is an ineffective insulator and cannot resist heat. However, coatings applied to glass reduce its thermal transmission, which is important for keeping a barrier between the inside and the outdoor.
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 will take to keep the inside of the building comfortable. U-values are expressed as numbers between 0 and 1, with the lower end meaning better window thermal performance.
Understanding the Solar Heat Gain Coefficient (SHGc)
Like U-values, SHGc ratings are also expressed as numbers between 0 and 1. Where a U-value tells you how much heat can pass through a material, an SHGc (Solar Heat Gain Coefficient) rating is the direct measurement of how well a material can block heat specifically from the sun.
The lower this number, the more efficiently that product blocks heat gain caused by the sun, which is vitally important in places like Florida, Southern California, and the Deep 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 and save on heating in the colder months.
Visible Transmission or VT Ratings
Finally, visible transmission ratings are important for understanding how much light comes through a product, and it’s a very important piece of information to have when buying windows. Though VT has little to do with window thermal performance, it is a very important number for consumers who are energy-conscious. The higher this number, the better the window let’s 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.
Overall Window Thermal Performance
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 Tribute Double Hung Windows with the high solar heat gain ENERGY STAR package from Harvey Building Products are 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.
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 the most important window thermal performance data for energy efficiency, 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.
To learn more about our window thermal performance data, and to compare the differences in these ratings as they relate to our window styles, types, and glass packages, you can view our thermal performance data for windows and patio doors.