When evaluating window energy ratings, the most popular figure is known as the U-factor. The three other window energy ratings are the Solar Heat Gain Coefficient (SHGc), Visible Transmittance (VT), and air infiltration or air leakage. Understanding a bit about the window energy label, its 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-factor (sometimes called the transfer coefficient) is another measurement of a product’s effectiveness as an insulator. U factor is prominent on the NFRC window energy label. R-value defines the measure of a product’s thermal resistance. Whereas, U-factor measures a product’s thermal transmission (the amount of transference through solid material.)
U-factor is much better for rating windows because glass is an ineffective insulator and cannot resist heat. However, applied glass coatings such as low-E reduce thermal transmission. Coatings help keep a barrier between the inside and the outdoors but may impact visible transmittance.
The lower a material’s U-factor, 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-factor, the less energy it takes to keep the inside comfortable. U-factors is a value between 0 and 1, with the lower end meaning better energy efficiency.
Solar Heat Gain Coefficient (SHGc)
Like U-factor, Solar Heat Gain Coefficient or SHGc is a value between 0 and 1. The SHGc rating is the direct measurement of how well a material blocks solar heat from the sun.
The lower the SHGc the better the product blocks sun-generated heat. A low SHGc is essential 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 utilize the suns heat. A higher SHGc can help save on heating in the colder months.
Visible Transmittance (VT Rating)
Finally, visible transmittance ratings are essential for understanding how much light comes through a product. Therefore, VT is a critical piece of information to have when buying windows. Like the other two numbers, VT is expressed as a number between 0 and 1. 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. A window with a high VT rating will help reduce the amount of artificial lighting needed – 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 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.
Air Infiltration (or Air Leakage) Rating
The air infiltration 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 or watch our explainer video below.
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 indicates that some of that solar heat is allowed in. A low U-factor means 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-factor 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-factor and SHGc.
Optional features on the Tribute double-hung window change the ratings significantly. This package 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-factor dips to an incredible 0.19, stopping almost all heating/cooling loss. You can still enjoy a VT of 0.44, which allows in a great deal of sunlight.
It’s Now Easier to Compare
Now that you have a better understanding of the window energy label and its ratings, its easier to compare windows side-by-side and make the best possible choices. Harvey takes energy-efficiency in fenestration very seriously. That is why we have developed entire lines of windows, including our Tribute and Classic lines, that offer up low U-factors and SHGc ratings alongside high VT ratings.
To learn more about our window thermal performance data, and to compare our different glass package options.