SHGC Window Ratings Explained
Posted on August 18, 2021
Posted in Windows
Homeowners think a lot about style, size, and placement when picking out windows. However, windows also have important energy ratings that can help show how specific models can keep you cool in the summer, save money on energy bills, and more. Let's take a closer look at window energy performance ratings like the SHGC and how they work.
What is the Solar Heat Gain Coefficient?
Solar Heat Gain Coefficient windows, usually abbreviated SHGC, is a rating that tells buyers how much solar heat is blocked by the window. All solar energy contains a portion of thermal energy, and at least some of it passes through glass. This is important when planning a building or replacing your windows, because that heat can increase the temperature inside the building: That may make air conditioners work harder and cost more money to run, which is why some homes benefit from windows that block lots of solar heat.
The SHGC rating is typically given as a number between 1 and 0. The number 1 represents the maximum amount of solar heat passing through the window, and the number 0 represents the least possible amount of heat making it through. This can be easily translated into a percentage - for example, an SHGC of 0.40 means that 40% of solar heat can pass through that window, and the window blocks 60%. A lower SHGC number means better solar heat-blocking capabilities.
What Affects the SHGC Window Rating?
Materials and manufacturing decisions have the greatest impact. Important factors for SHGC ratings include:
- How many panes the window has
- The type of glazing the window uses, especially if tinted glass or films are used
- The nature of the spacers between the windowpanes
- The window frame material and how well-sealed it is
- The general type and size of the window
Is a Low SHGC Rating Better for My Home?
It can be. Low SHGC ratings are typically thought of as energy-efficient and environmentally friendly. However, a lot depends on your climate.
In climates where it's generally colder and heating is important while air conditioning is less common, a low SHGC rating isn't as important. It's simply too cold for solar heat to make much of a difference in the long run, and home furnaces may benefit from all the help they can get.
In warmer climates, or climates with hot seasons where air conditioning is very important, a low SHGC rating can be much more useful, keeping homes cooler and reducing AC bills. That's why it's a good idea for Texas homeowners to look for windows that are rating 0.40 or lower - 0.30 or lower is a good idea if you have windows that get lots of sunlight in the summer months.
Is There Any Official Testing for These Ratings?
Yes, NFRC, or National Fenestration Rating Council, has performed these tests since 1993. It is a non-profit organization with the only independent rating and labeling system for energy ratings related to windows. The EPA and other federal departments using the NFRC's ratings because of their reliability.
What is the U-Value When It Comes to Windows?
This gets a little more complex. The U-value, also called the U-factor, is a separate rating from the SHGC.
Instead of measuring how much solar heat can pass through a window due to sunlight, the U-value shows how well windows block other types of heat. This can include heat in the air, and is greatly affected by the size of the window, how much hotter the outside/inside air is, and how many panes the window has. Think of U-factor windows as showing how good they are at general insulation.
Like SHGC, U-values are shown from 0 to 1. The lower the number, the better the window is at keeping outdoor temperatures at bay. Sometimes U-values are split into specific conditions, like a nighttime U-value to show how good the window is keeping heat inside a home during colder days.
What About R-Value? Is That Important?
R-value is another term you may see pop up when discussing home insulation, energy efficiency, and lowering your air conditioning bills. The big difference is that U-values are used specifically to rate the insulation capabilities of windows, while R-values are used to rate…nearly everything else. Walls, floors, rooftop materials - anything else that stands between indoor air and outdoor air is given an R-value. This allows you to see how insulated a home is, but it's not especially helpful when picking out windows.
However, you can calculate the U-value to R-value if you want to convert between values. Let's say you have a triple-pane window with a U-value of 0.30. Since U-values and R-values are reciprocal, you can simply divide 0.30 by 1, giving you a triple-pane window R-value of 30. This is of limited usefulness for homeowners but can sometimes be useful when planning a building or meeting building codes.
Which Rating Should I Pay Attention to The Most?
The good news is that SHGC ratings and U-values are closely related. A window that has a low SHGC rating is likely to have a low U-value as well. However, SHGC ratings are more connected to shade, window films, and generally how much light is let inside a home, which could be important for aesthetic reasons.
If you have any other questions about
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