Homeowners think a lot aboutstyle, size, and placement when picking out windows. However, windows also haveimportant energy ratings that can help show how specific models can keep youcool in the summer, save money on energy bills, and more. Let's take a closerlook at window energy performance ratings like the SHGC and how they work.

What is the Solar Heat GainCoefficient?

Solar Heat Gain Coefficientwindows, usually abbreviated SHGC, is a rating that tells buyers how much solarheat is blocked by the window. All solar energy contains a portion of thermalenergy, and at least some of it passes through glass. This is important whenplanning a building or replacing your windows, because that heat can increasethe temperature inside the building: That may make air conditioners work harderand cost more money to run, which is why some homes benefit from windows thatblock lots of solar heat.

The SHGC rating is typically givenas a number between 1 and 0. The number 1 represents the maximum amount ofsolar heat passing through the window, and the number 0 represents the leastpossible amount of heat making it through. This can be easily translated into apercentage - for example, an SHGC of 0.40 means that 40% of solar heat can passthrough that window, and the window blocks 60%. A lower SHGC number meansbetter solar heat-blocking capabilities.

What Affects the SHGC WindowRating?

Materials and manufacturing decisions have the greatest impact. Important factors for SHGC ratingsinclude:

  • 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 Betterfor My Home?

It can be. Low SHGC ratings aretypically thought of as energy-efficient and environmentally friendly. However,a lot depends on your climate.

In climates where it's generallycolder and heating is important while air conditioning is less common, a lowSHGC rating isn't as important. It's simply too cold for solar heat to makemuch of a difference in the long run, and home furnaces may benefit from allthe help they can get.

In warmer climates, or climateswith hot seasons where air conditioning is very important, a low SHGC ratingcan be much more useful, keeping homes cooler and reducing AC bills. That's whyit's a good idea for Texas homeowners to look for windows that are rating 0.40or lower - 0.30 or lower is a good idea if you have windows that get lots ofsunlight in the summer months.

Is There Any Official Testingfor These Ratings?

Yes, NFRC, or NationalFenestration Rating Council, has performed these tests since 1993. It is a non-profit organization with the only independent ratingand labeling system for energy ratings related to windows. The EPA and otherfederal departments using the NFRC's ratings because of their reliability.

What is the U-Value When ItComes 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 muchsolar heat can pass through a window due to sunlight, the U-value shows howwell windows block other types of heat. This can include heat in theair, and is greatly affected by the size of the window, how much hotter theoutside/inside air is, and how many panes the window has. Think of U-factorwindows as showing how good they are at general insulation.

Like SHGC, U-values are shown from0 to 1. The lower the number, the better the window is at keeping outdoortemperatures at bay. Sometimes U-values are split into specific conditions,like a nighttime U-value to show how good the window is keeping heat insidea home during colder days.

What About R-Value? Is ThatImportant?

R-value is another term you maysee pop up when discussing home insulation, energy efficiency, and loweringyour air conditioning bills. The big difference is that U-values are usedspecifically to rate the insulation capabilities of windows, while R-values areused to rate…nearly everything else. Walls, floors, rooftop materials -anything else that stands between indoor air and outdoor air is given anR-value. This allows you to see how insulated a home is, but it's notespecially helpful when picking out windows.

However, you can calculate theU-value to R-value if you want to convert between values. Let's say you have atriple-pane window with a U-value of 0.30. Since U-values and R-values arereciprocal, you can simply divide 0.30 by 1, giving you a triple-pane windowR-value of 30. This is of limited usefulness for homeowners but can sometimesbe useful when planning a building or meeting building codes.

Which Rating Should I PayAttention to The Most?

The good news is that SHGC ratingsand U-values are closely related. A window that has a low SHGC rating is likelyto have a low U-value as well. However, SHGC ratings are more connected toshade, window films, and generally how much light is let inside a home, whichcould be important for aesthetic reasons.

If you have any other questions aboutenergy-efficiency windows, or what replacing your windows would look like, contact Expo Home Improvement today. We can chat, arrange for a quote, and answerquestions while respecting your safety and home