Windows are designed to allow light to enter your home and protect the interior of your home from wind, rain, and critters. Most of us love bright spaces so lots of windows and lots of natural light is great but there’s more to light than what we can see. Sun rays also emit heat, in some climates additional heat entering through the windows is great but in hot climates, the less heat that enters the home the better. Heat can also travel through your window glass and frames causing heat gain and loss. We'll talk more about that and the values that represent those processes in this post comparing SHGC vs U-Value.
Solar heat gain coefficient is represented by a number which tells us how much solar radiation enters a home through a window, door, or skylight. The Efficient Windows Collaborative (EWC)↗ defines SHGC as, “The fraction of solar radiation admitted through a window or skylight, both directly transmitted, and absorbed and subsequently released inward. The solar heat gain coefficient has replaced the shading coefficient as the standard indicator of a window’s shading ability. It is expressed as a number between 0 and 1. The lower a window’s solar heat gain coefficient, the less solar heat it transmits, the greater its shading ability. SHGC can be expressed in terms of the glass alone or can refer to the entire window assembly.”
U-Value, or U-Factor, tells us how much heat is gained or lost through a window (or door or skylight). EWC defines U-value as, “a measure of the rate of non-solar heat loss or gain through a material assembly. It is expressed in units of Btu/hr-sq °F(US) or W/sq m- °K (European metric). Values are normally given for NFRC/ASHRAE winter conditions of 0° F (18° C) outdoor temperature, 70° F (21° C) indoor temperature, 15 mph wind, and no solar load. The U-factor may be expressed for the glass alone of the entire window, which includes the effect of the frame and the spacer materials. The lower the U-factor, the greater a window’s resistance to heat flow, and the better the insulating value.”
SHGC and U-value seem similar because we talk about heat but they are different. SHGC has to do with “solar” heat while U-value has to do with “non-solar” heat.
Solar heat gain coefficient measures the amount of solar radiation that passes through a window through reflection, absorption, and transmittance. Regardless of the temperature outside, windows can still absorb heat through solar radiation. A high SHGC value means the window allows greater amounts of solar radiation to pass through and heat up the home.
Reflection is when a ray of light approaches a surface and makes impact and bounces back. “The ray that bounces back is called the reflected ray↗.”
When light waves are absorbed by the glass is is converted to heat
Light waves that pass through an object, in this case light waves that pass through the windows and into a building, are said to be transmitted
U-value measures how much non-solar heat energy is transferred through a window. Heat can be gained or lost through conduction, convection, and radiation↗. This is also known as “insulating value.”
Conduction is the process by which heat transmits through a window from the hotter side to the cooler side.
Radiation is the transmission or emission of energy waves through space or an object
Convection is the transfer of heat by movement in air or water. Hot air rises and cool air falls.
For many homeowners most windows all seem the same, understanding window ratings like SHGC or U-value gives homeowners insight into the value of some windows over others. The truth is, many window companies will try to sell you just one type of window discounting countless other available options, some great, some not so great, and some that need to be left in the past. Windows that meet or exceed energy efficiency requirements are high performance windows. SHGC and U-value ratings are important factors that will help you understand the performance quality of your window options. Understanding these values will also help you determine if the windows you are choosing are worth the price tag.
Quality windows should have a rating label from the National Fenestration Rating Council (NFRC). The NFRC↗ is a non-profit organization that establishes standardized ways to test the performance of windows and doors. The council also provides consumers with energy performance ratings allowing consumers to verify information provided by window dealers and manufacturers.
For more information on window ratings check out our related blogs at the bottom of this page.
Brennan Enterprises is a home exterior remodeling company specializing in replacement windows and doors. Explore BrennanCorp.com to learn more about products and services available to Texas homeowners.
Windows play a huge factor in balancing heat levels inside your home. Whether you’re in the process of building a new home or looking to upgrade before Mother Nature turns the heat up, it’s important to consider the SHGC Factor for the windows you may be considering and how that rating will affect your comfort year round.