How to Calculate Solar Heat Gain Coefficient for Windows

Nowadays, windows are more than just sources of natural light and relaxing outdoor views. They’re also an excellent way to upgrade your home’s energy efficiency, a major theme that’s been trending in the fenestration industry for a long time now.

Window solar heat gain coefficient ratings are one of the most crucial factors to consider. In this article, we talk about why SHGC matters and how most energy efficiency organizations measure it. 

What Is Solar Heat Gain Coefficient?  

Solar heat gain coefficient indicates the fraction of solar radiation transmitted or absorbed through a window glazing and released into the room afterwards. SHGC ratings are commonly used to measure how well fenestration products, including windows, doors, and skylights, can block the sun.

Understanding solar heat gain coefficient is important because it affects your living space in many ways. Aside from achieving optimal indoor temperatures, it can also prevent exposure to the sun’s UV rays, reducing risks of skin cancer in humans and cause colour fading in furniture and interiors. 

Windows with a lower SHGC rating have less solar heat transmission and better shading capability. These types of windows effectively block the sun, lessening the need for cooling, which can be of greater benefit to homeowners living where there’s a longer summer season.

On the other hand, windows with a higher SHGC rating collect more solar heat, keeping your home warm and cozy during cold winter months while reducing your heating expenses. 

Determining SHGC Rating for Windows  

According to The National Fenestration Rating Council (NFRC), SHGC ratings factor in the total solar energy transmittance of a window or door as a whole, factoring in the glass, frame material, sash, divided lite bars, and screens. 

To get the bigger picture of how SHGC works and is measured, consider that when the sun hits your windows…

• A portion of the solar energy is directly transmitted through glass or glazing.
• Some are reflected, bounced off, or absorbed into the window. 
• The heat absorbed is then emitted outwards from the surface of the glass or inwards into your home.Considering the dynamics of glass and solar energy, the simplest formula experts use to calculate the value for solar heat gain coefficient is:

Solar heat gain coefficient = portion of solar energy transmitted + portion of absorbed solar energy that is emitted inside

Solar heat gain coefficients are expressed as decimals from 0.00 to 1.00. This is only the theoretical range. In real life, values of SHGC for windows are never a perfect 0.00 (no solar radiation gets through) or 1.00 (all the solar radiation is absorbed).

Most organizations that test for the SHGC either use simulation models or manually record the heat flow through a window by using a calorimeter.

What Makes Solar Heat Gain Coefficient So Unique?

Fortunately, buyers do not need to calculate SHGC themselves when shopping for windows, thanks to energy performance labels by the NRFC and CSA. 

SHGC ratings can be easily spotted in most energy performance labels in Canada along with other energy efficiency metrics like:

• U-factor or U-value
• Visible transmittance
• Air leakage
• Energy rating
• R-value (not present in most labels, but many contractors include them)

What makes SHGC so unique from the rest is that it’s the only one that can also give homeowners a glimpse of the amount of solar radiation absorbed by windows. On the other hand, other energy performance metrics tell you more about the insulation properties.

High or Low, What Is a Good SHGC Rating? 

High or low, there is generally no good or bad SHGC rating as it all depends on several factors that are specific to your property. These include:

• Climate – Homes in colder regions will benefit more from a higher SHGC that absorbs higher solar heat energy to maintain a warm temperature with minimal heating use. The opposite can be said for those living in hot and humid climates all year long, which should go for products with lower SHGC. In Canada, the recommended SHGC rating for windows can go from 0.44, 0.36, and 0.30, depending on your specific location.For areas with both cold and hot weather for different times of the year, a mid-range SHGC is preferable. You can also opt for smart glass windows that can intuitively change their settings. 
• Home orientation and window placement – South and southwest windows get more sunlight and relatively more solar heat. Choose windows with lower SHGC ratings when ordering replacement windows for rooms with this orientation.

• Presence of shading elements near the window – A lush tree or tall structure can block the sun and affect the amount of heat that can get through your window. 
• Personal desire to protect interiors from fading – If you own vintage furniture, oil paintings, and other valuable pieces, it may be wise to consider getting windows with a higher SHGC to prevent UV light damage.

Conclusion 

Now that you know the importance of the solar heat gain coefficient, those decimal numbers on energy performance labels will never look the same again. Ready to shop for energy-efficient windows? Check out our actively updated list of trusted window companies today.