If you’re in the market for windows, whether you’re building new or renovating, you have likely been engrossed in a number of terms such as U-factor (or U-value), visible transmittance, condensation resistance, and solar heat gain coefficient.
These are all ratings that help describe the efficiency of the window, which is useful when doing comparison shopping. However, they can sometimes be confusing.
This article explains what the solar heat gain coefficient (SHGC) is and why it’s important, particularly in warm climate zones.
But first, a brief background as to why energy efficient windows are important.
Efficient building materials lower energy demands
Buildings consume a great deal of energy. In fact, 40% of energy consumption in the US goes toward residential and commercial buildings (source). Therefore, using building materials that reduce our need to heat and cool buildings help lower energy demands.
The US Department of Energy has a program, the Building Energy Codes Program (BECP), dedicated to improving the efficiency of America’s residential and commercial buildings. Part of the BECP’s mandate is to raise the efficiency standards of the International Energy Conservation Code (IECC). The goal is to achieve 30% increased energy efficiency by 2012.
Baseline efficiency standards for windows are prescribed in the IECC, including a minimum SHGC value.
What is the solar heat gain coefficient?
SHGC is simply a measure of how well the window blocks heat from sunlight. It is expressed as a fraction of how much heat comes through the window relative to the total heat hitting the window. As it’s a fraction, the SHGC is a value between 0 and 1. The closer the value is to 0, the better the window is at blocking solar heat. If the SHGC is 0.3, it means 30% of the heat hitting the window is being transmitted.
This video helps explain further:
What are the SHGC standards as outlined by the IECC?
Though the BECP website states that all windows should have an SHGC less than or equal to 0.4 to be compliant in climate zone 1 to 3 (source), the IECC2009 standards give a slightly more efficient value of 0.3 (source).
Who rates the SHGC for windows?
The National Fenestration Rating Council (NFRC) is a non-profit independent labelling organization that has a standardized labelling system for windows. They will rate all certified products.
Where do I find the SHGC value for my window?
If the window has been certified by the NFRC, it will have a standardized label fixed to it by the manufacturer that gives the SHGC, U-factor, visible transmittance and other efficiency measures.
If the window does not have this label, you can either ask the manufacturer for this information or you can look it up on the NFRC certified products directory.
If you live in a hot climate, having windows with a low solar heat gain coefficient can go a long way to reducing your energy needs. If your windows don’t need replacing, there are other things you can do to reduce the amount of heat being transmitted through your windows. Awnings and shades are a great way to shade windows, as are strategically planted trees. The key is to try and stop the heat before it hits the window and ultimately enters the home. Little changes can make a big impact on your home’s efficiency.