Solar heat gain coefficient, or SHGC, is the fraction of the sun’s energy that passes through a window, door, or skylight and enters your home as heat. It’s measured on a scale from 0 to 1, where 0 means no solar heat gets through and 1 means all of it does. A window with an SHGC of 0.30, for example, lets in 30% of the solar energy that hits it. This single number plays a major role in how comfortable your home feels and how hard your cooling and heating systems have to work.
How SHGC Works
When sunlight strikes a window, some of it passes straight through the glass, some bounces back outside, and some gets absorbed into the glass itself. That absorbed energy heats the glass, which then radiates warmth both outward and inward. SHGC captures all of these pathways in one number: the total share of solar energy that ends up inside your home, whether it arrived directly or was absorbed first and then released as heat.
A lower SHGC means the window blocks more solar heat, giving it better shading ability. A higher SHGC means the window lets more solar heat through, which can be useful if you want free warmth from the sun in winter. The “right” value depends entirely on your climate and which direction the window faces.
SHGC vs. the Older Shading Coefficient
Before SHGC became the standard, the window industry used a metric called the shading coefficient (SC). The shading coefficient compared any given window’s solar heat gain to that of a single pane of ordinary double-strength glass. SHGC replaced it because it measures the actual fraction of incoming solar energy rather than a comparison to a reference pane. If you come across older spec sheets that list a shading coefficient, you can roughly convert it: SHGC equals the shading coefficient multiplied by the solar heat gain factor of that reference glass, which is about 0.87. So an SC of 0.50 translates to an SHGC of roughly 0.44.
What the Numbers Mean in Practice
Most residential windows sold today fall somewhere between 0.20 and 0.70. Where your ideal number lands depends on geography. In hot climates where air conditioning dominates your energy bill, you want windows with a low SHGC, typically 0.25 or below, to keep solar heat out. In cold climates, a higher SHGC on south-facing windows lets the sun help heat your home for free during winter months. Passive solar home designs often call for south-facing windows with an SHGC above 0.60 to maximize that benefit.
The impact on cooling costs is real. Research on residential buildings has shown that upgrading from standard single-pane glazing to high-performance double glazing can reduce peak cooling loads by roughly 13%. Even smaller changes, like switching to reflective coated glass, can cut cooling demand by 3% to 4% depending on the home’s orientation. Those reductions also mean you may need a smaller air conditioning system, which saves money upfront and over the life of the equipment.
Energy Star Requirements by Climate Zone
Energy Star’s Version 7.0 specification sets different SHGC thresholds depending on where you live. The requirements reflect the basic trade-off: block solar heat where cooling is the priority, allow it where heating matters more.
- Northern Zone: SHGC of 0.17 or higher (a minimum floor, because you want some solar heat gain)
- North-Central Zone: SHGC of 0.40 or lower
- South-Central Zone: SHGC of 0.23 or lower
- Southern Zone: SHGC of 0.23 or lower
Skylights follow a similar pattern. In the Northern Zone, there’s no SHGC cap. In every other zone, skylights must come in at 0.25 or below. These numbers were tightened in the latest specification, with the South-Central and Southern zones getting stricter limits than previous versions required.
Notice that the Northern Zone is the only one with a minimum SHGC rather than a maximum. That’s because in cold climates, a window that blocks too much solar energy can actually increase your heating costs by eliminating free warmth from the sun.
How Low-E Coatings Control Solar Heat Gain
The technology behind most low-SHGC windows is a microscopically thin low-emissivity (low-e) coating applied to the glass surface. These coatings contain layers of metallic material, often silver, sandwiched between protective dielectric layers. The silver reflects infrared and ultraviolet light, which carry heat, while the surrounding layers are tuned to let visible light pass through. The result is a window that looks clear but blocks a significant share of heat energy.
Early low-e coatings used a single layer of silver. Double-silver coatings improved on that by blocking over 30% more solar heat gain while maintaining the same amount of visible light transmission. The latest triple-silver coatings can transmit nearly 70% of visible sunlight into a building while blocking up to 75% of infrared and ultraviolet energy. This is what “spectrally selective” means in window marketing: the coating selectively filters the parts of the solar spectrum that carry heat while still letting you see clearly and enjoy natural daylight.
By adjusting the thickness and composition of these coating layers, manufacturers can produce windows across a wide SHGC range to suit different climates and orientations. A south-facing window in Minnesota might use a high-solar-gain low-e coating (SHGC around 0.50 to 0.60), while the same manufacturer offers a solar-control low-e version (SHGC around 0.20 to 0.25) for homes in Texas.
Where to Find the SHGC on a Window
Every new window, door, and skylight certified through the National Fenestration Rating Council (NFRC) carries a temporary label with standardized performance ratings. SHGC is one of the primary numbers listed, alongside the U-factor (which measures insulation) and visible transmittance (how much light comes through). The label looks like a nutrition facts panel for windows: a simple grid of values you can compare across brands and products.
When shopping, compare SHGC values only between products tested to the same NFRC standard. Manufacturer-reported values that aren’t NFRC-certified may use different testing conditions, making apples-to-apples comparison unreliable. The NFRC label ensures you’re looking at numbers generated under identical, standardized conditions.
Choosing the Right SHGC for Your Home
If you live in a cooling-dominated climate (roughly the southern two-thirds of the United States), prioritize a low SHGC, ideally 0.25 or below. This is especially important for west-facing windows, which take the brunt of intense afternoon sun during summer.
If you live in a heating-dominated climate, the decision is more nuanced. South-facing windows benefit from a higher SHGC to capture winter solar heat, while north-facing windows get little direct sun regardless, so their SHGC matters less than their U-factor. East and west windows in cold climates are a judgment call: they gain some useful winter heat but can also cause overheating on sunny summer afternoons.
In mixed climates where you run both heating and air conditioning for significant portions of the year, a moderate SHGC in the 0.25 to 0.40 range often strikes the best balance. Some homeowners in these zones choose different SHGC values for different sides of the house, pairing high-solar-gain glass on the south with low-solar-gain glass on the west.