SHGC stands for Solar Heat Gain Coefficient, a number between 0 and 1 that tells you how much of the sun’s heat passes through a window. A window with an SHGC of 0.25 lets in 25% of the solar energy that hits it, while a window rated 0.75 lets in 75%. It’s one of the most important numbers on a window’s energy label, and choosing the right SHGC for your climate can meaningfully affect your heating and cooling bills.
How SHGC Is Measured
SHGC accounts for all solar heat that enters through a window, not just the sunlight you can see streaming in. Some heat passes straight through the glass (direct transmission), while some is absorbed by the glass itself and then re-radiated into the room as warmth. The SHGC number captures both pathways in a single rating.
The scale runs from 0 to 1. Zero would mean no solar heat gets through at all, and 1 would mean every bit of the sun’s energy passes inside. Real windows fall somewhere in between. Standard clear glass typically has an SHGC around 0.7 to 0.8, meaning it lets most solar heat in. High-performance coated glass can drop that below 0.25.
Why SHGC Matters for Energy Bills
In hot climates, solar heat pouring through windows is one of the biggest drivers of cooling costs. South-, east-, and west-facing windows take the most direct sun and contribute the most unwanted heat. Choosing windows with a low SHGC reduces how hard your air conditioner has to work. ENERGY STAR estimates that installing certified windows, doors, and skylights can cut heating and cooling costs by an average of up to 13% compared to non-certified products.
In cold climates, the math flips. You actually want solar heat coming through your windows during winter to reduce your heating load. A higher SHGC means more free warmth from the sun, which is why energy codes in northern regions set a minimum SHGC rather than a maximum.
SHGC Requirements by Climate
ENERGY STAR’s upcoming Version 7 specification makes the climate divide clear. In the Northern zone, windows need an SHGC of 0.17 or higher to ensure they still capture useful solar warmth in winter. In the Southern zone, the requirement flips to 0.23 or lower, prioritizing heat rejection to keep cooling costs down. These numbers represent tighter standards than previous versions, reflecting improvements in glass technology.
If you live in a mixed climate with hot summers and cold winters, the choice gets trickier. Some homeowners use different SHGC values on different sides of the house: lower SHGC on west-facing windows that catch intense afternoon sun, and higher SHGC on south-facing windows where winter sun is most useful.
How Low-E Coatings Control SHGC
The main technology behind modern SHGC control is low-emissivity (low-E) glass, which uses microscopically thin metallic coatings to filter different wavelengths of light. There are two broad types, and they do opposite things.
Solar control low-E coatings are designed to block solar heat from entering the building. These work best when applied to the second surface of the glass (the inside face of the outer pane in a double-pane window), where they intercept solar energy before it can penetrate deeper. These are the go-to choice in warm climates.
Passive low-E coatings are designed to let solar heat in while preventing interior heat from escaping back out through the glass. They work best on the third or fourth surface (the inner pane, furthest from the sun). These are ideal for cold climates where you want to trap the sun’s warmth inside.
The type of coating is the single biggest factor determining a window’s SHGC. Conventional clear glass has relatively uniform transmittance across the solar spectrum, but coated glass acts like a filter, selectively blocking infrared radiation (heat) while allowing visible light through.
SHGC vs. Visible Transmittance
A common concern with low-SHGC windows is whether blocking heat also means blocking daylight. The answer: somewhat, but not as much as you might expect. Visible Transmittance (VT) is a separate rating that measures how much visible light passes through the glass. Good solar control coatings are engineered to block the infrared portion of sunlight (which carries heat but is invisible) while still allowing most visible light through.
The ratio between VT and SHGC is called Light-to-Solar Gain, or LSG. A higher LSG means the glass does a better job of letting light in while keeping heat out. If you want bright, naturally lit rooms without the cooling penalty, look for windows with a high LSG ratio, typically above 1.0.
Reading the NFRC Label
Every window sold in the U.S. through major retailers carries a label from the National Fenestration Rating Council (NFRC). The label lists five performance ratings: U-factor (insulation value), SHGC, Visible Transmittance, air leakage, and condensation resistance. SHGC appears as a decimal, and it reflects the performance of the entire window unit, including the frame, not just the glass alone. This is important because frames don’t transmit solar heat the way glass does, so the whole-unit SHGC is always lower than the glass-only value.
When comparing windows, make sure you’re comparing whole-unit SHGC numbers from NFRC labels rather than glass-center values that manufacturers sometimes highlight in marketing materials. The NFRC number gives you the realistic picture of how the window performs once it’s installed in your wall.
Choosing the Right SHGC for Your Home
For most homeowners, the decision comes down to three factors: your climate, which direction the window faces, and how much natural light you want.
- Hot climates (Southern U.S.): Aim for SHGC of 0.25 or lower, especially on south- and west-facing windows. This keeps cooling costs in check during the months when your AC runs hardest.
- Cold climates (Northern U.S.): Look for SHGC of 0.30 or higher on south-facing windows to capture free solar heating in winter. North-facing windows get little direct sun regardless, so SHGC matters less there than U-factor (insulation).
- Mixed climates: Consider varying SHGC by window orientation. Lower values on west-facing glass, moderate to higher values on south-facing glass, and prioritize U-factor on north-facing windows.
SHGC is just one piece of the window performance puzzle, but it’s the one that most directly affects how much solar heat enters your home. Paired with a good U-factor for insulation and a high VT for natural light, the right SHGC can make a noticeable difference in both comfort and energy costs.