A good solar heat gain coefficient (SHGC) depends on where you live. In hot climates, you want a low SHGC of 0.25 or below to block solar heat. In cold climates, a higher SHGC of 0.40 or above helps warm your home for free in winter. SHGC is measured on a scale from 0 to 1, where 0 means no solar heat gets through and 1 means all of it does.
What SHGC Actually Measures
SHGC represents the fraction of the sun’s energy that passes through a window and ends up as heat inside your home. That includes both the sunlight that passes straight through the glass and the portion that gets absorbed by the glass and then radiates inward. A window with an SHGC of 0.30 lets in 30% of the sun’s heat energy. A window at 0.60 lets in twice as much.
This is different from visible light transmittance (VT), which measures only the light you can see. A well-designed window can block a large share of solar heat while still letting in plenty of daylight. The ratio between these two numbers, called the light-to-solar-gain ratio, tells you how efficiently a window delivers brightness without excess heat. The higher that ratio, the more natural light you get per unit of heat gain.
Recommended SHGC by Climate Zone
ENERGY STAR’s Version 7.0 specifications break the U.S. into four climate zones with different SHGC targets for certified windows:
- Northern zone: SHGC of 0.17 or higher (no upper cap). These homes benefit from solar heat gain in winter, so the goal is to let warmth in, not block it.
- North-Central zone: SHGC of 0.40 or lower. A moderate limit that balances winter heating benefits against summer cooling loads.
- South-Central zone: SHGC of 0.23 or lower. Cooling costs dominate, so blocking solar heat is the priority.
- Southern zone: SHGC of 0.23 or lower. Same aggressive heat rejection as the south-central region.
The International Energy Conservation Code follows a similar pattern. Its 2015 edition requires an SHGC of 0.25 or below in zones 1 through 3 (the hottest parts of the country), allows up to 0.40 in zone 4 and 5, and has no SHGC restriction at all in zones 6 through 8, where winter heating is the dominant concern.
Why Cold Climates Need Higher SHGC
If you live in a northern state, a low SHGC can actually cost you money. South-facing windows with a high SHGC act as passive solar heaters, collecting free warmth on winter days and reducing your heating bill. The National Renewable Energy Laboratory recommends that south-facing glass in cold climates have a high SHGC, ideally paired with a roof overhang that shades the same windows in summer when the sun is higher in the sky.
The catch is that high-performance windows with very low U-factors (meaning excellent insulation) often come with low SHGC ratings too, because the same coatings that prevent heat loss also block incoming solar energy. Finding a window that insulates well and still admits solar heat requires shopping specifically for “passive” or “high-solar-gain” low-E glass. East- and west-facing windows, even in cold climates, should still have an SHGC below 0.40 because the low sun angle on those sides creates glare and overheating that’s hard to shade with overhangs.
How Low-E Coatings Affect SHGC
The coating on modern windows is the single biggest factor controlling SHGC. Low-E (low emissivity) coatings come in two broad categories: soft coat and hard coat. Soft-coat low-E is applied in a vacuum chamber and offers more control over which wavelengths of light pass through. Hard-coat low-E is baked onto the glass during manufacturing and tends to be more durable but less selective.
The difference in SHGC between coating types is dramatic. Pilkington’s Energy Advantage, a hard-coat product, has an SHGC of 0.68, meaning it lets in most solar heat. Cardinal’s LoĒ²-270, a soft-coat option, drops that to 0.37. Vitro’s Solarban 60 lands at 0.41. That spread shows why choosing the right coating matters more than choosing the right frame material or number of panes when it comes to managing solar heat.
For hot climates, a solar-control soft-coat low-E with an SHGC in the low 0.20s is typical. For cold climates, a passive low-E coating that keeps the SHGC above 0.50 on south-facing windows is the better play.
Window Film as a Retrofit Option
If replacing your windows isn’t in the budget, aftermarket solar control film can meaningfully reduce SHGC on existing glass. Research from the Florida Solar Energy Center suggests targeting an SHGC below 0.30 to 0.50 after applying film for effective solar heat rejection. Spectrally selective films reflect the infrared portion of sunlight, which carries nearly half the sun’s heat energy, while still allowing most visible light through. This means you can cut solar heat gain roughly in half without making your rooms noticeably darker.
Energy Savings You Can Expect
Choosing the right SHGC for your climate pays off on your energy bill. ENERGY STAR estimates that certified windows, doors, and skylights reduce heating and cooling costs by an average of up to 13% compared to non-certified products. In hot climates where air conditioning drives most of the energy bill, the cooling savings from a low-SHGC window are especially noticeable because you’re directly reducing the amount of heat your AC has to remove. In cold climates, high-SHGC south-facing glass offsets furnace run time during sunny winter days.
Certified windows also reduce UV-related fading of furniture, flooring, and artwork by up to 75%, a side benefit of the same coatings that control SHGC. This happens without a significant drop in visible light, so rooms stay bright even as solar heat and UV radiation are filtered out.
Picking the Right Number for Your Home
Start with your climate zone and ENERGY STAR’s recommendations as a baseline. Then refine by window orientation. South-facing windows in heating-dominated climates should have the highest SHGC you can find in a well-insulated frame. East- and west-facing windows in any climate benefit from lower SHGC because the low morning and afternoon sun is hard to shade and causes the most overheating. North-facing windows receive minimal direct sun, so SHGC matters less there, and you can prioritize U-factor and visible light transmittance instead.
If you’re comparing products, look at the light-to-solar-gain ratio (VT divided by SHGC) as a tiebreaker. A higher ratio means the window is more efficient at letting in daylight without the heat penalty. Two windows might both have an SHGC of 0.25, but if one has a VT of 0.45 and the other 0.35, the first window will give you a noticeably brighter room for the same thermal performance.