Solar Heat Gain Coefficient (SHGC) for Windows

A window's solar heat gain coefficient, or SHGC, measures how much of the sun's heat the window lets into the house. It is a number from 0 to 1 on the NFRC label: low numbers resist solar heat and high numbers allow it ¹. The U.S. Department of Energy puts it the same way: the lower the SHGC, the less solar heat the window lets in ⁶.

Whether you want a low or a high number depends mostly on climate. ENERGY STAR caps SHGC at 0.23 in its South-Central and Southern zones, where keeping the sun's heat out matters most, and sets a minimum of 0.17 in its Northern zone, where some winter sun is welcome ⁴. The trade-off to watch is daylight: some glass that blocks solar heat also lets in less visible light, so read SHGC next to visible transmittance (VT).

What SHGC measures

NFRC's glossary defines SHGC as the fraction of solar radiation admitted through a window, counting both what passes straight through and what the glass absorbs and then releases inward ³. That second part means heat soaked up by the glass itself can still count toward the number.

SHGC is one of the mandatory ratings and sits at the top right of every NFRC label ². It is separate from the U-factor, which covers heat that moves because one side of the window is warmer than the other, not heat from sunlight. The U-factor, and the label as a whole, are covered in window U-factor explained; this article stays with the sun.

DOE's Energy Saver Guide advises looking at whole-unit SHGC and U-factor rather than center-of-glass figures, because the whole-unit numbers reflect the entire product, frame included ⁷. Keep that in mind when a glass maker's sheet and a window's label show different numbers for what sounds like the same glass.

Why SHGC matters more in the South

In a hot climate, sun through the glass is heat the air conditioner then has to remove. NFRC's consumer site says that in hot climates a window with a low SHGC can reduce cooling costs and keep rooms from overheating in the afternoon sun ². DOE's Energy Saver Guide says low SHGCs reduce heat gain in warm climates, and suggests both a low U-factor and a low SHGC in temperate climates with hot and cold seasons ⁷.

ENERGY STAR's criteria put that into numbers: an SHGC of 0.23 or lower in the South-Central and Southern zones, 0.40 or lower in North-Central, and a floor of 0.17 or higher in the Northern zone, where a window with an SHGC of 0.35 or 0.40 or higher may have a slightly higher U-factor ⁴. ENERGY STAR's criteria page links a search for finding your climate zone ⁵. The full zone table, U-factor limits included, is in window U-factor explained.

Orientation changes the answer

DOE's consumer guide goes one step further and suggests choosing glass by the direction each window faces ⁶. In a cold climate it suggests the highest SHGC you can afford on south-facing windows, and a low SHGC or shading on east- and west-facing ones. In a warmer climate it suggests a low SHGC on the south, east and west sides, with shade on the south. For north-facing windows it gives only U-factor advice. The guide suggests selecting specific windows for different sides of the house ⁶, so it is reasonable to ask whether a quote can use different glass on different sides.

SHGC and daylight: the trade-off

Visible transmittance measures how much daylight comes through, also on a 0 to 1 scale, and higher means more natural light ¹. NFRC says that efficient windows can let light in while keeping out unwanted heat, because coatings on the glass allow different amounts of light and heat through ². Glass that passes much of the visible light while reflecting other parts of the solar spectrum is called spectrally selective ³, and NFRC's glossary also defines a light-to-solar-gain ratio as a measure of how well a glazing provides light without excessive solar heat gain ³.

Two glass products from Cardinal, a glass maker, show the range. Both figures are for the glass in a double-pane unit with 90% argon, not a whole window ^{8 9}:

Going from the first to the second cuts SHGC by more than half while visible light falls by a much smaller share ^{8 9}. That is the point of a solar-control coating: keep out heat while giving up less light. Still, some solar-control glass does look and feel darker, so if bright rooms matter to you, compare VT on the labels of the exact units you are quoted. How the coatings work is covered in low-E glass explained.

What a gas fill does to SHGC

Very little. A gas fill mainly lowers the U-factor. Cardinal's own bulletin on argon shows that in a clear double-pane unit, swapping air for argon left SHGC unchanged at 0.78 ¹⁰. The coating is what moves it. For more on fills, see argon vs krypton window fill.

How to use SHGC when you compare quotes

1. Find your zone with ENERGY STAR's search ⁵.
2. In the two southern zones, look for 0.23 or lower on the window's label ⁴.
3. In the north, do not assume lower is better. ENERGY STAR sets a floor there, not a cap ⁴.
4. Read VT next to SHGC if daylight matters ¹.
5. Ask for whole-unit numbers for the exact size and type quoted, not glass-only figures ⁷.
6. Consider orientation for big east- and west-facing windows ⁶.

The broader picture is in energy-efficient windows explained. When you are ready, you can compare quotes from window installers and ask each one for the SHGC and VT of the units in the quote.

We are not affiliated with the manufacturers named.

About these figures

• ¹ Official record: NFRC label descriptions of SHGC and visible transmittance, both on a 0 to 1 scale, checked October 26, 2026.
• ² Official record: NFRC consumer page on SHGC as a mandatory top-right label rating, low SHGC in hot climates, and coatings that pass light while limiting heat, checked October 26, 2026.
• ³ Official record: NFRC glossary definitions of SHGC, spectrally selective glazing and light-to-solar-gain ratio, checked October 26, 2026.
• ⁴ Official record: ENERGY STAR SHGC limits by climate zone (0.17 or higher Northern, 0.40 or lower North-Central, 0.23 or lower South-Central and Southern) and the Northern alternate paths, read in the official criteria PDF, checked October 26, 2026.
• ⁵ Official record: ENERGY STAR criteria page and its climate zone search, checked October 26, 2026.
• ⁶ Official record: DOE consumer guide (2021 PDF) on what SHGC measures and glass choice by orientation and climate, checked October 26, 2026.
• ⁷ Official record: DOE Energy Saver Guide (2022 PDF) on low SHGC in warm climates, low U-factor and SHGC in mixed climates, and whole-unit vs center-of-glass ratings, checked October 26, 2026.
• ⁸ Reported: Cardinal LoĒ-180 product sheet (SHGC 0.69, 79% visible light, glass U-factor 0.26, double-pane with 90% argon), checked October 26, 2026.
• ⁹ Reported: Cardinal LoĒ³-366 product sheet (SHGC 0.27, 65% visible light, glass U-factor 0.24, double-pane with 90% argon), checked October 26, 2026.
• ¹⁰ Reported: Cardinal IG technical bulletin IG02 (August 2022) showing SHGC unchanged at 0.78 when argon replaces air in a clear double-pane unit, checked October 26, 2026.