What Does SHGC Mean for Windows? Solar Heat Gain Coefficient Explained

Updated: October 2026.

What SHGC measures

Solar heat gets through a window in two ways. Some radiation is transmitted directly through the glazing, and some is absorbed in the glass and then admitted indoors as heat. The DOE Building America guideline on window selection adds that radiation absorbed by the frame also counts toward the total. SHGC adds these up as a fraction of the solar radiation that reaches the window. NFRC's own summary is short: "Low numbers resist solar heat, high numbers allow solar heat."

The scale runs from 0 to 1 in theory. Real products sit in a narrower band; DOE's Federal Energy Management Program (FEMP) says the SHGC of actual fenestration products is "typically between 0.20 and 0.80."

The rating comes from a standard procedure, NFRC 200, whose full title is Procedure for Determining Fenestration Product Solar Heat Gain Coefficient and Visible Transmittance at Normal Incidence. Normal incidence means sunlight hitting the glass straight on. So SHGC is a fixed reference condition for comparing products. It doesn't predict what one west-facing window will do on an August afternoon.

Whole window vs. center of glass

You will sometimes see two SHGC values for what looks like the same window. The DOE guideline explains the difference: "NFRC's SHGC rating method is for the whole window, including the effects of the frame. The area of a frame has a very low SHGC, so the overall window SHGC is lower than the center-of-glass value." A center-of-glass SHGC describes the glazing alone.

Two practical points follow. Because the frame counts, the same glass can give a different whole-window SHGC in a different frame, so a label belongs to one product and one glass package. And a center-of-glass figure can't be compared with an NFRC label or with a code or ENERGY STAR limit. Those limits use NFRC-certified ratings; the energy code requires SHGC to be determined under NFRC 200 (2021 IECC, Section R303.1.3).

SHGC, U-factor and VT: three different numbers

The NFRC label carries three mandatory ratings. NFRC defines them like this:

U-factor and SHGC move independently. U-factor covers non-solar heat flow, the heat that leaks through whenever inside and outside temperatures differ, day or night. SHGC only covers sunshine. A window can insulate well and still let in plenty of solar heat, and ENERGY STAR's Northern zone builds on that: it accepts a U-factor of 0.25 or 0.26 if the SHGC is 0.40 or higher.

VT matters because cutting solar heat can also cut daylight. FEMP's advice: "When purchasing windows with a low SHGC, verify that the VT is not too low." A related figure, light-to-solar gain (LSG), compares VT with SHGC. A higher LSG means more daylight for the same solar heat, though it is not always provided.

How low-E coatings change SHGC

FEMP names "low-e coatings and tints" as the features that affect SHGC. It says low-E products come in low, moderate and high solar gain versions: low gain (less than 0.4) products are designed for hot climate zones, and high gain (greater than 0.55) products for cold ones. For hot climates, FEMP adds that low-E coatings, "especially spectrally selective coatings, are effective at reducing summer heat gain."

For scale, the DOE guideline puts a typical double-pane insulating glass unit (IGU) at an SHGC of around 0.70. A tint lowers that somewhat, and a low-solar-gain low-E coating can lower it substantially. So two windows that look the same from the street can differ a lot in SHGC because of a coating you can't see. The label is the reliable way to tell. See low-E coating in the glossary.

Choosing SHGC by climate: the ENERGY STAR limits

ENERGY STAR Version 7.0 took effect on October 23, 2023. A window qualifies only with NFRC-certified ratings that meet its zone's limits and air leakage of 0.3 cfm/ft² or less.

In the Northern zone the SHGC figure is a floor, not a ceiling. A higher SHGC collects more solar heat in winter, and the equivalent-performance rows let a window with a slightly higher U-factor qualify if it brings in more sun. Everywhere else the limit is a maximum. ENERGY STAR assigns its zones by county and ZIP code, so check yours with the ENERGY STAR climate zone finder. More on reading the label against this table: window energy ratings. For the South, see windows for hot, humid climates.

The code minimum: IECC SHGC limits

The energy code is a separate, legal bar. The residential International Energy Conservation Code (IECC) sets a maximum SHGC for glazed fenestration by climate zone, and the values are the same in the 2021 and 2024 editions:

The IECC map is not the ENERGY STAR map, and states adopt different editions with their own amendments. The limits also apply when existing windows are replaced (2024 IECC, Section R503.1.1.1). For U-factor limits and which edition applies where, see What U-factor do I need?; Florida's rules are covered in Florida window energy code.

Orientation, and checking your own house

NFRC's consumer guide says "the right ratings for your home depend on your climate and the orientation of each window." DOE's consumer guide to windows gives the short version for south-facing windows. In a cold climate, it suggests the highest SHGC and lowest U-factor you can afford; in a warmer climate, a low SHGC "and lots of shade."

1. Find your ENERGY STAR zone, and ask your building department which IECC edition it enforces.
2. Read the SHGC on the NFRC label, which is the whole-window value. Confirm the exact product and glass package in the NFRC Certified Products Directory or the ENERGY STAR Product Finder.
3. Among windows with similar SHGC, compare VT.
4. Model your case. The GlassBase window energy calculator shows an estimated whole-window SHGC and U-factor for the frame, glazing and spacer you pick, and models heating and cooling energy in kWh for your ZIP code and wall direction. Change only the glazing and you can see whether a lower SHGC saves more cooling than it costs in heating on that side of the house. The results are engineering estimates, not NFRC ratings.

GlassBase does not sell or install windows. To find companies that do, browse window companies by city.

Check SHGC options for your ZIP code

Sources

• NFRC, What does the NFRC label tell you? (U-factor, SHGC and VT definitions and ranges) and Certified Products Directory
• NFRC, NFRC Document Public Review Through April 6, 2026 (title of NFRC 200)
• NFRC consumer guide, Window ratings (SHGC range; climate and orientation)
• US EPA, ENERGY STAR, Certification criteria for residential windows (Oct 2023), Residential Windows, Doors, and Skylights Version 7.0 (effective October 23, 2023) and climate zone finder
• US DOE, FEMP, Purchasing Energy-Efficient Residential Windows, Doors, and Skylights (typical SHGC range, low-E solar gain types, VT advice)
• US DOE, Building America, Measure Guideline: Energy-Efficient Window Performance and Selection (NorthernSTAR for NREL, November 2012; Section 2.2, whole-window vs. center-of-glass SHGC)
• US DOE, Guide to Energy-Efficient Windows ("Shop for Performance" by orientation)
• International Code Council, 2024 IECC, Chapter 4 [RE] (fenestration SHGC by climate zone), Chapter 5 [RE] (Section R503.1.1.1), and 2021 IECC, Chapter 3 [RE] (Section R303.1.3)