Understanding Solar Heat Gain Coefficient

What SHGC means and why it matters.

Modern home with large glass windows beside a swimming pool

The short answer

Solar heat gain coefficient, or SHGC, describes the fraction of solar energy entering through a window system as heat. Lower values indicate less solar heat gain under the stated test conditions. Compare figures for the same glass setup, and assess daylight, climate and winter heating needs alongside summer comfort.

The definition

SHGC is the "fraction of incident solar radiation" admitted through a window, either transmitted directly or absorbed and then released inward as heat. It ranges from 0 to 1. A lower SHGC means less solar heat is transmitted, which is better for cooling; a higher SHGC can provide passive solar heating in cold climates (Penn State EGEE 102).

Think of it as a percentage. An SHGC of 0.40 means about 40 percent of the solar energy hitting that window becomes heat inside.

The two-part definition matters for film. A film that absorbs solar energy heats the glass, and part of that heat flows into the room. That's why SHGC is more meaningful than "percent of heat reflected" claims (Whole Building Design Guide).

Typical values

Glass or assembly	Approximate SHGC
Clear single-pane glass (1/8")	0.86
Single-pane with exterior storm window	~0.46
Example ENERGY STAR-labeled low-e double-pane window	0.23
Clear glass + neutral solar film	0.37
Clear glass + ceramic film (36% VLT)	0.45
Clear glass + low-e film (26% VLT)	0.25

Sources: film and clear-glass values from LLumar/Eastman measured on 1/8" clear glass; storm window from U.S. DOE; low-e window example label from Penn State EGEE 102. Your windows will differ.

SHGC's companions on a label

The National Fenestration Rating Council (NFRC) label shows up to five ratings (Penn State EGEE 102):

U-factor: rate of non-solar heat transfer. Lower is better. Typical residential range 0.20 to 1.20.
SHGC: solar heat gain. Lower blocks more sun.
Visible transmittance (VT): how much light passes through, 0 to 1. Typical windows are 0.20 to 0.80.
Air leakage: cfm per square foot; 0.3 or less meets most codes.
Condensation resistance: 0 to 100; higher is better.

Light-to-solar-gain ratio (LSG) is VT divided by SHGC. An LSG above 1.0 means the glass admits more light than heat, which is ideal for hot climates (Penn State EGEE 102). "Spectrally selective" films and coatings aim for high LSG.

Choosing SHGC for your climate

ENERGY STAR criteria vary by climate zone (ENERGY STAR). As a practical guide, the NJ Green Building Manual cites ENERGY STAR recommendations of SHGC 0.40 or lower for homes with moderate air-conditioning needs, and 0.25 or lower for homes with significant air-conditioning costs or summer overheating (NJ Green Building Manual).

Hot and cooling-dominated climates: low SHGC almost everywhere.
Mixed climates: low SHGC on east and west glass and skylights; moderate on south.
Cold climates: higher SHGC on south glass can reduce heating. The National Trust for Historic Preservation notes that low-SHGC films in heating climates can reflect away beneficial winter solar energy (National Trust for Historic Preservation).

Lawrence Berkeley National Laboratory's analysis for ENERGY STAR illustrates the trade-off: in a northern-zone example, a 0.01 change in U-factor was roughly equivalent to a 0.05 change in SHGC, and higher minimum SHGC produced larger savings (LBNL for ENERGY STAR).

Orientation changes everything

SHGC describes the window, but how much sun it receives depends on direction. Unshaded east and west windows can get up to twice the midsummer solar heat per square foot of south windows (Building America Solution Center). The same SHGC matters far more on a west window than a north one.

SHGC for shades and attachments

The Attachments Energy Rating Council rates blinds, shades and storm windows on SHGC, U-factor, visible transmittance and air leakage, and publishes Warm Climate and Cool Climate ratings (Building America Solution Center). That makes it possible to compare a shade with a film on the same terms.

Shading coefficient: an older number

Some film spec sheets still list shading coefficient (SC), an older measure that compares a window to clear 1/8-inch glass. SHGC has largely replaced it. As an approximation, SC is about SHGC divided by 0.87. Use SHGC when comparing products.

Common misunderstandings

"Low SHGC means dark glass." Not necessarily. Spectrally selective films and coatings can have low SHGC with relatively high visible light.
"Lower is always better." Not in cold climates on south glass.
"The spec sheet number is my window's number." Film data is usually measured on single-pane clear glass; results on double-pane and low-e windows differ (LLumar/Eastman).

Questions to ask

What is my current window SHGC, and what will it be with this product?
Is that figure for my glass type, or for single-pane clear glass?
What is the visible transmittance and LSG?
How does this SHGC affect winter heating in my climate?

Sources

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Updated October 20, 2026. Educational guidance from Tint My Home Windows, powered by Sun Stoppers. Film selection, glass compatibility and warranty terms require project-specific confirmation.

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