Thermal
Solar Heat Gain Coefficient
SHGC
The fraction of solar radiation striking a window that ends up as heat inside, from 0 to 1. Whether a low number is desirable depends on climate, orientation and shading.
In more detail
Solar heat gain coefficient is the fraction of solar radiation striking a window that becomes heat inside — transmitted directly, plus absorbed and re-radiated inward. It runs from 0 to 1; the European equivalent is the g-value.
Low-emissivity coatings are how it is controlled. Their placement and composition determine how much of the solar spectrum is reflected, and modern coatings are selective enough to separate visible transmittance from solar gain to a degree — though not without limit, which is why very low SHGC glazing tends to be somewhat darker or more reflective.
Whether a low SHGC is desirable depends on the situation, and this is where a single whole-house specification does the most damage. A south-facing window in a cold climate with a properly sized overhang collects useful winter heat and is shaded in summer, so a higher SHGC can be an asset. A west-facing window in a hot climate takes low-angle afternoon sun that no overhang shades, so a low SHGC is close to unambiguous. NFRC rates SHGC for the whole assembly, and exterior shading generally outperforms glazing selection where both are available.
Why it matters
SHGC drives cooling load, afternoon overheating, and the localized discomfort of sitting in direct sun indoors — and it is fixed when the windows are ordered. Getting it wrong by orientation produces a room that is unusable at a predictable time of day, and the remaining remedies are exterior shading, interior shades that block the view, or applied film. Orientation-specific glazing costs little more than one specification and cannot be added later.
The common misunderstanding
That lower is always better, or that SHGC is the same as tinting. It is a climate-and-orientation decision, and useful winter solar gain is real in heating-dominated climates. Spectrally selective coatings also reach low SHGC while passing most visible light, so a low number does not imply a dark or mirrored window.
Related terms
On Compound
Where this term does real work — and where the decisions get made.
Sources
Last reviewed July 2026 by Compound Editorial. Definitions are educational and not project-specific engineering or medical advice.