28/03/2026
Solar gain coefficients — or g‑values — often sit quietly in the background of glazing design, even though they have a major influence on how a building performs.
As glass areas increase, the conversation naturally moves beyond U‑values and toward what actually happens when sunlight hits the façade.
The same solar energy that can be a welcome source of passive heat in winter can just as easily push a building toward overheating in summer. That’s exactly why the g‑value matters.
A g‑value describes how much of the sun’s energy passes through the glazing and into the building. A value of 0.60 means roughly 60% of that energy enters; a value of 0.30 means only about 30% gets through. It accounts for both direct solar transmission and the heat the glass absorbs and re‑radiates inward.
This single number influences comfort, overheating risk, and heating demand. And despite common assumptions, a lower g‑value isn’t automatically the better choice.
The right value depends heavily on context — climate, orientation, shading, and the role the glazing is meant to play.
Coatings tend to have the biggest impact, especially solar‑control layers designed to keep gains down. Triple glazing usually reduces g‑values compared with double glazing, and tinted glass can cut solar energy but at the cost of daylight. Even the frame‑to‑glass ratio and the building’s orientation matter, with south, west, and unshaded façades naturally
more exposed.
Typical products span a wide range: clear double glazing often sits around 0.6–0.75, solar‑control glass around 0.25–0.45, and triple glazing somewhere between 0.4–0.6. Higher g‑values can brighten interiors and support passive heating, while lower ones help keep summer temperatures under control and make compliance with overheating regulations more achievable.
The real challenge is that the g‑value can’t be chosen in isolation. It needs to be considered alongside daylight, ventilation, shading, and orientation.
One of the most common pitfalls is specifying a single g‑value for the entire building. A more thoughtful approach tailors the glazing to each façade — often lower on east and west, more carefully balanced on the south, and generally less critical on the north.
In the end, the g‑value isn’t just a product metric. It’s a design tool. Used well, it supports comfort and reduces energy demand. Used poorly, it leads to buildings that overheat or underperform.
