Solar Gain and g-Values Explained

U-value and g-value are often in tension. A coating that improves thermal insulation (low-e) can also reduce solar transmission, and a highly solar-controlled glass that keeps a room cool in summer can also reduce welcome free heat and daylight in winter. There is no single correct answer — the right balance depends on orientation, room use and how much glazing is involved.

Large expanses of south or west-facing glazing — a common feature of contemporary kitchen extensions and garden rooms — bring a genuine risk of summer overheating if specified purely for U-value without regard to g-value. Overheating assessments are increasingly requested by building control on larger extensions for this reason.

• Specify solar control glass on south and west-facing large glazed areas
• Consider external shading — deep overhangs, brise soleil or louvres — for the most exposed aspects
• Combine with adequate background ventilation and, where possible, cross-ventilation
• Do not rely on internal blinds alone; by the time heat is inside, most of the problem has already occurred

On south-facing elevations in winter, solar gain can make a genuine contribution to space heating, which is why some passive design approaches favour higher g-value glass on south aspects and lower g-value on west aspects, where low, glaring summer evening sun is the greater problem. This is a useful conversation to have with a designer at the concept stage of an extension, not a decision to make from a glazing brochure alone.