BS EN 410BS EN 1096CIBSE TM59
Solar Heat Gain (g-Value) & Selectivity
The solar factor (g-value) represents the total fraction of solar energy transmitted through the glazing. Selectivity (Light Transmittance LT % / Solar Factor g) measures the ability to admit daylight while rejecting infrared heat.
UK Regulatory Mandate:Approved Document O (Overheating in non-domestic and residential buildings) and CIBSE TM59/TM52 modeling.
1. The Selectivity Ratio (LT / g)
Natural daylight provides ~100,000 lux. High-performance double-silver and triple-silver magnetron coatings achieve selectivity ratios above 2.0 (e.g. 70% light transmission with a g-value of 0.35).
2. Thermal Stress Risk on Coated Panes
Solar-control coatings absorb substantial solar radiation. If part of the pane is shadowed by an overhang while the center is in direct sunlight, severe thermal stress develops. Thermal stress calculations are mandatory; toughening is typically required.
Critical Specification Pitfalls to Avoid
- •Placing the solar-control coating on Surface #3 instead of Surface #2
- •Using dark tinted body glass that cuts daylight and increases artificial lighting electricity consumption