Solar factor (g) / Light transmission (TL)

The solar factor (g) and light transmission (LT) are two key properties of glazing. g (from 0 to 1) indicates the proportion of total solar energy that passes through the window (radiation + re-emitted heat): the higher the g value, the greater the solar heat gain; the lower the g value, the more overheating is limited. LT (from 0 to 1) expresses the proportion of visible light that enters: a high LT ensures good natural lighting and visual comfort. The art lies in matching g and LT to the orientation, climate, and intended use, often with the help of external solar shading and a well-configured building management system (BMS).

Solar factor & TL: definitions and uses

The solar factor (g) is sometimes called SHGC (English). A g = 0.60 allows 60% of incident solar energy to enter; g = 0.30 only allows 30% to pass through. g is adjusted via selective coatings (solar control), the glazing composition (double/triple), the spacer, and possibly gases (argon).

Light transmission (LT) measures the transmission of visible light (without heat). A LT of 70% provides good brightness; a LT of 40–50% may be suitable for very sunny facades if glare is a concern. Selectivity (LT/g) is sometimes referred to as selectivity: the higher the value, the more light the glazing lets in while blocking heat (useful for offices/schools).

In development:

South : aim for a moderate to high g to take advantage of the winter sun + external controlled protections for summer (BSO, awnings).

West : favour low g (low summer sun critical) + essential external protections.

North : g not very critical, maximize TL for diffuse light.

East : intermediate (morning contributions), arbitrate according to use (housing vs offices).

Always combine glazing with a high Uw value, airtight installation and, on the operational side, BMS scenarios (automatic closure in heatwaves, early opening in mid-season).

Choosing the right g and TL: advantages, limitations, key points

Interests

• Winter comfort: g adapted to the South = reduced heating.
• Summer comfort: low temperature + external protection = limited overheating.
• Lighting quality: High TL = pleasant daytime, less artificial lighting.
• Overall performance: good selectivity (TL/g) = light without excess heat.

Boundaries

• g too low on the South side = lost winter heat gains → heating ↑.
• TL too low = dark environments, artificial lighting ↑.
• Highly selective layers can tint the light (color rendering).
• Glazing alone does not replace effective external protection.

Points to consider

• Cross g/TL with orientation, use (offices vs housing) and climate.
• Aim for high selectivity for offices with extensive glazing; accept a slightly higher g-value in south-facing housing if external venetian blinds are present.
• Treat glare (blinds, sunshades) independently of heat gains.
• Measure in STD/SED the DH impact (summer comfort) and the kWh heating/lighting.
• Pay attention to the installation (frames, thermal breaks): good glazing that is poorly installed loses its benefit.
• Plan for the maintenance of external protections (BMS control, cleaning).

Anecdote — “Soft light on the Lyon peninsula”

In Lyon – Presqu'île, southwest-facing glass-fronted offices suffered from glare and intense summer heat. The team opted for selective double glazing (TL ~ 60%, g ~ 0.35, selectivity ≈ 1.7), complemented by external adjustable sunshades connected to the building management system (closing based on sunlight and interior temperature). The result: comfortable natural light year-round, reduced artificial lighting, and a significant decrease in summer humidity levels without oversizing the air conditioning. The director summed it up: "We kept the daylight... but not the heat."