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The solar factor: what does it tell us?

When looking for glazing, one of the key indicators of its efficiency is the solar factor.

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The solar factor: definition

The solar factor, or solar coefficient, is a measure of the proportion of solar energy transmitted into a room by a window or glazing. The g-factor is expressed as a percentage or hundredth between 0 and 1. The higher the g-factor, the greater the solar energy transmitted.

solar factor diagram

The solar factor includes both the transmission of energy through the glass and the heat absorption capacity of the glass.


  1. The glazing receives the sun's rays;

  2. Some are reflected outwards by the glazing components;

  3. Others pass through the glass and are transmitted into the room;

  4. Some of the radiation is transformed into heat;

  5. Some is rejected to the outside;

  6. The rest enters the room.

Solar factor Sg or g = transmitted energy (3) + input heat (6).

The solar factor g therefore corresponds to the energy that a glass allows through in relation to the energy transmitted by the sun.

What's the difference between Sg and Sw solar factor?

When we talk about solar factor, we can look for two related, although different, results.

Solar factor g or Sg

The solar factor g or Sg corresponds to the solar factor of the glazing alone, i.e. without the joinery. This is the indicator that glazing manufacturers pass on.

Solar factor Sw

When we talk about the solar factor sw, we mean that we want to know the solar energy transmitted by the window: the glazing and the joinery. This data is supplied by a joiner or facade builder.

How to recognize a good sun factor?

We're looking for a higher or lower solar factor to limit overheating in summer and reduce heating needs in winter. The solar factor varies according to the glazing manufacturer, the type of glazing, its composition and the joinery chosen to integrate it.

Solar factor of single-glazed clear glass

The g-factor is around 0.86 for single glazing, although this value may vary depending on the glass used.

The solar factor of single-glazing is very high, so it may look good in winter, but depending on the exposure of the facade, the room will overheat in summer and require air-conditioning.

Solar factor of ITR double glazing (with reinforced thermal insulation)

The solar factor of double glazing is 0.65, lower than that of single glazing. The advantage of ITR double glazing is that it improves the building's thermal insulation to keep the heat out in winter. This glazing is fitted with a low-emissivity layer and the space between the two glasses is filled with argon gas. This reduces solar gain in summer, preventing the room from overheating.

However, this solar coefficient may vary according to the composition of the double glazing.

Solar factor of triple glazing

Triple glazing is a little less widespread, but can be useful in very cold geographical areas to limit heat loss inside a room. The solar factor of triple glazing is around 0.50.

Although it's important to know the solar factor of a glazing unit, double- and triple-glazed units are mainly designed to improve the insulation coefficient (Ug).

How to choose the right glazing?

It's essential to know the solar factor of the glazing, but you also need to know whether the glazing is suitable for the building or the room. In other words, you need to take into account the location of the building and the exposure of the facade.

What's the point?

It's a double-edged sword: if you opt for glazing with a high solar factor, it will benefit you in winter, but penalize you in summer. So it's easiest to make a decision based on the facade to be fitted. A south-facing façade will be very exposed to the sun, so we recommend choosing a solution with a low solar factor. For a north-facing facade, which is more difficult to heat in winter, glazing with a high solar factor will be more attractive.

If you wish to improve the solar factor, particularly to limit overheating in summer, you can install solar protection devices such as solar control glazing. IMMOBLADE's solar control glazing is suitable for the tertiary sector. Find out more

There are other coefficients linked to glazing, such as light transmission or insulation coefficient, which provide important information on the qualities of the glass.

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