Selectivity in Glass – Why the Ratio of Light to Heat Is Decisive

Beyond g-value and Ug-value: the figure that truly reveals glass quality

In contemporary architecture, the glass façade is no longer a passive component but a precisely tuned filter system. It has to manage the balancing act between maximum transparency and energy efficiency. Planners and clients usually look first at the Ug-value (thermal insulation) or the g-value (solar control). Yet a third figure says the most about the real performance of a glazing unit: the selectivity value. It describes how intelligently a coating works – and determines whether a room overheats in summer or stays bright and naturally lit despite effective solar protection.

1. What selectivity actually describes

Selectivity (S) is not a standalone physical unit but a ratio. It relates the light transmittance Tv to the total solar energy transmittance g (g-value). Under the European standard DIN EN 410, the formula is:

S = Tv / g

Light transmittance (Tv): the percentage of visible light (wavelength 380 to 780 nm) that passes through the pane into the interior.

Total solar energy transmittance (g-value): all incoming energy – the directly transmitted radiation plus the heat re-radiated inwards by panes that have absorbed and warmed up.

The goal is a high numerator (Tv) with a low denominator (g-value). If a glass lets in 70% of the light but only 35% of the heat, the result is a selectivity of 2.0.

2. From tinting to high-performance coatings

For decades, solar control was a compromise. Early glasses were either heavily tinted or metallically mirrored – as the g-value dropped, light transmittance always collapsed with it. The consequence: occupants sat cooler but had to switch on artificial light even during the day. A poor deal both energetically and for the feel of the room.

The breakthrough came with the magnetron sputtering process. In a high vacuum, ultra-thin precious-metal layers – usually silver – are applied to the glass. Depending on the layer structure, different performance classes emerge: double-silver coatings reach selectivity values of around 1.8 to 1.9, while modern triple-silver technologies exceed 2.1 in selected high-performance products.

These coatings act as spectral filters: they let short-wave visible light pass largely unhindered while reflecting a large part of the long-wave infrared radiation – the heat. This combines plenty of daylight with effective summer heat protection.

3. Why 2.0 serves as a benchmark

In professional planning, the selectivity value is a key performance indicator for solar control glass. Which value makes sense depends on use, building type and façade concept. Especially for demanding glazing, high selectivity values are gaining importance.

Daylight is the cheapest and healthiest light source in a building. Studies on biophilic design show that natural light reduces absenteeism and boosts concentration. A highly selective glass lowers cooling loads and noticeably increases daylight autonomy. Broad product families – such as the more than 60 variants of the SOLARLUX® range – make it possible to choose exactly the right S-value for each project: from highly neutral solutions like SOLARLUX® E71 to reflective variants for special design requirements.

Glazing types in an energy comparison

4. Standards and the GEG

The German Buildings Energy Act (GEG) and DIN 4108-2 (summer thermal protection) set clear limits for the solar gain coefficient. With increasingly hot summers, demonstrating summer thermal protection often becomes the biggest hurdle in the approval process.

Highly selective glasses – for example from the SOLARLUX® series – are often the only way to realise large glass surfaces without permanently hiding the façade behind closed blinds. Selectivity thus bridges legal requirements and the desire for transparency.

5. Planning notes: where selectivity makes the difference

• South and west façades: variants with high selectivity and a low g-value.
• North façades: glasses focused on light transmittance.

Because the coatings within the series are technologically matched, the façade looks visually uniform on all orientations despite different technical values.

Solar Control Glass in Passive Houses – Which Values Matter Most?

6. Sustainability: climate resilience through glass

With global warming, cooling is becoming the new heating. In modern office buildings, the energy demand for cooling often already exceeds that for heating. A highly selective glass is therefore an investment in the building's climate resilience.

Developments such as blinds within the cavity or radio-transparent coatings show that the selectivity value forms the basis for a multifunctional building envelope.

Quality reveals itself in the ratio. A good planner does not only ask about the g-value, but about selectivity. Anyone designing a building that should still be energy-efficient and user-friendly in twenty years has to master the balance between light and heat – and selectivity is the most precise way to measure it.

Author: Arnold Glas Marketing Department