For many years, discussions around window performance in Europe centred primarily on U-values and thermal insulation. Today, however, changing climatic conditions, overheating risks and evolving building regulations are shifting attention toward another critical technical indicator: the solar factor, or g-value.

Defined under EN 410, the g-value represents the total fraction of solar energy transmitted through glazing into the interior. It includes both direct solar transmission and the secondary heat transferred inward after absorption by the glass. In practical terms, a higher g-value allows greater solar heat gains, while a lower g-value reduces overheating risk and cooling demand, a balance that has become essential in contemporary façade design.

Despite decades of innovation, a large share of Europe’s building stock still relies on outdated glazing systems. Many installations continue to operate with single glazing or early generations of uncoated double glazing installed during the 1970s and 1980s. Modern glazing solutions are typically five to ten times more efficient in overall performance, yet windows remain in buildings for around 40 years. As a result, each renovation decision effectively locks in energy performance for decades.

This makes specification choices increasingly critical. Renovation alone does not guarantee progress: replacing outdated windows with products that merely satisfy minimum requirements, or that overlook solar performance, risks repeating the same inefficiencies for another generation of buildings.

Across many EU markets, regulatory assessments still focus mainly on thermal insulation. While U-value addresses heat loss, it provides little indication of how much solar energy enters a building, a factor that is becoming increasingly significant as cooling demand grows. This gap between regulation and real performance leads to several challenges:

• Windows with acceptable insulation values may still contribute to overheating.
• Minimum standards are sometimes set close to the performance of the least efficient products available.
• Solar gains, orientation and façade exposure are often absent from national requirements.

Industry experts increasingly highlight energy balance, combining heat losses and solar gains, as the most accurate way to assess real window performance. Countries such as Denmark and the United Kingdom have begun applying this principle more systematically, reflecting a broader move toward performance-based evaluation.

Italy provides one of the clearest regulatory examples focused directly on solar control. Since 2015, Italian renovation rules requiring a town-planning permit have introduced:

• A maximum g-value of 0.35 for glazing installed on façades oriented from east through south to west,
• Exclusion of north-facing orientations where solar gains are minimal,
• Climate-zone-specific application reflecting regional solar exposure,
• The possibility of integrating movable shading devices within compliance calculations.

This framework demonstrates how orientation and climate conditions are increasingly shaping building policy beyond insulation-focused metrics alone.

The growing emphasis on g-value is also reshaping strategies across the façade supply chain. Solar control coatings are moving from optional upgrades to core specification requirements. Architects and façade consultants are required to evaluate orientation and shading earlier in the design process, while fabricators must communicate solar performance alongside thermal data. HVAC design is becoming more closely linked to glazing selection, as reduced solar gains can significantly lower cooling loads.

In highly glazed commercial buildings and modern residential projects, balancing daylight transmission with solar protection has become a defining design parameter.