Window with excellent g-value, Ug-value and LT-value

Glass specifications for an optimal energy balance

Glazing from swissFineLine scores top marks in respect of its energetic quality. In order to achieve an optimal energy balance, glazing solutions must be tailored to the building’s specific needs.

They should help to cool the building in summer, heat it in winter, and always keep the interior bright and filled with light. This ideal scenario places tough demands on the energetic properties of construction materials, and particularly transparent elements. For many years, striking the right balance between heat loss and solar heat in the form of visible light has posed a significant challenge, especially for large window façades.

Frameless sliding windows from swissFineLine score top marks in respect of total solar energy transmittance (g-value), thermal transmittance (Ug-value) and light transmittance (LT-value). However, it’s not possible to simply define optimal values that apply across the board. Instead, architects take numerous factors into account to define suitable values for a building, or specific areas within a building.

The g-value

The total solar energy transmittance value describes how much energy can penetrate the pane and thus deliver an energy gain for the building’s interior.

  • It is composed of direct solar radiation that passes through the pane (1) and secondary heat dissipation (2)
  • The lower the g-value, the less energy reaches the building’s interior
  • Benefit of a low g-value in summer: building interiors heat up less
  • Benefit of a high g-value in winter: lower heating costs
  • The optimal g-value is determined in each case by the architect
Graphic illustration of the g-value (total solar energy transmittance) of a pane of glass

Solar energy gain: if the g-value is 0.5, this means that 50% of solar energy reaches the building’s interior

The Ug-value

The thermal transmittance coefficient of glass quantifies the amount of energy a building loses through a window element.

  • Ug-value determines heat loss
  • The lower the value, the better the glass insulation
  • Triple-glazed insulation glass, which is used in swissFineLine elements as standard, achieves values of up to 0.5 W/(m2K)
  • Bullet-resistant panes in resistance class FB6-NS use double-glazed insulation glass, which can achieve Ug-values of approx. 1.0 W/(m2K)
Graphic illustration of the thermal transmittance coefficient (Ug-value) of glass

The LT-value

The light transmittance level indicates the proportion of visible light that passes through the pane.

  • Visible light has a wavelength of 380nm to 780nm (based on the light sensitivity of the human eye)
  • The higher the LT-value, the more visible light passes through to the building’s interior
  • Factors including the glass thickness, reflectivity and coatings affect the LT-value
  • LT-values: up to 90% for float glass, up to 70% for insulated glass
  • Despite the higher LT-value, it is still possible to achieve up to 99.8% UV protection > for more details, see UV protection
Graphic illustration of the light transmittance level (LT-value) of glass

Light transmission: an LT-value of 80% means that 80% of visible light passes through a pane and into a building

All at a glance.

Download the factsheet on glass specification as a PDF now.

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99% reduction in aggressive UV radiation.


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Noise Protection

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