The total solar energy transmittance of glazing, commonly called the g-value, is an important parameter in energy-efficient building design. It indicates the proportion of incident solar energy that enters the interior through the glazing, both directly and through secondary heat transfer.
The g-value affects the balance between useful solar gains and the risk of overheating. Understanding it helps investors, designers and building owners compare glazing options in the context of the whole building.
What is total solar energy transmittance?
Terms such as g-value, solar factor and total solar energy transmittance describe the same basic glazing property: how much solar energy is transmitted into the interior.
The value includes energy transmitted directly through the glass and the inward-flowing part of the energy absorbed by the glazing. In general, a higher g-value allows more solar heat to enter, while a lower value provides greater protection against solar gains. Neither is universally better.
In winter, useful solar gains can reduce heating demand in some conditions. In summer, a high g-value can increase indoor temperatures and cooling demand. The result depends on factors including orientation, glazing area, shading, climate and the building’s thermal characteristics.
What determines the g-value?
The solar-energy performance of glazing is influenced by several factors. The type of glass and its coatings and treatment can raise or reduce solar transmittance. The number of panes and the configuration of the glazing unit also affect the result.
Glass tint and transparency are relevant as well. Solar-control and selective coatings are used to limit solar gains, while other glazing configurations allow a greater proportion of solar energy to pass through.
The choice should also consider the orientation and shading of the façade. Large south- or west-facing glazed areas may need stronger solar control, depending on the design, whereas useful winter solar gains may be part of the energy strategy in another building. The correct g-value is therefore a project-specific decision.
Requirements for window g-values
There is no single g-value that is appropriate for every window and every building. The expected performance depends on the building’s use, location, façade orientation, glazing area, shading system and heating and cooling strategy.
The design must balance energy performance with occupant comfort. In a highly glazed building, lower solar transmittance or effective external shading may be needed to control overheating. In another project, a higher g-value can contribute useful heat gains during the heating season.
The g-value should not be assessed in isolation. It forms part of a broader evaluation that can include the window U-value, visible-light transmittance, shading, airtightness, installation quality and the building’s energy model.
Assessing the solar-energy transmittance of windows with TERMOCENT
Solar-energy transmittance is one component of window and façade performance. A documented assessment can help verify the characteristics of installed glazing and investigate whether the glazing is consistent with the design assumptions.
TERMOCENT performs building and energy-related measurements for investors and property professionals. The appropriate method and conclusions depend on the purpose of the assessment, the available product documentation and the condition of the installed glazing.
The result provides technical evidence for further design, commissioning or diagnostic decisions. It should not be treated as a universal guarantee of the overall quality of the window or façade.
Speak to a specialist: +48 530 105 398
or email biuro@termocent.pl
