Last edited 08 Feb 2018

Solar transmittance (gtot)

Solar heat gain in buildings can be beneficial in the winter, as it reduces the need for heating, but in the summer it can cause overheating.

The total solar heat transmittance into a space is the sum of the solar heat that is transmitted directly through its fabric (such as solar radiation transmitting through glass), and the solar heat that is absorbed by its fabric and then re-emitted into the enclosed space.

Total solar energy transmittance (gtot) relates to the performance of solar protection devices such as blinds and shutters and is the percentage of solar energy incident on the fabric of a blind or shutter that is transmitted to the interior of the building, including both the transmittance of the solar protection device itself and of the glazing.

According to the EN 14501 standard (Blinds and Shutters, Thermal and Visual Comfort, Performance Characteristics and Classification), it is measured on an index of 0 to 1. The closer the fabric’s index is to 0, the more efficient it is at protecting against heat and reducing solar energy; whereas an index of 1 means that all the incident solar radiation is transmitted.

Two methods are used for calculating the gtot of a solar protection device:

  • Simplified method of EN 13363-1: Calculates approximate values for the gtot of glazing and shading combined, using the solar integrated optical and thermal parameters. This can be calculated easily in a spreadsheet.
  • Detailed method of EN 13363-2: Calculates more precise values using spectral transmission and reflection data. This requires specialised software.

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