Solar Gain Coefficient (G-Value)
In the context of windows and glazing, the g-value, also known as the solar heat gain coefficient (SHGC), represents the fraction of solar radiation (both directly transmitted and absorbed and then reradiated) that enters a building through a window or glazed surface. It is expressed as a number between 0 and 1, with higher values indicating a greater ability to transmit solar heat into a building.
G-Value use in Part O Overheating
The default g value for our simplified overheating assessment method is 0.60. We can use tailored values within dynamic thermal modelling simulations using TM59 methodologies.
What is a G Value implication on SAP?
In SAP/ HEM Calculations, solar gains are measured as part of overall gains on the space heating demands/ energy use requirements.
The more solar energy that enters a building, the less space heating that is required in the winter. But, in the summer this can lead to overheating.
Like everything in energy modelling, there needs to be a balance. That is why overheating needs to be assessed at planning stage and a full design is carried out to avoid issues further down the building process.
What are other considerations of G Values?
- Acoustic Attenuation
- U-values
- Off-set/ Thermal bridging
- Passive Ventilation
- Purge Ventilation
- Means of Escape
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