When selecting a window in terms of U-value and g-value, both values must be considered and weighed up against each other.
A purely value-based comparison when choosing the glazing should, however, be avoided. Instead, when planning your window purchase, it is advisable to consider where and for what purpose the windows will be installed. Factors such as the surrounding masonry, solar radiation or the compass direction, as well as the purpose of the room to be insulated, all play a role in balancing the U-value and g-value:
The g-value is only significant for windows that also receive substantial solar radiation in winter. For windows on a north-facing façade it can be disregarded, whereas it is important for a sunny south-facing façade.
The colder the climate at the location in question, the more important the U-value. Heat loss is higher in cold locations, whereas solar heat gain does not depend on the outdoor temperature.
For these reasons, windows for north-facing façades and for very cold locations are chosen primarily according to the U-value, whereas on a south-facing façade somewhat poorer U-values can certainly be accepted in favour of higher g-values.
Please note that high g-values also contribute to warming in summer, so that stronger solar shading may become necessary in order to prevent the room behind the glazing from overheating too much in the case of large glass façades. Special solar-control glass with a particularly low g-value below 50% (e.g. by means of a thin silver coating) is often used for fully glazed façades, as well as for car windscreens. In winter they can of course be a disadvantage if the low g-value is not compensated for by a very good U-value.
