Now that windows with good thermal insulating properties have become "state of the art," as reflected in some national building codes, attention has shifted to their total solar energy transmittance (g value) as the property of interest. In contrast to the u-value, where it is clear that lower values are advantageous under almost all climatic conditions, the optimisation direction for g values depends strongly on the prevailing climatic conditions. A high value, which allows maximal solar gains to contribute to space heating in winter and the transitional seasons, will cause major discomfort due to overheating in summer. By contrast, a constant low value acts against the primary aim of using solar energy in buildings to replace conventional, fossil-fuelled heating. The solution to this dilemma is to vary the g value according to requirements, which may well change several times during one day, depending on the location, building orientation and occupations.
