Glazing specification

The g-value of glass is used to describe how much solar heat energy (short wave infrared radiation) can pass though a material. It is a factor on a scale from 1 to 0. Where 0 represents no solar gain and 1 represents maximum solar gain. For residential schemes a g-value of 0.5 is optimum. Reducing the g-value to mitigate the risk of overheating is not recommended. This can lead to larger proportions of glazing than is necessary, because glass has a higher conductive heat loss than an insulated wall, large amounts of glazing can greatly reduce the energy efficiency of the building.

Design Actions

• Use a g-value of 0.5 (for schools this can lowered to between 0.4-0.5)
• Avoid tinted glass or glazing films, with g values of below 0.5 as these reduce useful solar gain in winter. If overheating is a problem, it is better to reduce the proportion of glazing or use solar shading.

How much glazing should there be?

Getting the right glazing-to-wall ratio on each façade is a key feature of energy efficient design. Windows and doors conduct more heat than an external wall, even when they are super-efficient. It is useful to think about how much energy a window will lose compared to how much solar energy it will capture across the year. This will tell you whether you should really reduce the amount of glazing to a minimum (important on north facing elevations) or provide larger glazing areas (important in south facing elevations). Don’t forget that glazing is expensive - being careful about where glazing is used is beneficial for energy, thermal comfort, and the project budget. Rooflights can be useful for providing daylight to spaces, but they should be used conservatively. They are often less energy efficient than standard windows and harder to shade, contributing to overheating risk.

Design Actions

• Use LETI Climate Emergency Design Guide recommended glazing proportions.