Thermal performance values of glazing can be confusing, so here is a post to outline the basics. The two key values used when assessing the thermal performance of windows are the U-value and the Solar Heat Gain Co-efficient (SHGC). U-value is a measure of heat transmission through a window. Windows are the only building product that use U-value as the measure of transmittance. All other building products use R-value, which is the opposite of U-value. R-value describes thermal insulation (or resistance) of building materials.
In a cool climate (such as Tasmania), the smaller the U-value, the better. The smaller the U-value, the better the window is at preventing heat loss and the higher its thermal performance. Typical U-values range from 7.0 (for clear single glazing in a standard aluminium frame) to 1.0 (for high performing triple glazing in a composite timber/aluminium frame).
Solar Heat Gain Coefficient (SHGC) is a measure of the fraction of solar radiation that enters a building through the window as actual indoor heat gain. SHGC is presented as a decimal fraction between 0 and 1. It depends which climate you live in as to whether you may want to use a higher or lower SHGC. Typical values are between 0.2 for a dark tinted glass, to 0.75 for clear single glazing. In a cool climate such as Tasmania, typically clearer glass is more beneficial as it allows a greater percentage of solar radiation to enter and warm the building. However, each window needs to be considered individually with respect to the building type and the space that the window is serving. There will be instances where a lower SHGC may be beneficial to mitigate overheating.
Values for U and SHGC must be based on the whole window design, including the frame. Glazing manufacturers will often quote the U and SHGC values of their glazing units, but this is before the glazing has been put into a frame. The final U and SHGC values will likely vary greatly depending on the frame that the glazing unit is put into, the operability of the window, and the size of window pane.
More information about thermal performance can be found at the Australian Government's Your Home website. If you would like to talk more specifically about thermal performance on your own building or renovation project, contact us at RED Sustainability.
AUTHOR: STEVE WATSON
Steve is Director at RED Sustainability and specialises in sustainable buildings and energy efficiency.