What a U-value measures
A U-value is the rate at which heat passes through a material. It is the amount of heat, in watts, that moves through one square metre for every degree of temperature difference between the inside and the outside. The unit is W/m²K. Lower is better: a low U-value means less heat escapes through the window in winter. (Heat from sunlight shining through the glass is measured separately.)
Single vs double glazing
Typical figures make the difference clear:
- a clear single-glazed aluminium window: about 5.6 W/m²K
- a clear double-glazed aluminium window: about 2.8 W/m²K
These are typical figures for the type of window, not test results for any one product. Double glazing puts a sealed gap between two panes, and that gap slows the heat down.
How heat gets through a window
Heat moves through a glazed window or door in four main ways:
- Air leakage — air getting in or out around opening frames that do not seal.
- Conduction — heat passing directly through the glass and the frame.
- Convection — warm air rising and cool air sinking against the glass, which moves heat along with it.
- Radiation — heat travelling as infrared through the glass.
Low-e glass
Low-emissivity (low-e) glass has a coating that cuts the heat radiated through the pane. It can be used on its own or as part of a double-glazed unit.
Where SANS 10400-XA comes in
SANS 10400-XA deals with energy usage in buildings. For new builds and extensions it covers fenestration — the windows and doors — so glazing choices can matter at plan stage. If you are building or extending, mention it when you ask for a quote.
Getting a quote
Tell us which rooms, roughly how big the openings are, and whether you are thinking single or double glazing. WhatsApp is the easiest way to start.
