Windows and heat loss

Windows have a considerable impact on a building’s energy use, because glass is significantly affected by solar radiation and airflow, outside and indoor temperatures, wind and occupant use. As a result, it is important for the window industry to understand how energy efficient a window is by measuring its thermal transmission, known as its U-factor. The U-factor is simply described as the rate of heat transfer through a window. The lower the U-factor, the lower the heat loss, and the better the window will insulate a building.


The U-value is measure in W/m2.K
W = amount of heat transmitted in watts
m2 = one meter squared of the material
K = each temperature difference across the face of, or through, the material in °C 

Measuring the U-factor takes conductivity into account as well as the glass’s emissivity, the ability of a window to absorb energy and then radiate that energy away from it. A window with high emissivity transfers over 84 % of its energy from a warm room to the cold air outside. Therefore, the lower the conductivity and emissivity of the glass, the less the rate of heat loss and the lower the U-factor.

The National Fenestration Rating Council (NFRC) developed a standard for rating windows, doors and skylights for U-factor, known as NFRC 100. NFRC 100 has standardised environmental conditions, product sizes, and testing requirements for allowing those involved in construction, as well as consumers, to compare the performance of different glass products fairly and accurately. The South African Fenestration and Insulation Energy Rating Association (SAFIERA) is the country’s representative of the NFRC. When choosing products, always looks for those that have U-factors determined in accordance with NFRC 100, or that have SAFIERA Certified Performance.

Compiled by Daniella Favis for Builders’ Space. Source: SAGGA.

For the fenestration requirements regarding energy-efficiency, see Energy-efficient fenestration.