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Thermal Properties of Double Glazed Units


Whether there is direct sun on a window or not, heat is still pouring through a conventional glass window. Heat can travel in three ways:

  1. Radiant heat is felt from the direct sun
  2. Convection is the transfer of heat by a current of air (e.g. either ‘natural' convection - warm air rises, or 'forced' convection - a breeze by a window, both of these can result in a draught by a window
  3. Conduction is heat moving through an object (i.e. heat will travel through an aluminium, or steel frame more easily than through a timber or PVC frame)

U – Value

U Value – measurement unit is watts per m2 per degree Celsius (W/m2°C) and is a measure of the rate of heat gain or loss through glazing due to environmental differences between outdoor and indoor air.

U represents the heat transfer through a window irrespective of direct sunlight. For example, heat still moves through a closed window at night-time. The U-value is specific to windows and glass and is the inverse of the commonly used R-value, which is used for insulation properties of walls and other building materials. For this reason the lower the U-Value the better the insulation performance. The U-value is important for measuring the heat transfer both in and out of the window. The more heat transfer that the window can resist the better an insulator it is. It can either stop the heat from coming in on a hot summer day or stop the heat escaping on a cold winter evening

Solar Heat Gain Coefficient (SHGC)

SHGC (Solar Heat Gain Coefficient) – the proportion of total solar radiation that is transferred through the glass at normal incidence. It comprises the direct solar transmission and the part of the solar absorption dissipated inwards by radiation and convection. The lower the number the better the solar performance.

Is a fraction that is used to indicate the amount of radiant heat that can travel through a window. This coefficient is largely affected by direct sunlight, which is radiant heat. The lower the SHGC, the better the material will stop solar heat coming into the building through the window. A low SHGC is like having a very effective shade over the window, except that the view will be less restricted. In the summertime it is important for this number to be as low as possible as we are trying to minimise unwanted solar heat gain. In a winter climate we are trying to keep this number as high as possible as it is important to let the solar heat in but not let it out.



In winter the ideal situation is to let in as much of the sun's heat and then trap it inside to maintain a comfortable environment in your home or office. This will also greatly reduce heating costs and directly relate to money savings. The ideal window for winter will have a high SHGC to let the sun's heat in and a low U-Value to prevent the heat from escaping. Clear windows without shading films are better suited for winter climates.


In summer the ideal situation is to minimize the heat coming in from outside and prevent the cool air inside from escaping through the window. This will make your home more comfortable and reduce your air-conditioning costs. The ideal window for summer, or for a large west-facing window, will have a low SHGC to limit the heat from the sun's direct rays, as well as, a low U-Value to prevent the heat in the air outside from moving through the window and warming the cool conditioned air inside. It is better to have a shading film on the first (outer) glass pane to limit the heat that enters the double glazed unit.

Glass Thickness U SHGC Reduction in Heat transfer Reduction in radiant heat
Clear 3mm glass 5.9 0.86 N/A N/A
Clear 4/12/4 2.7 0.85 54.24% 1%
Bronze 4/12/4 2.7 0.58 54.24% 32%
Lowe E-Clear 4/12/4 2.1 0.55 64.41% 36%
Lowe E-Bronze 4/12/4 2.1 0.43 64.40% 50%

Reductions are worked on compared to 3mm float glass
Using Argon instead of air there can be a further 3% improvement

Detail Thermal Properties of Glass

Glass Performance Data - Energy