Glass
Glass allows visible light and short-wave infrared radiation to pass through. The measure of how transparent glass can be is called its transmittance and is defined as the fraction of incident light that can pass through the glass.
Glass is most obviously used for windows, in fact it is used anywhere where it is required to let light through. An example of a use other than as a window is a garden coldframe or even a greenhouse. In both these cases letting light through is important but keeping heat in and minimising thermal loss is just as important.
Double-glazed windows are recommended to minimise heat loss. But there are other factors to consider.
The first is the glass itself, for different types of glazing. The U–Values are calculated as the amount of heat lost through one square meter of glass for each degree of temperature difference between the outside temperature and the inside temperature. There are standard U-values for the different types of glazing. For example a single-glazed window has a U-value of 5 W m-2 0C-1.
Example: calculate the heat loss through a 2.5 square metre double-glazed window with a U-value of 3 on a day where the indoor temperature is 10OC and the outdoor temperature is 25OC.
This is 2.5 x 3 x (25 - 10) = 112.5 W.
Over 24 hours then the total heat loss would be 112.5 x 24 = 2.7kWh.
If the above window was single-glazed with a U-value of 5 then
the calculation is 2.5 x 5 x (25 - 10) = 187.5 W.
Over 24 hours then the total heat loss would be 187.5 x 24 = 4.5kWh.
It is also important to realise that the window frame has a U-value as well.
 Parabolic Trough Collector system image from Wikimedia Commons |
 Solar power plant at Kramer JunctionSolar power plant at Kramer Junction image from Wikimedia Commons |
