Heat flow is unavoidable when two objects of different temperatures come into contact. Think of wrapping your cold hands around a hot cup of coffee. Windows, doors and skylights gain and lose heat in three different ways:
Solar radiation plays an important role in heating living and working spaces. Sunlight enters the inside via the glass panes and is absorbed by things like carpets, curtains and furniture. This stored heat is later slowly released, warming up the air in the room. Special glazing ensures that long wave radiation (i.e. the heat) does not leave by reflecting it back inside. This helps maintain a consistent temperature and prevent the need for additional heating. However, this has no effect on visible light which can pass back and forth unobstructed.
In plain English, the U-factor is how much warmth an object transfers from its warm side to its cold side. It is given in W/m²K (Watt per m² Kelvin) for those on the metric system. The lower the value, the better the insulation. A U-factor of 0.6 W/m²K is one of the best possible today, followed by U-values of 0.7, and up. Triple glazing for example, typically produces U-Factors between 0.4 and 0.8 W/m²K
The difference between single glazing (i.e. one pane) windows and modern double and triple is so great that single glazing is extremely rare today and not even allowed in many places. Switching to two and three panes of glass can reduce heat loss by 20% or more and dramatically improves a home's insulation.
What is the G Value ?
The G-value is a measure of how much solar heat (infrared radiation) is allowed in through a particular part of a building. Therefore a low value indicates that a window allows a very low percentage of solar heat to enter. This may be positive in very hot climates but not in cooler ones. In winter, with less sunlight, the U-value will actually be higher than the G-value. When sufficient sunlight is available, both the G-value and the U-value will be high. In this case, however, the higher and somewhat worse U-value is actually better.
The R-value is the measure of thermal resistance. This means the ability of energy to move from hot to cold areas through materials such as insulation, walls, windows and doors. The higher the R-value is, the more that material prevents heat transfer. A material's R-value is based on its resistance to conduction as well as its thickness. Considering the example of camping. Sleeping directly on cold ground will quickly make you cold. However, sleeping on a foam pad makes it more difficult (due to the higher R-value) for the cold to move from the ground to your body.
In short, for insulation in windows, high R-values are good. Low U-values are good and a balanced G value is the best for most climates.