Heat Insulation R and G-Values
The difference between single glazing (i.e. one pane) windows and modern double and triple is so large that single glazing is rare today and even forbidden in many building codes. 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 measures how much solar heat (infrared radiation) is allowed in through a specific part of a building. Therefore, a low number indicates that a window allows a very small percentage of solar heat to enter. This may be desirable in 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 will be high. In this case, however, the higher and somewhat worse U-value is actually better.
What is the R-value?
The R-value is the measure of thermal resistance. This means the ability of energy to move from hot areas to cold areas via materials such as insulation, walls, windows and doors. The higher the R-value is, the better 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.
U, R and G Value Summary:
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.