Layers of Glass and Air Spaces
Single-pane. This has very little insulating value with (approximately r-1). It provides only a thin barrier to the outside and can account for considerable heat loss and gain.
Double-pane. To improve a window's energy efficiency the number of glass panes in the unit is doubled, because multiple layers of glass increase the window's ability to resist heat flow.
Types of Glass
There are various types of glass manufacturers use in constructing windows.
Clear glass has been the primary material available for window panes in homes. However, in recent years, the market for glazing--or cutting and fitting window panes into frames--has changed significantly. Now several types of special glazing's are available that can help control heat loss and condensation.
Low_emissivity (low-e) glass. This has a special surface coating to reduce heat transfer back through the window. These coatings reflect from 40% to 70% of the heat that is normally transmitted through clear glass, while allowing the full amount of light to pass through.
Heat-absorbing glass. This contains special tints that allow it to absorb as much as 45% of the incoming solar energy, reducing heat gain. Some of the absorbed heat, however, passes through the window by conduction and re-radiation.
Reflective glass. This has been coated with a reflective film and is useful in controlling solar heat gain during the summer. It also reduces the passage of light all year long, and, like heat-absorbing glass, it reduces solar transmittance.