Most window manufacturers use r-values in rating their windows. Traditionally, clear glass has been the primary material available for window panes in homes. Now several types of special glazing's are available that can help control heat loss and condensation. Reflective glass 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.
Plastics can be stronger, lighter, cheaper and easier to cut than glass.
Some plastics also have higher solar transmittance than glass. However, plastics tend to be less durable and more susceptible to the effects of weather than is glass. The simplest type of storm window is a plastic film taped to the inside of the window frame. These films are usually available in prepackaged kits. Although plastic films are easily installed and removed, they are easily damaged and may reduce visibility.