In contrast to double glazed windows single glazed windows consist of a single pane of glass, which if not protected by a film of some sort is very prone to heat loss and noise transmission.
Window insulation film is a plastic film which can be applied to glass windows to reduce heat transfer. There are two types in common use designed to reduce heat flow via radiation and convection respectively.
Solar control film:
This works by reflecting the infra-red component of solar energy and absorbing the UV component. Typical absorption for a silvered film is 65% for visible and infra red with 99% for UV. This type of film sticks directly onto the glass.
Convection control film:
This film is attached to the window frame using double sided pressure sensitive tape. The effect is to create a double glazed system with a still air layer about 0.5 inches thick between the film and the glass. This restricts the convective air flow which efficiently transfers heat onto the inside glass surface. The reduced heat flow also can prevent condensation which is triggered when the temperature of the inside surface falls below the dew point.
The film is a heat-shrink plastic which allows a hair dryer to be used to remove creases and improve optical clarity. Since the thermal conductivity of air is much lower than glass the heat flow via conduction could be theoretically reduced by 97.5% though this is limited in practice by slow air movement in the convective cell formed between the film and the glass.