The biggest news in window technology is “low-e” films for low-emissivity. These thin metal coatings allow the shortwave radiation of solar energy to pass in, but block most of the long-wave thermal energy tying to get back out.
Low-e films block most solar heat gain while transmitting most visible light. Windows treated with low-e film help in both the winter and summer, with little heat leakage coming inside on a hot summer day, or leaking outside in winter.
A low-e coating is virtually invisible from the inside, but most brands tend to give windows a semi-mirror appearance from the outside.
Low-e windows are available ready-made from the factory, where the thin plastic film with the metal coating is suspended between the glass panes, or, low-e films can be applied to existing windows.