Upgrade leaky windows. Wood and vinyl are better insulators than aluminum.
Wood frames are more expensive than aluminum and vinyl.
Double glazing insulates almost twice as well as single glazing. Adding a third or fourth layer results in additional efficiency gains. The air space between double panes of glass in a double-glazed window has a major impact on the efficiency performance. A very thin air space does not insulate as well as a wider air space. These new glazing reduce the solar heat gain without reducing visibility.