Many people think that to improve their home's energy efficiency, they need to replace all of their antique windows with newer ENERGY STAR windows. These new windows typically have double or triple panes, are filled with gasses like argon for insulating purposes, utilize low conductive materials, and have low emissivity (known as "low e") coatings that can either absorb heat into a house or reflect heat away from the house. With a huge budget, these energy efficient windows are a great addition to a whole house remodel project.
The problem, however, is that high efficiency windows are quite expensive, typically a couple hundred dollars per window plus installation costs. A much more cost-effective upgrade you can make to an existing home is air sealing around the windows. The problem with many homes is that the original windows were never installed properly, so there are large gaps in the framing around the window itself. This allows air to travel around the window and enter through the cracks, irrespective of what type of window you have in place.
To prevent the air from "sidestepping" the window, you need to remove the window trim and spray foam the sash weight cavities and in between the framing, and then caulk around the outside of the trim. In the end, you'll spend significantly less money on air sealing than you would replacing all your windows, while the results are similar.