To save energy, most people think old windows have to be replaced. Not true and your return on investing in "saving" your windows will give you so much more in return. Let's review the facts. First, in general terms if you're seeking energy improvements in a building, windows and doors generally only account for 5-10% of the entire building's thermal performance. Next, there are four key areas to address when it comes to window efficiency and the means by which one can bring an "old" window up to a modern, two-pane standard: good glazing, good weatherstripping, non-drafty weight pockets, and a good, well-installed storm window. All of those things combined, when well done, will give you equal or maybe even better energy performance compared to a new, modern two-pane window. Good glazing condition or technique means your individual panes of glass will not leak. Weather stripping is a requirement, whether your windows are old or new and is designed to stop/slow down air infiltration at around your window sashes. We use solid metal, interlocking weather stripping as it's historic looking and it outlasts all others and stops air infiltration. It works beautifully and it also allows your windows to move freely. Leaky weight pockets, part of the old pulley-and-weight balance system, should be addressed where their interior and exterior casings sit to make a seal at the wall surfaces. Last, a good quality, well-installed storm window will bring the needed complement to an old window that's in good condition to create a window assembly whose standard is of equal energy performance to modern replacements.