Sash windows are a common form of window made of an over and under arrangement, comprised of at least one set of movable panels, the sashes, that are arranged to multiple panes of glass separated by muntins. These are commonly found in homes built in the Victorian and Georgian style houses, designed specifically with two sets of sashes containing six panes of glass each. The only problem with this style of window is that they were traditionally single glazed, which isn't the most efficient arrangement for energy efficiency, and allows a fair amount of heat to get into, and escape from, the home.
One possible solution to this, if the windows in your home can handle it, is to try to get your windows double glazed. This will provide an extra layer of protection and insulation from the variant temperatures of the year. It can pay to have this done in your home by reducing the heating and cooling bills you may be suffering from now. One of the primary issues with having this done involves the structural integrity of your existing windows.
Room has to be made to accommodate the second glazing, and not all existing window panes are going to have the room available to rout out that second space. If you do have room, it's a simple matter of making enough room in the existing muntin and sash, and using a good window sealant to seal it in. However, you'll want to keep an eye on this, when you do this some protection will generally be lost, and that can lead to sealant drying out and pulling away. With all of these things under control, double glazing can provide you with superior energy efficient in your home, and save you hundreds of dollars a year.