Simulated stained glass static-cling window film can be changed and reused on another window, and will filter UV rays.
Old single-pane windows typically have significant heat loss and cold-air gain because of poor caulking and weather stripping (if there is any to begin with). When you're near them, you often feel chilly during cool months and warm when you're near them in hot months. The most significant heat loss and chilly feeling occur on clear winter nights. The R-value — a higher-the-better number that shows the ability of insulation to resist the transfer of heat — of a single pane of glass is only R-1, as compared to an insulated wall at R-13.
Fortunately, there are many things you can do on a limited budget to improve year-round efficiency of old windows. First, check the caulking and weatherstripping on the windows and ensure the framing is not deteriorated. Fix any subpar conditions or your next improvements won't be worth much. Sealing all air leaks around your existing windows is a priority and will result in the biggest saving.
Adding storm windows, either interior or exterior, can more than double energy efficiency. Custom-made, multi-track storm windows can often cost almost as much as totally new windows. You can even make your own using clear acrylic sheets. Also, acrylic blocks most of the sun's fading ultraviolet rays. Exterior storm windows can be made with 1-by-2-inch lumber, acrylic sheet, and foam weather stripping. If you size them to fit inside the wall opening and paint them to match your existing window frames, they will look like part of your windows.
To install interior storm windows, use a kit with magnetic seals. The magnetic section of the seal attaches to the acrylic sheet with an adhesive backing, and the steel strip attaches to the window frame. This allows you to easily remove them during summer for ventilation, but if you use air conditioning most of the summer, just leave them up year-round.
Another option is to install insulating window shades or curtains to increase the overall insulation level of the window opening and to block the radiant heat loss through the window. Something as simple as adding a pull-down pleated shade or making sure to close your Venetian blinds can block your skin's exposure to the cold outdoors.
Some of the most efficient window shades can add R-6 insulation to your windows. These are multi-layer roll-up shades with a heat-reflecting, airproof inner film layer to greatly reduce radiant heat loss (or gain during summer).
The newest energy-saving permanent window films are also effective for reducing wintertime heat loss. These films have just a very slight tint so they can't be detected and use the same type of microscopically thin low-emissivity metallic coating as expensive replacement windows. Simple vinyl static-cling film will also help a bit. But before installing anything on double-pane windows, check the window manufacturer's warranty regarding film application.
Do-it-yourself energy-saving film installation kits are available at most home improvement stores. You may want to select a darker tint if summertime heat gain is your most significant concern. Because the sun is higher in the sky during summer, installing window awnings for shade and a lighter film on south-facing windows will allow for some passive solar heating from the lower wintertime sun.
A final option is to install a tilt-in double-pane, sash-only replacement kit. If your existing frames are in good condition, this will convert your old windows into very efficient ones. This option also allows you to clean both sides of the window glass from indoors.