If winter left you cold, now’s the time to prepare for next year. Start by caulking completely outside and in, including: the crack between the jamb and fixed windows or windows you never open… along window stops… and along window casing where it meets the wall and the window frame. (You can also remove trim and spray in foam insulation.) Check glazing putty for cracks, and weatherstrip moving windows (more on weatherstripping another time).
We lose body heat to cold surfaces, so icy glass chills us even if the room is comfortably warm. You can keep the inside surface warm by adding a second layer of material. Some options:
Shrink-wrap plastic works well and installs fairly easily with a blow dryer, but it’s easy to puncture and you have to apply it every year.
Exterior storms are reasonably effective, but a big job in spring and fall. Fixed storms save work, but alter your house’s look. Non-yellowing Plexiglass, affixed directly to the window or trim makes a fairly unobtrusive all-season storm.
(Never caulk an outer storm window. Moist air from the house can get trapped and condense.)
It’s easier to install homemade or manufactured Plexiglass interior storms in the winter, but they require good, reusable seals to prevent condensation.
Curtains or blinds stay warm, but can actually increase condensation by trapping moisture near the glass.
Sealed, insulated windows have a more substantial one-time cost, but are your best bet. These have at least two panes of glass, factory-sealed with at least 1/4″ air space (and sometimes a special gas sealed in). Low-E glass reduces the amount of heat transferred through the glass.
You can buy new windows, but it’s also possible to retrofit old single-pane glass with an insulated unit. (This will be at least 1/2″ thick).