Drafty windows are a major source of heat loss and cold spots. Most can be sealed cheaply, buying years before replacement makes sense.
Find the leaks
On a windy day, run your hand or a lit incense stick around the window edges and sash to find where air moves. The gaps between the sash and frame, and between the frame and wall, are the usual leak points.
Seal the gaps
Add weatherstripping where the sash meets the frame, and caulk the stationary gaps between the frame and the wall. For single-pane and older windows, interior shrink film adds an insulating air layer for winter — invisible once installed.
When to replace
If the window has a failed seal (fog between panes), rotted frame, or you're sealing the same drafts every year, replacement may be worth it for comfort and efficiency. Until then, sealing is a cheap, effective fix.