Windows are among the largest sources of heat loss in a home, accounting for 25-30% of heating and cooling energy waste. Weatherization involves detecting air leaks (using a candle, incense stick, or thermal camera), then sealing them with the appropriate method: weatherstripping for moving parts like sashes, caulk for fixed joints between frame and wall, and window film or storm windows for single-pane glass. Threshold and sill plate gaps are often overlooked but are some of the worst offenders because they connect directly to the wall cavity and exterior.
On a cold, windy day, slowly move a lit incense stick around the perimeter of each window — the smoke will deflect visibly wherever air is infiltrating. Mark each leak with painter's tape, then address them by type: replace cracked or missing weatherstripping on operable sashes, re-caulk hardened exterior joints, and apply shrink-fit window film to single-pane windows as a low-cost insulation layer.
You already know how to apply caulk and weatherstripping. Window weatherization puts those skills to work on the most energy-lossy part of a typical home's envelope. Understanding why windows lose so much energy helps you choose the right fix. There are two distinct problems: air infiltration (drafts — cold outside air physically entering through gaps) and conductive heat loss (heat radiating through the glass itself from the warm inside to the cold outside). Weatherstripping and caulk address air infiltration; window film and storm windows address conductive loss. Confusing these two mechanisms leads to money spent on the wrong solution.
Air infiltration is the place to start because it's the cheapest to fix and often delivers the greatest comfort improvement. The key diagnostic tool is a draft test: on a cold windy day, hold a lit incense stick or piece of tissue paper slowly around the full perimeter of each window — along the sash edges, the frame-to-wall joint, and especially the sill plate at the bottom. Any smoke deflection or tissue flutter marks a gap. The appropriate fix depends on where the leak is. Weatherstripping goes on moving parts — the sashes (the sliding or hinged panels of glass) where they meet the frame. The sash compresses the weatherstripping when closed, creating the seal. If the seal is failing, the weatherstripping is either worn, compressed flat, or missing. Caulk goes on fixed joints — where the window frame meets the surrounding wall, both inside and outside. These joints don't move, so caulk's rigid seal is appropriate; don't caulk operable sashes, or you'll seal them shut.
For single-pane windows, air sealing alone may not be enough. A single pane of glass is an extremely poor insulator — cold air against the outside surface cools the glass itself, which then radiates cold into the room and can cause condensation. Heat-shrink window film solves this economically: a sheet of thin plastic is attached to the window frame with double-stick tape, then shrunk taut with a hair dryer until invisible. It creates a dead-air buffer between the room and the cold glass — the same principle as a double-pane window's gas fill, just with air instead of argon. Film typically reduces conductive heat loss by 35–50% through single-pane windows. Interior storm window inserts — rigid panels that pop into the frame — provide even better performance and are reusable season to season.
The sill plate and threshold gaps are the most commonly overlooked locations and often the most severe. At the base of each window, cold air can enter not just through the frame but down through the sill plate, into the wall cavity, and up through the floor. Exterior caulk at the window base — where the frame meets the siding or masonry — is critical and often absent or cracked in older homes. Inside, rope caulk pressed into gaps around the sill is a removable option for renters or seasonal use. When all interventions are complete, the payback is fast: window weatherization typically costs $50–200 per window to do thoroughly, while new windows cost $600–1,500 each. For most homes, a full weatherization pass recaptures most of the energy loss with a payback period measured in months, not years.