Air leaks around windows, doors, and penetrations (pipes, wires entering exterior walls) account for a significant fraction of home heating and cooling losses. Caulk is used for stationary gaps — around window frames, where trim meets wall — while weatherstripping is used for moving parts like door edges and thresholds. Silicone caulk is waterproof and flexible for wet areas (tubs, exterior); paintable latex caulk is used for interior trim. A $5 tube of caulk applied correctly can meaningfully reduce utility bills.
Run a hand along door and window edges on a cold, windy day to feel air infiltration — or hold a lit incense stick near gaps and watch the smoke. Start with the most obvious leaks first.
Air infiltration is one of the largest and most overlooked sources of energy loss in a home. The Department of Energy estimates that air leaks account for 25–40% of heating and cooling costs in a typical house. Unlike insulation failures (which slow heat transfer), air leaks create direct pathways for conditioned air to escape and unconditioned outdoor air to enter. Sealing these leaks is one of the highest-return maintenance tasks you can do — materials cost a few dollars and the energy savings accumulate every month. From your prerequisite work with basic hand tools, you already have the skills to handle a caulk gun, which is the primary tool for this job.
The first distinction to make is between caulk and weatherstripping, which address different kinds of gaps. Caulk is for stationary joints — gaps that don't move. Common examples: where a window frame meets exterior siding, where a door frame meets the wall, where pipes or wires enter through an exterior wall, and around the perimeter of a bathtub or shower. Weatherstripping is for moving joints — the edges of doors and operable windows that open and close repeatedly. Door weatherstripping typically comes as foam tape, V-strip, or door sweeps, each suited to different gap locations (the sides and top of the door vs. the threshold). Using caulk on a moving joint will cause it to crack and peel within weeks.
Choosing the right caulk type matters. Silicone caulk is waterproof, remains flexible as surfaces expand and contract, and lasts 20+ years — it's the right choice for exterior joints exposed to weather and for wet interior areas like tubs and sinks. The downside: it cannot be painted and requires mineral spirits for cleanup. Paintable latex caulk (also called acrylic latex or painter's caulk) is the right choice for interior trim — baseboards, crown molding, gaps around window and door casings. It cleans up with water, accepts paint readily, and provides a clean finished appearance. It is not waterproof and will fail outdoors.
Application technique determines how long a caulk joint lasts. Cut the tube nozzle at a 45-degree angle, starting with a small opening (you can always cut more). Hold the gun at 45 degrees to the joint and push — don't pull — so the caulk is pressed into the gap rather than dragged over the surface. Apply a continuous bead in one pass. Then tool the joint: run a wet fingertip or a caulk-smoothing tool along the bead to press it in and create a concave surface that sheds water. Wipe excess immediately. Before applying fresh caulk to an existing joint, remove all old caulk with a utility knife and/or caulk remover — applying new caulk over failed old caulk creates a cosmetic patch that will crack along the original fault lines within a season.