Every autumn the same complaint turns up: one room runs cold, the next runs warm, and the heating bill explains nothing. Drafty windows rarely announce themselves with a whistle. They show up as a cold ankle under the desk, a curtain that stirs with the radiators off, a thin line of condensation along the bottom of the glass. None of that means the window is finished. Most air leaks come from a handful of predictable places, and one windy afternoon with the back of your hand is enough to find them.

Start With the Hand Test

Choose a day when the wind is actually doing something. Close and lock the window first — an unlocked sash sits a millimetre or two off its seal and will read as a leak even when the weatherstripping is fine.

Now run the back of your hand slowly around the perimeter of the frame. The back of the hand, not the palm: it is far more sensitive to moving air. Work in a loop — down the hinge side, across the sill, up the latch side, along the head — then repeat along the meeting rail where the two sashes close against each other. Pause for two or three seconds at each point. A leak feels like a cool thread running across the skin.

Mark every spot that reacts with a small piece of painter's tape. It saves you from rediscovering the same gap three times, and it turns a vague sense of "this window is draughty" into a list you can work through.

On a still day, substitute a thin strip of tissue or a smouldering incense stick held close to each joint; watch for the smoke to bend or flutter. Skip the candle. Curtains and draughts are a poor combination.

Where Leaks Actually Hide

Air takes the easiest route, and around a window that route is usually one of a few familiar paths:

• The gap between the sash and the jamb — where the operable part slides or swings. This is the most common single source, and the one weatherstripping is designed to close.
• The meeting rail on a double-hung window, where the upper and lower sashes overlap. A worn or flattened strip here lets air straight through the middle of the window.
• Along the sill, especially where the interior sill meets the frame and where the exterior sill has pulled away from the wall.
• The perimeter of the frame, where trim meets plaster or drywall. Gaps here are often hidden behind a bead of old, cracked caulk.
• Behind the trim, where the rough opening was never insulated properly by whoever installed the window.
• Door thresholds and the bottom corners of exterior doors, which behave exactly like window leaks and are usually solved the same way.

Look for the visual clues too: a dark line of dust collected along one edge of a frame, stained paint at a joint, or a strip of condensation that always forms in the same place. Air carries moisture with it, and it leaves a signature.

Test the Seals Systematically

Feeling for a draft tells you where air gets in. These checks tell you why, which determines the fix.

1. The dollar bill test. Close the window on a banknote so that it is trapped between sash and frame, then try to pull it out. On a well-sealed window it should drag noticeably. If it slides out with no resistance, the weatherstripping has lost contact.
2. Check the compression. Open the window and look at the strip along the jamb and the meeting rail. Is it flat, torn, brittle, or peeling at the corners? Closed-cell foam that has hardened and vinyl that has split are both past saving.
3. Look for daylight. Stand inside on a bright day with the room lights off. Any visible sliver of light is a gap you can fix.
4. Test the hardware. A latch or lock that no longer pulls the sashes tight together will leave a gap at the meeting rail no matter how good the strip is. If the window rattles when you press it, the seal is not being compressed.
5. Check the frame itself. Look for soft wood, bubbling paint, or staining at the bottom corners — signs that water has been getting in alongside the air.

Weatherstripping: For the Parts That Move

Anything that opens and closes needs a flexible seal, and that means weatherstripping rather than caulk. Match the product to the gap rather than buying whatever is on the shelf.

V-strip, in vinyl or spring bronze, is the workhorse for sash edges on double-hung and casement windows. It compresses into a thin gap and springs back, so it survives years of opening and closing. Tubular or bulb seals suit wider or uneven gaps, particularly on doors and on older sashes that no longer sit perfectly square. Door sweeps and threshold seals handle the bottom of exterior doors, where a V-strip has nothing to press against. Self-adhesive foam tape has its place for a quick seasonal fix on a basement window you rarely open, but it compresses flat and crumbles within a season or two — treat it as a stopgap.

Clean the surface with a damp cloth and let it dry before applying anything adhesive. Cut strips slightly long rather than short, and miter the corners so they meet cleanly instead of overlapping and holding the sash open.

Caulking: For the Joints That Shouldn't Move

Caulk belongs on static joints — the frame to the wall, the trim to the plaster, the exterior sill to the masonry. It does not belong anywhere a sash has to slide, and it certainly should not be run around a closed window to "seal it up," because you will be painting it shut.

Remove the old bead first; fresh caulk over failing caulk peels away within months. For gaps deeper than about six millimetres, press in a foam backer rod before caulking, so the sealant has something to grip and does not slump into the cavity. Choose acrylic latex for interior joints that will be painted, and a exterior-grade silicone or polyurethane for outside work, where it must cope with sun, rain and movement. Tool the bead with a damp finger or a caulking tool for a neat finish, and work within the temperature range on the tube — most sealants misbehave in the cold.

Finally, look at the outside of the window while you are at it. Gaps between frame and wall, cracked sealant under the sill, and missing mortar all let water in behind the frame, which is a bigger problem than a draft.

When to Stop and Call a Professional

Some leaks are not a tube of caulk away. Call someone if the sash is visibly warped or will not sit flush when locked; if the wood is soft or the paint is blistering at the bottom corners; if condensation is forming between the panes, which means the sealed glass unit has failed and no amount of stripping will help; or if hinges have pulled loose from the frame. Windows high on an upper floor, and any work involving ladders or the exterior of a building, are better left to someone insured to do it.

If you rent, small fixes like weatherstripping are usually worth doing yourself, but structural repairs and glass replacement are a conversation with the landlord rather than a weekend project.

A Two-Hour Routine Before the Cold Arrives

Do this once in early autumn and the difference is immediate. On a windy day, walk the house with painter's tape and mark every reacting joint. Test each window with the bill trick, note which ones have flattened or missing strips, and note any that no longer latch tight. Then buy only what you need: a roll of V-strip for the sash edges, a couple of tubes of sealant and a backer rod for the frames, a sweep for the back door. Work through the worst offenders first — usually the north-facing and windy-side windows — and leave the rest for a second pass. A single afternoon of this, done before the first hard frost, keeps the warm air where you paid for it.

Photo: cottonbro studio / Pexels