Find the gaps first

Most draught-proofing goes wrong before a single strip of foam is unrolled. People buy a pack of self-adhesive seal and head straight for the front door because that is the obvious place — and the front door probably does need attention — but the cold coming through the hallway at ankle height is rarely just the door. Seal it perfectly and you may notice almost no difference, because the real leak is under the floorboards, or behind a redundant pipe hole, or inside a skirting board nobody has looked behind in thirty years. The job is not to seal: the job is to find the gaps first, and then to seal.

You need a windy day and a smoking taper. Everything else follows from that.

The smoking taper survey

A draught-proofing survey costs you nothing and takes about ten minutes per room. The principle is simple: move a thin thread of smoke slowly along every joint, gap and surface you suspect, with all windows closed and the loft hatch shut. Where the smoke bends sharply, air is moving. Where it drifts straight up undisturbed, there is no draught worth treating. The smoke makes visible what your hand cannot reliably feel, especially near carpet or curtains that already buffer the air.

Close the room up properly before you start. A window left on the latch half an inch will pull air across the whole room and confuse the picture entirely. Do this on a day when there is genuine wind outside — a still day in summer will show you almost nothing, because the pressure difference that drives cold air in is simply not there. The best conditions are an overcast day with wind from any direction, the kind of day when you can already sense cold coming from somewhere and cannot quite locate it. That is exactly the day to run this survey.

Move methodically. Start at floor level — crouch down and work the taper along the skirting boards, pausing at every internal corner, every section where pipes or cables enter the room, and every section where the skirting meets a chimney breast or an exterior wall. Then work up: run it along window frames, door frames, and the joint where ceiling meets wall. Finally check the obvious large openings: the letterbox, the keyhole, any cat flap. The order matters because air at floor level is usually colder and denser, and skirting-level leaks are the ones most commonly missed.

Write down what you find as you go — a rough sketch of the room with marks at each leak point is enough. Memory is unreliable once you start actually sealing things.

What the smoke actually reveals

The gaps that show up are usually surprising. Floorboard gaps are one of the most common sources of cold air entering a room, and yet almost nobody thinks to look there first — they go straight for the window frames instead. Skirting boards often have a small but continuous gap at their base, particularly on exterior walls, and behind them the void connects directly to the underfloor space. That continuous gap, running the full length of one wall, can shift more cold air than a loose door seal. If you have floorboard gaps and uninsulated space beneath, the floor itself is worth surveying on its hands and knees.

Chimney breasts catch many people out. Even a boarded-up fireplace with a decorative surround often has an air path — sometimes through the board itself, sometimes around its edges. An unused chimney flue is essentially a dedicated vertical wind tunnel open at both ends, and the pressure difference on a windy day is enough to pull cold air in at the base even when the fireplace appears shut. If the taper bends strongly toward a boarded fireplace, that opening deserves proper attention before anything else.

Pipe and cable entries are a perennial source. Where a radiator pipe comes through a floorboard or a wall, the hole cut by the original plumber is almost always slightly larger than the pipe itself, because cutting exact is harder than cutting approximate. The gap is small, but on an exterior wall it is open to cold air. The same is true of telephone and television cable entries, extract fan surrounds, and any place where a wall was cut and not fully made good. Electrical back-boxes set into external walls sometimes have no insulation behind them at all, which you can feel with a hand but can confirm definitively with the taper.

One thing the survey will also reveal — and this is as important as finding the gaps — is where you must not seal. Airbricks and trickle vents are not draughts: they are deliberate ventilation paths that protect the fabric of the building and serve combustion appliances. If the taper bends near an airbrick, that is the airbrick working. Leave it. The what-not-to-seal guide covers this distinction in full, because misreading a ventilation path as a gap to fix is one of the genuinely consequential mistakes in this kind of work.

From the survey to the list

Once you have gone round the whole room — and ideally every room that feels cold — you will have a list that almost certainly looks different from the list you started with in your head. The front door is probably on it, but so is a two-metre run of skirting board on the north wall, and the gap where the soil pipe exits through the kitchen wall, and the loft hatch you walked past on the landing. That list is now a priority order: the largest, most open paths come first, and small finishing gaps come later once the main routes are closed.

Do not be tempted to seal as you go on the survey day. Survey the whole house, then step back and look at the full list. You may find that three gaps in one room are actually part of the same air path, and that closing one upstream point fixes all three. Air finds the path of least resistance; closing one gap can redirect it rather than stop it, until you close the next one. Understanding the whole picture before you act means fewer return visits with the taper.

A draught-proofing survey costs you nothing and takes about ten minutes per room.

The taper survey is worth repeating after you have done a round of sealing, again on a windy day, because new gaps sometimes reveal themselves once the strongest air path is closed. It is not a single-pass job. Think of the first survey as reconnaissance: it tells you where the battle is, not where you assumed it was.

The ten minutes you spend with a lit taper, working quietly round a cold room, will save you from sealing the wrong things with the wrong materials in the wrong order — which is how most draught-proofing ends up half-done and largely ignored by the next winter.