Every draught is warm air you have paid to heat sliding out of the house, with cold air pulled in behind it to replace it. Sealing those gaps is among the cheapest jobs you can do, needs no skill, and is one of the few improvements you feel the same evening you finish it, because a room without draughts feels comfortable at a lower thermostat setting than a draughty one. That lower setting is where the saving quietly compounds, winter after winter.
First, find the draughts
You cannot seal what you have not found, and some of the worst gaps are not the obvious ones. On a cold, windy day, walk the house slowly with the back of a wet hand or, better, a lit candle or a stick of incense, and watch the flame or smoke. Where it pulls sideways or flickers, air is moving. Do this around door and window frames, along the bottom of skirting boards, at the loft hatch, around pipes coming through walls, at old vents and at the fireplace. The wet-hand method works too: bare skin feels moving air as a cool line even where you cannot see it.
Doors
External doors are usually the biggest single offender, and they leak in several places at once. Fit compression or brush draught strips around the frame so the door closes against a seal, a brush strip or weighted excluder along the bottom where the worst draught often is, a sprung flap or brush on the inside of the letterbox, and a cover over an unused keyhole. Internal doors to unheated spaces, an integral garage or an unheated porch, are worth sealing too. None of this is hard, and a complete door kit costs very little.
Windows
Openable windows leak around the opening edge, where self-adhesive foam or E-strip and P-strip rubber profiles seal the gap when the window shuts. Older sash windows are notorious draught sources and take a little more work, with brush strips in the runs and seals along the meeting rail, though the reward is large because they leak so much. Fixed panes that whistle around the frame can be sealed with a bead of frame sealant. For the cold radiating off the glass itself, which is a different problem from draughts, see secondary glazing and curtains for warmth.
Floors, skirting and service holes
In older houses with suspended timber floors, cold air rises straight up between the floorboards and along the gap where the boards meet the skirting. A flexible filler, a proprietary gap-filler strip, or simply a good rug takes the edge off, and the fuller treatment is on the underfloor insulation guide. Do not forget the small holes you never think about: where pipes and cables pass through external walls, behind kitchen units, around extractor ducting and waste pipes. A squirt of expanding foam or silicone seals each one in seconds, and together they add up.
Chimneys and the loft hatch
An open, unused chimney is effectively a hole in the roof through which warm air streams out around the clock; it can be one of the largest draughts in the whole house. A chimney balloon, an inflatable plug, or a sheep's-wool chimney draught excluder stops it, while still leaving the small amount of ventilation a flue needs to stay dry. Remember to remove it before ever lighting a fire. The loft hatch is another easily missed gap: fit draught stripping around its edge and a scrap of insulation to its back, so you are not letting heat pour up into the cold loft you took the trouble to insulate.
What you must leave alone
This is the part to get right, because over-sealing causes its own problems. A house needs a controlled amount of fresh air to carry away the moisture that cooking, washing and breathing put into it; block every route and that moisture condenses on cold surfaces and grows mould. So leave airbricks clear, leave the trickle vents in window frames open, and never seal the extractor fans in the kitchen and bathroom or the air supply to a fuel-burning appliance like an open fire or certain gas heaters. The aim is to stop the uncontrolled, wasteful draughts, not to make the house airtight.
Why it is worth doing first
Draught-proofing rarely tops the table for raw heat saved, since the gaps are small individually, but it wins on value because it is so cheap and so fast. The whole house can usually be done over a weekend for the price of a couple of takeaways, the payback is quick, and the gain in comfort is out of all proportion to the spend. Most importantly, a sealed room lets you sit comfortably with the thermostat turned down a notch, and as the thermostat settings guide explains, that lower setting is where the real, repeating saving lives. Do this alongside the loft and a turned-down boiler flow temperature and you have the three best-value heating jobs done for very little money.