Where the heat actually goes

Before you spend a penny, it helps to know where the warmth is leaving. In a typical British home, roughly a fifth of heat loss goes through the windows, and doors — especially old timber ones with a gap you could post a magazine through — account for a good deal more. On a still day you may not notice a thing. Add a stiff north-easterly and an ill-fitting sash window starts to behave like an open window with glass in it.

Some of the leakage is obvious: a rattling letterbox, daylight under the back door. Much of it isn't. Heat slips out through the narrow gaps between frame and wall, around tired sashes, along skirting boards and up an unused chimney. Cold air creeps in low, warm air drifts out high, and the room feels chilly even when the thermostat reads a respectable 20°C. The trick is to slow that movement down, not to seal the house into a plastic bag.

Draught-proofing: the cheapest win in the house

Start by finding the culprits. On a windy day, run the back of a damp hand slowly around window frames, door edges and skirting. You'll feel a cold kiss where the air is moving. A stick of incense or a smoke pencil does the same job and shows the direction of travel. Fix the biggest, most obvious gaps first — draught-proofing follows the law of diminishing returns, and the first ten pounds spent usually does more than the next fifty.

• Sash windows: fit brush or wiper seals in the staff beads and meeting rails, and a sash lock so the two halves pull tight against each other. If the sashes rattle in a breeze, that rattle is money leaving the room.
• Casement windows: press a self-adhesive rubber or EPDM seal into the rebate where the sash meets the frame. Check the hinges — a hinge that has dropped a few millimetres leaves a gap along the top that no amount of sealant will cure.
• Doors: fit a brush strip or hinged flap to the bottom, brushes around the frame, a double-flap letterbox with a brush, and a cover over the keyhole. Seal the gap between frame and wall with a flexible frame sealant that stays slightly rubbery.
• Chimneys: if a fireplace is genuinely out of use and no open-flued appliance depends on that flue, a chimney draught excluder or inflatable balloon stops a surprising amount of warm air vanishing upwards.
• Skirting and floors: run a bead of decorator's caulk along the top of skirting on external walls, and fill gaps between floorboards in the coldest room if the boards are exposed.

Curtains, blinds and the value of trapped air

It is the air gap, not the fabric, that does the insulating. A curtain hung three centimetres from the wall with daylight leaking around all four edges is largely decorative. So widen the track: extend it 15–20cm beyond each side of the window and fit it as close to the ceiling as the pelmet allows. Floor-length, interlined or thermal curtains trap a decent cushion of still air, and closing them at dusk makes a noticeable difference to how a room feels.

Be careful with radiators. Long curtains draped over a radiator send heat straight up behind the fabric and out through the glass, which is the worst of both worlds. Tuck them behind a shelf above the radiator, or fit a reflective foil panel on the wall behind it so radiant heat bounces back into the room rather than into the brickwork. On sunny mornings, open the curtains on south-facing windows and let the sun do some of the work for free.

Blinds work too. Thermal roller blinds and honeycomb styles reduce heat loss, but leave a small gap between blind and glass where you can — a sealed cavity performs better than one you've squashed flat against the pane.

Secondary glazing: quiet, warm and reversible

Secondary glazing adds a second pane inside the existing window, leaving an air gap of around 20–100mm. It can cut heat loss through a single-glazed window by half or more, and it takes the edge off traffic noise as a bonus. Professional systems are neat and sliding, but a DIY acrylic panel fixed with magnetic strip costs only a few tens of pounds per square metre and can be lifted out in summer. Shrink-film kits cost less than a takeaway and last a season — ideal for a spare room or a rented flat where you can't change the windows.

Two rules. Keep at least one opening section per room for ventilation, and never cover a window you rely on as a fire escape. While you're at it, look after what you've got: crumbling putty, cracked panes, a broken sash cord and a frame that no longer meets the wall all leak heat. A joiner can put those right, and it's money well spent before you spend anything on new glazing.

Seal the gaps, but never the air supply

Here is where heating advice meets gas safety. A gas appliance needs air to burn safely. Open-flued boilers, gas fires and cookers in small kitchens draw combustion air from the room, and if you block the air bricks, vents or trickle vents that feed them, the flame can burn incompletely and produce carbon monoxide — colourless, odourless and potentially fatal.

• Never block an air brick, wall vent or trickle vent, however cold the draught feels.
• Fit an audible carbon monoxide alarm to the current British Standard and test it monthly.
• Have every gas appliance checked annually by a Gas Safe registered engineer, and never use a gas fire or boiler you suspect.
• Draught-proofing a bathroom or kitchen solidly traps moisture, so keep extract fans working and open a window briefly after a shower or while cooking. Damp air costs more to heat and eventually brings mould.

A sensible order of work

Work outward from the biggest gaps and keep an eye on ventilation as you go.

• Walk the house on a windy day with a smoke pencil and note every gap, including the ones behind curtains.
• Fix the front and back doors, letterbox and window seals first — that's where the obvious losses are.
• Rehang curtain tracks wider and closer to the ceiling, and move curtains clear of radiators.
• Consider secondary glazing for the two or three coldest rooms before looking at full replacement.
• Book the annual boiler service, check the carbon monoxide alarm, and confirm every vent is clear.

Do those things and a room that needed the thermostat nudged to 22°C will often hold a comfortable 20°C — with the heating working less hard, the boiler happier, and the house a good deal quieter too.