Where the warmth actually escapes

If you stand close to a single-glazed window on a cold February evening, you can often feel the chill coming off the glass. That sensation is not imagination — it is heat leaving your home. In a typical UK house, windows and doors are responsible for somewhere between a quarter and a third of all heat loss, and draughts through gaps around frames, letterboxes and sills can quietly account for another sizeable slice of your heating bill.

The good news is that this is one of the most fixable problems in any home. You do not need to spend five figures on replacements. What you need is a logical approach: find the leakiest openings, seal them, then improve the glazing and dressings in the rooms you heat hardest. This guide walks you through that order of work, from a few pounds of draught excluder to properly specified replacement glazing.

Start with the leakiest openings first

Before you spend anything on new glass, spend an hour finding where cold air is actually getting in. On a windy day, hold the back of your hand near window frames, door edges, skirting and sill joints. A thin trickle of cold air will tell you immediately where the money is going.

Walk around the house and check these common culprits:

• Window frames — gaps between the frame and the wall, cracked external sealant, loose putty around older single panes, and worn beading.
• Opening sections — the gaps between casements and frames, and around sash windows where the two halves meet.
• Letterboxes and keyholes — a surprising amount of heat leaves through an unguarded front-door letterbox.
• Door bottoms and sides — check thresholds, especially on older back doors and external garage or utility doors.
• Chimneys and vents — an open chimney is a hole in your insulation, and unused vents can be capped (but never block a vent that serves a boiler, fire or gas appliance).

Seal these with brush strips, silicone sealant, foam strips, letterbox flaps and keyhole covers. A full draught-proofing kit for a whole house usually costs well under £100 and can cut heating bills by around £30–£50 a year, with the added comfort of no cold draught around your ankles.

Glazing: what actually pays back

If your windows are single glazed, you have three realistic options. The first is secondary glazing — a second pane or a rigid acrylic panel fitted inside the existing window, creating an insulating air gap. It works well, suits period properties, and is often the only acceptable option in a listed building or conservation area. Expect to pay roughly £300–£600 per window professionally installed, or £20–£40 for a DIY film and magnetic strip kit.

The second is full double glazing, ideally with a low-emissivity coating and argon fill. Modern units achieve U-values around 1.2 to 1.4 W/m²K, compared with roughly 5.0 for a single pane. That is a dramatic improvement. A typical semi-detached home heated by gas can expect to save somewhere in the region of £100–£150 a year, though the bigger benefit is often comfort and reduced condensation.

Triple glazing is worth considering on north-facing walls or in very exposed locations, but the extra cost rarely pays back in milder parts of the UK. Whatever you choose, insist on trickle vents being fitted or retained — a sealed-up house with no background ventilation will simply trade heat loss for mould.

Curtains, blinds and pelmets that earn their keep

Window dressings are the cheapest upgrade in this whole article, and most homes get them wrong. A thin cotton curtain hanging a few centimetres short of the sill does very little. A properly specified thermal curtain, closed before dusk, does a great deal.

• Choose thermally lined or interlined curtains that reach the floor or the sill, with as little gap as possible at the sides.
• Add a pelmot or curtain track with a valance to stop warm air flowing up behind the fabric and straight out of the glass.
• Close curtains at dusk and open them in the morning to let in solar gain on south-facing windows.
• Use honeycomb or thermal roller blinds in kitchens and bathrooms where full curtains are impractical.
• Never hang curtains over a radiator under a window — the heat simply rises up the cold glass. A radiator shelf or moving the curtain to the wall can help.

Doors: the openings people forget

Doors suffer more wear than windows, so seals degrade quickly. On external doors, fit a brush or wiper seal around the frame and a draught excluder or threshold plate at the base. A letterbox with a sprung flap and a keyhole cover cost pennies and take minutes to fit.

Consider a door curtain on a front or back door that opens directly into a living space — a heavy, floor-length curtain on a track above the doorway is remarkably effective. If your porch or conservatory is unheated, treat the door between it and the house as an external door, because that is what it is in thermal terms.

Inside, closing internal doors to unused rooms keeps heat where you want it. It costs nothing and works immediately.

Prioritise, then work through the list

If you do one thing this weekend, walk around your home and seal the three leakiest openings you can find. That is almost always the best value for money in home energy efficiency. Then, if budget allows, tackle the worst-glazed windows — usually those in the coldest, most used rooms — followed by heavier curtains throughout, and finally full replacement glazing where the payback and comfort gains justify it.

Keep ventilation in mind at every stage. A home that is airtight but poorly ventilated will develop damp, and damp feels colder than dry air at the same temperature. Leave trickle vents open, run extractor fans in kitchens and bathrooms, and keep your improvements working together rather than against each other.