Why the default answer isn't always to replace

Draughty windows are one of the most common complaints in British homes, and replacement is usually the first thing suggested. It's also often the most expensive thing you can do. A full window replacement typically costs between £500 and £1,500 per opening fitted, so a modest semi with eight windows can run into five figures before you've addressed the walls, loft or boiler.

The thing worth knowing is that heat loss and draughts from windows usually come from three separate places: the frame, the seals and ironmongery, and the glazing. Only one of those is expensive to fix. A frame in decent condition with worn hinges, perished gaskets and a failed double-glazed unit can often be brought back to comfortable, quiet and reasonably efficient for a fraction of replacement cost. If the timber is sound, the opening is square and the window still opens and closes properly, repair deserves serious consideration before you reach for the catalogue.

Assessing the frame: rot, cracks and wear

Start with a thorough inspection. With timber windows, the critical areas are the bottom rail, the sill, the lower parts of the stiles, and any joint where water sits. Push a bradawl or small screwdriver into the timber with moderate pressure. If it sinks in easily or crumbles, you've found rot.

• Localised rot at a sill or bottom rail is usually repairable. A joiner can cut out the affected section, splice in a new piece of matching timber, treat and prime it. Expect £100–£300 per window.
• Rot affecting multiple joints or the full length of a stile or rail is a much bigger job, and the economics start tipping towards replacement.
• uPVC frames can't be repaired in the same way. Look for hairline cracks, brittleness, discolouration that won't clean off, and failed welds at the corners. A cracked weld in a corner is a structural failure.
• Aluminium frames often outlast everything else, but check the thermal break and any gaskets, which perish just like any other.

Also check whether the window is square. Open it, look at the gaps around the edges, and see whether the sash binds or drops. A twisted or dropped frame leaks air no matter how good the seals are.

The cheap wins: seals, hinges and draught-proofing

Before condemning a window, price the small items. Perished rubber gaskets, worn hinges, tired handles and failed draught seals are all the usual suspects, and they're genuinely cheap to sort.

• Replacement gaskets for uPVC windows cost a few pounds per metre and can be pushed into the existing groove with a blunt tool.
• Hinges and handles that no longer pull the sash tightly against the frame are a common cause of draughts. On uPVC, adjusting or replacing the friction hinges so the sash compresses the seal often solves the problem entirely.
• Sash windows benefit enormously from brush seals in the staff bead and parting bead, plus a decent draught strip. Paired with re-cording and re-balancing, an overhaul runs roughly £250–£500 per window — well under replacement, and it keeps the original joinery.
• Trickle vents should be checked rather than sealed up. A home needs background ventilation, and blocking vents to stop a draught can lead to condensation and mould elsewhere.

Doing these jobs yourself is realistic for the confident DIYer; a carpenter or window specialist will charge roughly £150–£350 per window for a proper overhaul.

Glazing: when the pane is the problem, not the frame

Misted double glazing with condensation between the panes means the sealed unit has failed, not the window. The unit can be measured and replaced — typically £80–£200 per pane — while the frame stays exactly where it is. If you're going to the trouble, it's worth specifying a low-emissivity coating, argon fill and a warm-edge spacer, which will comfortably outperform the original glass.

Single-glazed windows can be improved too. Secondary glazing fitted inside is the least disruptive option and can cut heat loss noticeably; slim double-glazed units can sometimes be fitted into existing timber sashes where the rebate allows. Be aware that if you're renovating more than 25% of a thermal element such as a window, building regulations may require the work to meet a reasonable thermal standard, so it's worth a quick conversation with your local building control team before you start.

Period and listed properties: repair is usually the right answer

In a conservation area, or on a listed building, replacement windows usually require consent and are often refused outright. That's not a bureaucratic quirk for its own sake — original timber sashes and casements are a big part of what makes a period property valuable, and a well-maintained hardwood frame can last a century or more with periodic maintenance.

A properly overhauled and draught-proofed sash window, with secondary glazing added, performs far better than most people expect and looks right on the house. Replacement units, however well made, rarely do.

When replacement genuinely is the better call

Sometimes it is. Consider replacing rather than repairing when:

• Rot or decay affects the majority of the frame, or the structure has failed.
• Several things have gone wrong at once — failed glazing, broken hinges, twisted frame — and the combined repair bill approaches half the cost of new.
• The windows are old single-glazed units and thermal performance is your priority, particularly if you're also insulating elsewhere.
• You need different glass for safety, security or noise, or want to change the opening configuration.

As a rough rule, if repairs come in under 40% of a like-for-like replacement, repair is usually the sensible choice — and it's almost always the greener one, since a window frame already has its embodied carbon paid for. Get two quotes for repair and one for replacement, look at the numbers side by side, and decide with your head rather than the salesman's clipboard.