The seven signs in priority order
1. Condensation between the panes (the big one)
If you can see mist or water droplets between the two panes of glass — not on the inside of the room-side glass — the perimeter seal of the sealed unit has failed. The desiccant inside the spacer bar can no longer absorb moisture, and the unit has lost its insulating gas fill. There's no fix for this; the unit needs replacing. The frame itself may still be sound, in which case unit-only replacement is cheaper than a full window.
2. Draughts you can feel
Run the back of your hand around the frame perimeter, the cill, and the meeting rails on a windy day. Detectable air movement means the gasket has compressed or split. On a window under 15 years old, gasket replacement is often the right answer. On a window over 20 years old, the rest of the frame is likely close to end of life too.
3. Sticking, binding or stiff openings
Casement windows that won't shut without a shoulder, sashes that need both hands and a wedge, or tilt-and-turn handles that grind. Causes range from settled foundations and warped frames to worn hinges. Hinges are cheap to replace; a warped PVC frame is end-of-life.
4. Visible rot, swelling or peeling
Original timber frames can be repaired up to a point — splice and re-paint can get another 10 years out of a sound original window. Once rot is into the cill or the meeting rails, replacement is the right call. PVC doesn't rot but does warp and discolour; once it's bowed, there's no straightening it.
5. Heating bills creeping up year-on-year
Energy-bill creep with no other obvious cause is often glazing. A typical Essex semi with windows from 1995 will have whole-window U-values around 2.8–3.2 W/m²K. Modern A-rated replacements run 1.2–1.4 W/m²K — roughly half the heat loss through the same opening. The annual heating saving across a full house typically pays back the install over 8–12 years.
6. External noise more intrusive than it used to be
Worsening road or aircraft noise is rarely the road getting louder — usually the gaskets and seals losing their acoustic compression. Acoustic-laminated glass plus fresh gaskets can give 35dB+ reduction over old single-pane or basic double-glazed units.
7. Locks that no longer engage properly
Multi-point locks on PVC and aluminium windows are rated for ~50,000 cycles. Daily use over 15+ years gets close to that. If the lock won't engage smoothly or you're getting "lift to lock" failures, the gearbox is going. Standalone gearbox replacement is possible but on an old window often signals the start of a wider end-of-life.
What looks like a problem but isn't
Condensation on the inside of the room-side glass
This is a ventilation problem, not a window problem. New windows have lower internal-glass temperatures than old single-pane (because the outer pane is now insulating the inner pane), so condensation can sometimes get worse after a replacement if the house wasn't ventilated properly to start with. Trickle vents and decent extract in kitchens and bathrooms is the fix.
Cold air near windows
Cold radiating off cold glass feels like a draught but isn't. Hold a candle near the frame — if the flame doesn't move, it's radiation, not airflow. New A-rated glazing fixes radiation through warmer internal glass surface.
Cosmetic discolouration on PVC
Yellowing on white PVC frames is UV-driven and entirely cosmetic. The frame is structurally sound. Specialist PVC restorers can bring colour back at a fraction of replacement cost — worth trying first.
When you can wait
Single signs in isolation usually don't justify a full replacement. One sticky sash can be re-hinged. One misted unit can be replaced individually. One draughty meeting rail can be re-gasketed. Replacement makes sense when you've got three or more signs across multiple windows — that's when individual fixes start to add up to more than a coordinated replacement programme.
If you're seeing several of these signs across the house, our window installation service in Essex covers free survey, condition assessment, and an honest "repair vs replace" call on each window. The wider glazing and windows category covers double glazing and aluminium windows in detail.