Quick answer: One warning sign on its own usually means a £15–£200 repair. Three or more signs together, especially across multiple windows, usually means it’s time to budget for full replacement (£300–£700 per window). This guide lists the 9 warning signs that actually matter, ranked by how serious each one is, plus a simple decision rule for when to repair vs replace.
The 9 warning signs
1. Misting or condensation between the panes (high seriousness)
If you can see fog or droplets between the two panes of glass and can’t wipe it off from either side, the perimeter seal in the sealed glass unit has failed. The argon gas that used to insulate has escaped; your “double glazing” is now performing closer to single. Repair (£40–£80 per pane) is usually enough — the frame is fine. See our misted window repair guide for full pricing.
2. Draughts you can feel near the frame (medium-high)
Hold the back of your hand 5cm from the frame on a windy day. If you can feel air movement, the gaskets, hinges or frame seal have failed. Most draughts are fixable for £15–£40 in DIY parts. See our draughty windows guide. Replace the whole window only when 5+ are draughty AND the frames look knackered.
3. Visible damage to frames (high seriousness)
Cracks, rot, warping, splitting at the corner welds (uPVC), bubbling paint (timber), or visible holes. None of these are fixable cheaply — by the time you see them, the structural integrity is gone. This usually means full replacement. One or two damaged frames can be done individually; widespread damage means a full-house package will work out cheaper per window.
4. Difficulty opening, closing, or locking (medium)
Sashes that drop when open, locks that don’t engage, handles that wobble — these are usually hardware issues, not frame issues. Hinge or lock replacement: £15–£60 per item, fitted. Full hardware swap on an older window: £80–£150. Only escalate to full replacement if the frame itself is what’s stopping the sash from sitting square.
5. Heating bills creeping up year-on-year (medium)
If your usage hasn’t changed but your bills are rising on the same tariff, the windows might be losing more heat than they used to. Old (pre-2002) double glazing typically performs at ~2.0–2.8 W/m²K U-value; modern A-rated windows are 1.4 or better. Replacement of all windows in a 3-bed semi typically saves £100–£250/year at 2026 prices — payback time 20–50 years. Worth doing for comfort, rarely worth doing for energy alone.
6. Cold spots when you touch the glass (low-medium)
All glass feels cooler than your skin — that’s physics. But if you’re getting icy-cold spots near the edges of the glass, the spacer bar conducting cold from outside has failed (this is what “warm-edge spacers” prevent in modern units). Mid-glass cold spots usually mean argon escape — see warning sign 1 above.
7. Outside noise getting louder (low)
The acoustic seal is part of the same gasket system that stops draughts. As gaskets age, sound transmission goes up. Often fixed by replacing gaskets (£15–£25 per window in DIY parts). For homes near busy roads where noise is the main concern, consider acoustic glass on full replacement — typical sound reduction goes from ~30 dB to ~38 dB.
8. Your windows are pre-2002 or single-glazed (high)
Pre-2002 double glazing was made to a much lower standard than current Building Regs require. Single glazing is essentially uninsulated. Both are draught and noise nightmares regardless of how they look. Full replacement is the default. Heritage-property exception: if you’re in a conservation area or the property is listed, secondary glazing inside the original window is often required instead — half the cost and avoids planning issues.
9. Water ingress or visible damp around the frame (high)
Black mould, peeling paint, visible water on the cill, or a wet patch on the wall near the window — the frame seal has gone (or never existed properly). Tackle the cause first: re-bed the frame (£100–£200 per window, fitted) or full replace if the rot has reached the brickwork. Don’t paint over it. Damp around windows almost always gets worse, never better.
How many warning signs before I replace?
A simple rule we use:
- 1 sign on a single window → fix that one thing (£15–£200)
- 1 sign across multiple windows → systemic problem (e.g. all gaskets are aging at the same rate). Cheaper to fix the gasket on all of them at once than to wait and replace.
- 2–3 signs on a single window → repair quote vs replacement quote. Often replacing one window costs only ~50% more than the cumulative repairs.
- 3+ signs across 3+ windows → time for full-house quotes. The economics are now in favour of replacement.
- Sign 3 (visible damage) or sign 9 (water ingress) on any window → that window needs to come out. Full replacement is usually the only safe option.
Cost expectations for full replacement
- Single uPVC casement window: £300–£500 supplied and fitted
- Bay window (typically 3-pane): £900–£1,800
- Sash window (uPVC): £700–£1,200
- Whole 3-bed semi (8–10 windows): £4,500–£8,000
- Whole 4-bed detached (10–14 windows): £7,000–£12,000
- Heritage / conservation area work: add 30–50%
For a precise estimate based on your home’s size, postcode, and window count, try our double glazing cost calculator. It runs in 30 seconds, no contact details required.
When NOT to replace
- You’re moving within 12 months — the buyer almost never pays back what you spent. Repair cosmetic issues only.
- Only 1–2 windows have a single fault each — fix those, leave the rest.
- The windows are under 15 years old AND the only complaint is energy bills — the savings won’t justify the cost. Better invested in loft/wall insulation, which has much faster payback.
- You’re in a listed building or conservation area — replacement may not even be permitted; secondary glazing is usually the legal alternative.
Frequently asked questions
Should I replace one bad window or wait until they all fail?
If only one is bad, replace just that one — full-house pricing only beats single-window pricing when 3+ windows need doing at once. Most installers will quote a single window happily. Just confirm the FENSA certification and warranty cover the single unit.
Will my house lose value if I leave old windows in?
It depends. In most UK markets, modern double glazing is now considered standard — older single or pre-2000 units count against the EPC rating, which buyers do check. But the £6,000 you spend rarely comes back as £6,000 in sale price. Replace because you want comfortable, quiet windows; don’t replace purely on the assumption you’ll recoup the cost on sale.
Are pre-2002 windows always worse?
For energy efficiency, yes — Building Regs Part L tightened sharply in 2002 and again in 2010 and 2022. A 1995 sealed unit is typically losing 60–80% more heat than a 2024 A-rated unit. For aesthetics and durability, the variation between brands matters more than the year of install.
If only the seals failed, can I just leave them?
Yes for the short term — the window still keeps the wind out, just not the cold. But you’ll lose 30–50% of the unit’s insulation performance, which means visible higher bills in winter. Most people fix within 12–18 months once the misting becomes obvious enough to bother them daily.
How long do new windows last?
Frames: 20–35 years for uPVC, 30–45 for aluminium, 50+ for properly maintained timber. Sealed glass units inside: 10–25 years. See our window lifespan guide for the full breakdown.
Get installer quotes
If you’ve ticked off three or more warning signs above, our quote service connects you with up to 4 FENSA-registered local installers within 24 hours. Free, no obligation, no pressure (or we drop the installer from our network).
Sources: BWF (British Woodworking Federation), Energy Saving Trust draught and glazing data (2026), FENSA Building Regs guidance, Pilkington WER ratings, Hazlemere Windows lifespan analysis. Pricing is UK 2026 estimate. Last reviewed: April 2026.
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