Most homeowners replace their windows for one of two reasons: either something visibly fails — a frame rots through, a unit blows and fogs up — or they’re renovating and the old windows look tired alongside everything new. Both reasons make sense, but they’re also the reasons most people end up either replacing windows too late, after months of wasted heat and discomfort, or too early, on the back of a salesperson’s pitch rather than actual need.
The honest answer is that windows tend to give plenty of warning signs before they fail. You just have to know what to look for.
The condensation test
Condensation on the inside of a window pane is normal in cold weather, especially in kitchens and bathrooms. Condensation between the panes of a double-glazed unit is not. It means the seal has failed, the inert gas (argon, usually) that gives modern glazing its insulating properties has escaped, and you’ve effectively lost the thermal performance you paid for.
A blown unit doesn’t always need the whole window replaced — sometimes the sealed unit alone can be swapped. But if multiple units in the same property have gone, or the frames are also showing their age, full replacement usually makes more economic sense than a string of repairs.
The quickest way to check is on a cold morning: walk around the house and look at each pane. If you can see moisture, mist, or a streaky residue trapped between the two layers of glass, that unit has failed.
Draughts you can feel without a candle
A common DIY test involves holding a candle near the frame on a windy day and watching the flame. It works, but you don’t need it. If you can sit near a window in winter and feel cold air on your face or hands, the seals around the sash or frame have degraded.
Older uPVC windows — anything fitted in the 1990s or early 2000s — used gaskets and rubber seals that harden and shrink over twenty-plus years. The window still closes and locks, but it no longer seals properly. Heating bills climb, rooms feel uncomfortable, and you end up over-heating the rest of the house to compensate.
Frame movement and visible damage
For timber-framed windows, soft or spongy wood around the sill is the obvious sign — usually rot starting from the bottom up where water has been collecting. For uPVC, look for hairline cracks, especially around the corners where the frames have been welded, and any yellowing or chalkiness on the surface. Once uPVC has degraded to that point, the structural integrity of the frame is compromised, and no amount of cleaning will bring it back.
Aluminium frames are generally the longest-lasting of the three, but their gaskets and thermal breaks can still fail. If you can see daylight around a closed window, or if the sash doesn’t sit flush in the frame anymore, the window is past its useful life.
The heating bill that won’t come down
This one is harder to spot because it creeps up slowly. If you’ve insulated the loft, replaced the boiler, fitted new radiators, and your bills are still higher than they should be for the size of property, your windows are a likely culprit. Old single glazing loses roughly four times as much heat as modern double glazing, and even early-generation double glazing performs significantly worse than today’s units.
The energy rating system makes this comparable. Modern A-rated or A+-rated windows have measurable U-values (a measure of heat loss) that are dramatically better than the windows fitted in most UK homes thirty years ago. If you’ve never checked what’s actually in your walls, it’s worth doing.
Security catches that no longer engage
This is the one homeowners often dismiss as “just needing a bit of jiggling”. A window that no longer locks cleanly, or where the multi-point locking system has stopped engaging on all points, isn’t a minor irritation — it’s a security weakness. Insurance policies often have specific clauses about working window locks, and a window that doesn’t lock properly is also unlikely to be sealing properly.
If you find yourself working a window handle harder than you used to, or relying on a single locking point when the window was designed to have three, the mechanism is on its way out.
What “replacement” actually involves
If multiple signs above apply, it’s probably time to get quotes. A few things worth knowing before you do:
Standard replacement windows fall under permitted development in nearly all cases, so planning permission isn’t normally required. Listed buildings and conservation areas are the obvious exceptions, and a reputable installer will tell you upfront which categories apply.
Any installer fitting replacement windows in England or Wales must be FENSA registered (or equivalent) so the work is certified for building regulations compliance without needing a separate building control inspection. Always ask to see the FENSA number before agreeing to work.
The other thing worth asking is how long the company has been trading. The industry has a high failure rate among smaller installers, and a window guarantee from a company that’s gone bust five years later isn’t worth much. Established firms — those who’ve been trading two or three decades — generally exist for a reason: they do the job properly enough that they’re still here.
Don’t wait for things to fall apart
The mistake most homeowners make is treating window replacement as an emergency response rather than a planned upgrade. By the time a unit visibly fails, you’ve usually been losing money on heating for several years already. The payback period on modern double glazing — even ignoring the comfort improvement — is shorter than most people assume.
If three or four of the signs above describe your windows, it’s worth getting a proper assessment. Reputable companies will quote without pressure and will tell you honestly if a repair makes more sense than a replacement.
If you’re in need of new Windows, consider Amazon Windows, a family-run home improvement company supplying and installing windows, doors, conservatories and solid roofs across Hampshire, Berkshire and Surrey since 1989.