Diagnosing a draught in five minutes
Work the perimeter with the back of your hand on a windy day. Around the opening edge means the door isn't compressing. Around the frame's outer edge means the seal to the brickwork has failed. Through the letterplate or keyhole means exactly what it says. Under the threshold means the weatherbar or brush strip has worn.
The paper test settles the compression question. Close the door on a sheet of paper and pull. If it slides out freely at one point on the perimeter and grips at others, the door isn't sitting evenly — that's an adjustment, not a new door.
| Where you feel it | Likely cause | Typical fix |
|---|---|---|
| Along the opening edge | Compressed or worn gasket | Replace the gasket |
| One corner only | Door out of adjustment or hinge sag | Hinge adjustment |
| All round, evenly | Lock not pulling the door in | Adjust keeps or lock cams |
| Frame to brickwork | Failed external seal | Rake out and re-seal |
| Under the door | Worn brush or weatherbar | Replace the strip |
| Through the letterplate | No internal flap or brush | Fit a draught-excluding letterplate |
The fixes that cost almost nothing
Gaskets perish. They're a rubber strip in a groove and they harden and shrink over ten to fifteen years, and they pull out and push back in as a length of new material. This is the single most common cause of a draughty uPVC door and it is not a reason to replace the door.
Hinge and keep adjustment is next. uPVC and composite door hinges adjust in two or three axes, and keeps can be moved a couple of millimetres to increase compression. A door that has 'gone draughty' after a hot summer usually just wants the keeps tweaked.
- Replace the perimeter gasket if it's hard, flattened or shrunk at the corners
- Adjust hinges to lift a dropped leaf back square
- Move the keeps to increase compression across the lock side
- Lubricate the multi-point lock so it pulls fully home
- Renew the external mastic between frame and brickwork
- Fit a brush or flap to the letterplate
When a draught does mean replacement
There are three cases. First, a frame that has moved because it was never properly packed — once the frame is out of square the lock can't be adjusted back into compression. Second, a slab that has bowed, which happens on hollow-core doors and can't be corrected. Third, a rotten timber frame, where there's nothing sound left to fix hardware to.
In those cases we'd say so and quote — uPVC doors from £799 fully fitted, composite from £1,099. In the far more common cases above, we'd tell you it's an adjustment.
Condensation: three different problems
Where the water sits tells you everything, and it's the only question that matters.
Inside the room, on the room side of the glass: warm moist air meeting a cold surface. That is a ventilation and humidity issue in the house, not a fault in the window. Outside, on the external face: the glass is colder than the outside air because so little heat is escaping through it. Counter-intuitively that's a sign the unit is working well. Between the panes: the sealed unit has failed and moisture has got inside the cavity.
| Where the condensation is | What it means | What to do |
|---|---|---|
| Room side of the glass | Humidity and ventilation | Ventilate, use trickle vents, extract at source |
| Outside face of the glass | Very low heat loss | Nothing — it's a good sign |
| Between the panes | Failed sealed unit | Replace the unit, not necessarily the window |
Internal condensation — what actually helps
A household generates a lot of moisture: cooking, showering, drying washing indoors, and simply breathing. If that moisture has nowhere to go it condenses on the coldest surface, which is usually a window.
The fixes are about moisture and airflow rather than glass. Use the extractor while cooking and for a while after, close the bathroom door while showering and extract, avoid drying washing on radiators, and open the trickle vents. If your windows have trickle vents and they're taped or painted shut, that's frequently the whole answer.
Where new windows do help is by raising the internal glass temperature, so the dew point is reached less often. An A-rated unit with a warm-edge spacer runs noticeably warmer at the edge than an old aluminium-spacer unit, which is why edge-of-glass condensation and black mould on the reveal often disappear after replacement. But if you never ventilate, the moisture will just find the next coldest surface.
Misted units: replace the glass, not the window
If the misting is between the panes, the perimeter seal has failed. No amount of ventilation touches it and nothing can be done to dry the cavity permanently.
The good news is that on a frame that's otherwise sound and locking properly, the sealed unit alone can be replaced. That's a fraction of the cost of a new window. We'd only recommend the whole window where the frame is also failing — hinges gone, locks unavailable, or the profile itself cracked or distorted.
The order to do things in
Diagnose the draught by location. Fix gaskets, hinges, keeps and seals first, because they're cheap and they solve most cases. Only look at replacement if the frame or slab has physically failed.
For condensation, decide which of the three it is before spending anything. Between the panes means a new sealed unit. On the room side means ventilation. On the outside means you can stop worrying.
Mould on the reveal: what it's telling you
Black mould at the edge of the glass or on the plaster beside the window is condensation that has been left in place long enough to grow something. It is a moisture problem, not a dirt problem, and cleaning it without changing the conditions simply resets the clock.
Two conditions produce it: humid air with nowhere to go, and a cold surface for it to settle on. Old sealed units with an aluminium spacer bar are a thermal bridge right at the glass edge, which is exactly where you see the mould appear first. That is why replacing units with warm-edge spacers so often clears a problem that no amount of wiping did.
Treat the surface, then treat the cause. Clean it off, then improve extraction at source and open the trickle vents. If it comes back in the same spot on the same window, look at the unit rather than the household.
Winter checks worth ten minutes
Almost everything on this page can be caught early with a short walk round before the weather turns. None of it needs tools beyond a cloth and a can of light oil.
- Run a hand round every external door edge on a windy day and note where you feel air
- Squeeze the gaskets — hard, shiny or shrunken at the corners means they're due
- Oil the hinges and the multi-point lock cams, then lock and unlock a few times
- Check the external mastic between frame and brick for splits and gaps
- Clear the drainage slots in window and patio door tracks with a brush
- Open every trickle vent and check none have been painted or taped shut
- Look along the bottom edge of each sealed unit for the first signs of misting
Get it diagnosed before you get it quoted
Describe where you feel the draught or where the condensation sits and we'll tell you honestly whether it's an adjustment, a sealed unit or a replacement. Plenty of these calls end with us not selling you anything.