How to replace the rubber seals on uPVC windows and doors
By Glazing London,
"Window seal" can mean two quite different things, and getting them confused is the most expensive mistake in this whole job. One is the rubber gasket that stops draughts; you can usually replace it yourself. The other is the edge seal of the glass unit; when that fails, only new glass will do.
The short answer
To replace a uPVC window gasket: identify the profile, pull the old gasket out of its groove from a corner, clean the groove, then press in a new continuous length starting at the top, easing it round the corners rather than cutting, and join it at the top. If you see mist between the panes instead of a draught, it is the glass unit that has failed, not the gasket.
Gasket or glass? Check first
| What you notice | Which seal | Fix |
|---|---|---|
| Draught or whistle round a closed window | Sash gasket | Replace gasket or adjust hinges |
| Rain getting in at the bottom of the sash | Sash gasket or drainage slots | Clean drainage, then gasket |
| Rattling sash in wind | Gasket shrunk, or keeps out of adjustment | Adjust, then gasket |
| Black rubber shrunk back from the corners of the glass | Glazing gasket | Replace glazing gasket |
| Mist or droplets between the panes | Edge seal of the sealed unit | New sealed unit, see the blown unit tests |
Before buying anything, close the window on a strip of paper. If it slides out freely, try adjusting the hinges and keeps first: our guide to fixing a uPVC window that won't close shows how. Sometimes the gasket is fine and the window simply is not pulling tight.
Step 1: identify the gasket profile
- Open the window and find the gasket on the sash or the frame, often both.
- At a corner, pull out a short length with your fingers.
- Look at the cut end. Note the foot (the part that sits in the groove) and the seal (a round bulb, a flat flipper, or a fin).
- Photograph it end-on next to a ruler and note the colour, usually black, grey or white.
Many window systems use their own shapes, so match the profile, not just the size. If you cannot match it, a glazier can.
Step 2: remove the old gasket
- Pull the gasket out of its groove all the way round. Old gasket often comes out in brittle pieces.
- Do not lever with a screwdriver against the uPVC face; it marks easily.
- Scrape out any bits left in the groove and vacuum the channel.
Step 3: fit the new gasket
- Start at the middle of the top, never at a corner.
- Press the foot into the groove with your thumb or a small roller, working along in short sections.
- At corners, ease the gasket round rather than cutting it. A continuous run has no joints to shrink and open up later.
- Do not stretch it. Stretched gasket shrinks back within a season and leaves gaps at the corners. Push it in slightly bunched instead.
- Join at the top, cutting the end square with a little extra length so the two ends butt firmly together.
- Close the window and test with the paper strip at several points. It should drag everywhere.
Step 4: finish and maintain
- Wipe the new gasket with a little silicone or soapy water so it does not stick to the frame in hot weather.
- Clear the drainage slots along the bottom of the frame so water does not sit against the new seal.
- Once a year, clean all gaskets with warm soapy water. Never paint over them or use solvent cleaners.
Worth knowing: a window that is still draughty with a new gasket is almost always out of adjustment. The gasket can only seal if the sash is pulled onto it, which is the job of the hinges and locking cams.
When to call us
Call in a glazier when you cannot match the profile, when the glazing gasket round the glass needs replacing (the beads have to come out and the unit must be handled safely), when the frame is warped, or when the unit has misted. Our uPVC window repair and uPVC door repair teams re-run gasket in one continuous length round the sash and set the hinges so it actually seals. Send a photo of the gasket end on WhatsApp or call 07915 903485.
Sash windows are different: they use brush pile and draught beads rather than rubber gaskets. See draughty sash windows.
Window seals, answered
Can you replace window seals yourself?
Often, yes. The rubber gaskets on most uPVC windows sit in a groove and pull out by hand, and new gasket presses in with a roller or your thumb. The hard part is matching the profile: there are many shapes, and the wrong one either will not seat or stops the window closing.
Can you just replace seals on windows rather than the whole window?
Yes. If the frame is sound and the glass is clear, new gaskets cure most draughts, rattles and whistles. Replacing the window is only worth it when the frame itself has warped or the hardware is beyond repair.
How do I know which window seal I need?
Pull out a short length from a corner and look at its cross-section: the shape of the foot that sits in the groove and the bulb or flipper that touches the sash. Take a photo of the end next to a ruler, and match that, plus the colour.
Is a failed window seal the same as a blown double glazed unit?
No. The rubber gasket round the sash stops draughts. The seal of a double glazed unit is the edge of the glass itself, and when it fails the unit mists inside. A misted unit needs new glass, not a new gasket.