Perished window seal: signs that the rubber failed

The casement still locks, yet a cold line runs along the meeting edge on a still evening. That is a perished window seal when the rubber has gone hard, flattened, cracked, or shrunk off the corners and no longer springs back. There is no year-count on the foam and gasket coils sold for this job, so you replace on that condition, not on a birthday for the window.

In short

A sound closing seal springs back when you press it. A failed one stays in the dent, and you can feel air at a sash that still throws its locks. A tired corner with no draught and a dry sill can wait. Which? says you can usually leave a broken seal unless it causes condensation, a draught, or a leak, and that some gaskets are not a separate part you can swap.

What a perished window seal looks like

A closing seal on a uPVC casement is sound when you press it and it comes back. Guides to uPVC window rubber seal replacement call a healthy gasket soft, and a failed one hard, flattened, cracked, split, or pulled away from the corners. Shut the sash so the cams are home, then run the back of a damp hand along the frame. A draught at a window that still locks, plus rubber that stays flattened, is the failed seal.

Look at the corners before the middle of the run. Rubber shortens along its length, so a window seal shrunk at corners shows as a bare few millimetres where two sides should meet. A split that has opened, or a bead gone shiny and brittle, will not recover from a wipe. Flattened rubber can still sit in the groove and look present from across the room. Up close it is a ribbon with no bounce. A corner you can feel on your wrist, on a sash that still locks, is when you plan a replacement.

Why these coils have no year count

How often you replace a uPVC gasket is a condition, not a date. Tangram's PVC-U maintenance note says gaskets and seals made from EPDM or TPE should remain effective for at least ten years, and that renewal should be done by installers using the manufacturer's materials. That is about rubber fitted with the window and looked after as the maker intended. It is not a lab result for a foam roll on amazon.co.uk, and it is not a certificate for a listing that shows no BS EN 12365 class.

The Energy Saving Trust is plainer about stick-on foam. It may not last long. Brush or wiper strips last longer, and foam is a poor seal for a sash. Historic England's HEAG084 treats foam as a short-life compression strip. None of those pages gives a life in years for these coils. On 15 October 2026 a YANKUIRUI foam listing was about £4.49, and Storm Patrol's push-in coil was about £20.18 for 25 m. EPDM on a foam page is a material word, not the Tangram interval.

The Products Compared editorial team read those notes and the amazon.co.uk listings on 15 October 2026. We did not lab-test the coils. Replace the closing seal when it no longer springs back, when it is hard, cracked, flattened, or shrunk off the corners, and you feel a draught at a sash that still shuts. The window seal comparison shows what the coils list. It does not invent a service life they omit.

A draught around a closed window

If the rubber springs back and you still feel a draught around a closed window, the gasket is the wrong suspect. Shut the sash with your hand on the handle side and hold it. If the draught stops while you hold it and returns when the handle takes the load, the cams are not drawing the sash home. Worn hinges, a dropped sash, or a tired locking pull do that with rubber that is still soft. A thicker strip is then a wad in a gap the hardware should have closed. It can stiffen the handle and stop the locks meeting. If a new strip does that, take it off the same day.

A dropped sash is a fitter's job. The sash sits low, scrapes the bottom, or needs a lift before the cams bite. Hinge wear and a twisted frame are the same family of fault. The seal did not drop the glass, and a push-in foot will not lift it.

If the lever spins with no resistance and the cams do not move, the rubber is not the failed part. The spindle or the gearbox has stopped driving the lock. That sits in when a window handle spins and nothing locks and in the window handle comparison. A gasket will not restore a handle that no longer pulls the sash. Check spring-back, then a hand-held close, then whether the cams move. Only the first of those points at a new seal.

Mist in the glass is not this gasket

A grey film you cannot wipe from either face is moisture in the sealed unit. The rubber this page is about is the other line: the gasket or strip that meets the sash when you close it. A whistle and a cold edge at a locked sash point at the closing gasket. Mist between the panes points at the unit. A failed window gasket is not misted glass. On an old window both can be true, so one coil does not clear every complaint.

Rubber bought for cloudy glass will not dry the cavity. If you can wipe water off the room side, that is condensation on the inner pane, not a gasket and not automatically a new unit. Water you cannot reach, between the panes, is the unit. Read mist between the panes before you order a coil for cloudy glass. A fitter quoting one price for the seals should say whether they mean the bead around the glass or the seal that closes on the frame.

Check the gap, then choose the coil

Once the rubber fails the spring-back test, measure the groove or the closed gap. A line that says a coil fits all uPVC is not a measurement. A push-in gasket belongs in a slot, and only after a short offcut seats there. Stretching it to the corner thins the rubber, and the ends creep back. Foot width is often missing even when a frame range is printed, so the sample is the evidence.

On 15 October 2026 Storm Patrol, ASIN B0CDVH313Y, was about £20.18 for 25 m on amazon.co.uk, with 4.5 stars from 306 reviews, when a sample seats in a frame stated as 3 to 5 mm. The table gives no foot width, so try that range in your slot and leave the coil if there is no groove. The comparison's stick-on top pick is the YANKUIRUI D-profile foam seal, 6 m, white, at about £4.49, for a stated 3–6 mm gap. Foam near that price may not last long. Which job you have is on foam or gasket, the sample fit is on replace a uPVC window seal, and the gap is on the buying guide. None of the nine listings states a BS EN 12365 class. A qualifying purchase can earn us a commission at no extra cost to you.

Leave trickle vents alone, and do not cover a grille that feeds an open flue. If you cannot tell which opening supplies combustion air, ask a fitter first. Call a fitter too when the rubber is under a glazing bead, when the frame is listed or twisted, or when water comes through the wall joint rather than past the sash. A listed sash is not a uPVC groove you pack from a retail coil.