Window seals create a weather-resistant barrier between the moving and fixed parts of a window. When a seal flattens, goes brittle, shrinks or splits, you start to notice draughts, wind noise, or water getting in around the opening sash.
The good news is that you can usually replace the seal without touching the window itself. The harder part is choosing the right one - two profiles can look almost identical from the front but have completely different stems, groove widths or compression depths.
This guide covers how to work out whether the seal is actually the problem, how to identify the profile you have, and how to measure it so the replacement fits first time. Once you know what you need, our companion guide walks through the fitting.
What Is a Window Seal?
A window seal, also called a gasket, is a flexible rubber or synthetic strip fitted into a groove in the window frame, sash or glazing system.
Depending on where it sits, a seal may:
- Compress between the sash and frame when the window closes
- Hold the glass securely within the glazing system
- Reduce draughts and wind noise
- Help prevent rainwater entering
- Limit movement between components
- Support the window’s overall weather performance
Is It the Seal, or the Sealed Glass Unit?
This is the first thing to establish, because the phrase “broken window seal” gets used for two completely different problems.
A worn rubber gasket is a cheap, replaceable strip. A failed double-glazed unit is the glass itself, and no amount of new gasket will fix it.
Signs Your Window Gasket Has Failed
The removable gasket may need replacing if you notice:
- A draught around the opening sash
- Rubber that is cracked, brittle or crumbling
- Gaps opening up at the corners
- A seal that has visibly shrunk back
- A flattened profile that no longer springs back when pressed
- Wind noise around the frame
- Localised water entry in bad weather
- A seal that keeps coming out of its groove
A serviceable gasket should still feel flexible, sit securely in its groove, and return towards its original shape after you press it.
Check the Hardware Before You Blame the Seal
A draught does not always mean the rubber is at fault. Worn hinges, a dropped sash, or a locking mechanism that is not pulling the sash firmly against the frame all produce very similar symptoms.
Close the window and look at how the sash sits. If it meets the frame evenly all the way round, the gasket is the likely culprit. If one corner sits further out than the rest, the problem is alignment or hardware.
Where the cold air comes in is a useful clue. A draught along the hinge side often points to worn window hinges, while a draught around the whole perimeter is more likely to be the gasket or overall compression. If the sash closes but never feels tight, check the window keeps and locking mechanism too.
How to Identify Which Gasket Profile You Have
The most reliable method is to remove a short sample and compare its cross-section against the replacement profile. Do not choose based only on the visible rounded section - the part that pushes into the groove matters just as much.
1. Open the window fully.
2. Find the joint where the two ends of the seal meet.
3. Pull out a short section carefully.
4. Cut off a small sample only if you need to.
5. Let stretched rubber relax back to its natural shape.
6. Photograph the profile end-on.
7. Compare the full cross-section against product diagrams and measurements.
Knowing the manufacturer or profile system can help narrow things down, but it is not conclusive - windows of similar age and appearance often use different gaskets.
Testing a physical sample beats judging by appearance every time. Our bubble and flipper gasket sample pack contains labelled profiles you can push straight into the groove to find the match, and the full gasket seals range covers bubble, flipper, wedge and E-gasket profiles for most window and door systems.
TRADE TIP: If you look after multiple properties or fit regularly, keep a sample pack in the van. Identifying a profile on site and ordering once is far quicker than a return visit with the wrong gasket.
Bubble, Flipper, Wedge and E-Gasket Profiles
Different gasket types do different jobs. Establish where yours sits and how it works before ordering.
Bubble Gasket
A bubble gasket has a rounded, hollow section that compresses when the sash closes against the frame. It is the usual weather seal between an opening window or door and its fixed frame, closing the gap while staying flexible enough for the window to operate.
Bubble gaskets come in different profile heights, shapes and stem sizes. Too small and it will not make proper contact. Too large and the handle becomes stiff to close, putting needless strain on the hardware.
Flipper Seal
A flipper gasket has a flexible blade or fin instead of a large rounded bubble. The fin bends and compresses against the adjoining surface as the window closes. Like a bubble gasket, it is normally found around opening sashes and frames.
Do not pick a flipper seal just because the fin looks the right height - the complete cross-section has to be compatible.
Wedge Gasket
A wedge gasket belongs to the glazing system rather than the opening sash. It sits between the glass and the frame or glazing bead, helping hold the unit in place and seal it.
Replacing one may mean removing a glazing bead or working directly around the glass, which is a different job from swapping a compression seal. If you have not worked on glazing systems before, this is one to hand to a professional fitter - incorrect removal can damage the bead, the glass or the frame.
E-Gaskets and Other Profiles
Some older or system-specific windows use E-shaped gaskets. Certain modern frames use co-extruded seals that form part of the window profile and cannot be replaced like a push-in gasket. Where the profile is unclear, remove a sample and compare directly before ordering a full length.
Measuring for the Right Fit
Measure the gasket relaxed, not stretched inside the frame. A gasket under tension reads shorter and thinner than it really is.
Stem or Foot Width
This is the part that pushes into the groove, often with an arrowhead or T-shaped retaining profile. It needs to hold securely without falling out or requiring excessive force to insert.
Groove Width
Where you can reach it, measure the opening of the groove itself and compare it against the stem shown on the product diagram. In practice the old gasket sample is usually more useful than measuring a narrow or recessed channel.
Overall Depth
Measure from the bottom of the inserted section to the highest point of the bubble or fin. This tells you how far the gasket will stand off the frame and how much it will compress when the window shuts.
Bubble Diameter or Flipper Height
For a bubble gasket, check the size and shape of the rounded compression section. For a flipper, measure the height and the direction the fin points - a small difference changes how the seal meets the frame.
The Whole Profile
Compare the entire end profile rather than one dimension. Two gaskets can share an overall height and still use completely different retaining stems.
TRADE TIP: Order a sample or short length and fit it to a small section of groove before committing to every window in the property. Check the window closes normally and the handle operates without force - that five-minute test prevents an expensive mistake.
How Much Window Seal Do You Need?
Measure the full perimeter of the area being sealed, not just the width and height of the opening.
For a rectangular sash, add together the two vertical sides, the top edge and the bottom edge. Then add an allowance for trimming and joining - ordering the exact measurement leaves no room for a mistake or a damaged section.
If several windows appear to use the same profile, check each one before ordering. Different window styles, or windows fitted at different times in the same property, often use different gaskets.
How Long Do Window Seals Last?
Most window seals last around ten years, and many window manufacturers offer a warranty of a similar length. It is worth inspecting them every five years or so for early signs of wear.
Actual service life varies with sunlight and UV exposure, temperature swings, the gasket material, how often the window is opened, how much the seal is compressed, cleaning chemicals, frame movement, and the quality of the original fit. Judge by condition rather than age alone.
Can a Window Seal Be Repaired?
If a seal has simply lifted out of its groove but is still flexible and undamaged, you can often press it back into place and get more life out of it.
Rubber that has gone brittle, permanently flattened, cracked or badly shrunk will not recover its shape, and patching individual sections rarely holds. Replacement is the reliable fix.
Avoid filling gaps in a removable gasket with silicone. It makes future repairs awkward and can stop the sash compressing evenly.
Ready to Fit Your New Seal?
Once you have identified the profile and ordered the right length, our step-by-step guide covers the fitting: how to replace the rubber seal on uPVC windows. It walks through removing the old gasket, cleaning the groove, feeding the new seal in without stretching it, and testing the window afterwards.
FixTrade supplies bubble, flipper, wedge and E-gasket profiles for most uPVC window and door systems, with free next day delivery available and trade accounts welcome. If you are repairing more than the seals, the full uPVC window and door range also covers hinges, handles, keeps and locking components. Contact us for further information.
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