Water Coming Through Your Windows? Common Causes and What to Do

Water coming through a uPVC double-glazed window is usually caused by blocked drainage channels, worn gaskets, failed external mastic, poor installation or a fault above the window. First, check whether it is a leak, condensation or a misted unit, clear the drainage channels, and call a professional if the water keeps returning.

Finding a wet sill or a damp stain after a rainy night is worrying. The good news is that the spot where water shows up is rarely where it gets in, and many "leaks" turn out to be condensation, blocked drainage or a problem with the wall above.

Most causes can be found with a few simple checks. In this guide, we explain what's happening with your uPVC double glazed window, what you can do today, and how to decide between repair and replacement.

Is It a Leak, Condensation or a Misted Unit?

Before you try any fix, work out which of three problems you have. Each has a different cause and a different answer.

• Condensation forms on the room-side glass or frame on cold, humid mornings. It doesn't need rain.
• Water ingress appears during or after rain, often worse in wind-driven rain, and usually from one point.
• Moisture between the panes means the sealed unit has failed. The glass unit needs replacing, though the frame can often stay.

Here's a simple rule of thumb. If the moisture only appears after rain, look outside the window. If it appears on cold, dry mornings, think ventilation and condensation. Either way, a double glazed window uPVC frame is rarely the whole story, so it pays to check the pane, the seals and the wall around it.

Persistent damp can also affect health, particularly where mould develops. If you rent out a property, or live in one, the government's guidance on damp and mould: understanding and addressing the health risks explains the risks and what landlords can do about them.

What Causes Water to Come Through uPVC Double Glazed Windows?

Water usually gets in because something that should keep it out has worn, blocked or been fitted badly. Here are the most common culprits.

Blocked Drainage Channels

Drainage slots in the outer frame let water escape to the outside. When dirt, cobwebs or paint block them, water backs up and spills indoors instead. Gently clearing the slots often solves the problem. Never seal drainage slots, as they're there for a reason.

Worn or Displaced Gaskets

Rubber gaskets seal the glass and the opening sashes. Over time they can shrink, harden or slip out of position, leaving small gaps for water and draughts to get through. A gasket that has simply popped out may press back in, but a brittle or shrunken one needs replacing.

Cracked or Missing External Mastic

The joint between the frame and the brickwork is sealed on the outside. When that mastic cracks, shrinks or comes away, wind-driven rain can get behind the frame and appear as damp on the inside. Small gaps are easy to spot from outside.

Poor Installation

A brand new window can still leak if it was fitted badly. Common faults include unsealed sill ends, unsupported sills, gaps behind the frame and weak detailing above the opening, such as a damaged cavity tray. A proper survey and careful fitting matter as much as the product itself.

Leaks in uPVC Double Glazed Sash Windows

Sashes have more moving parts, so water can collect in the tracks and around the meeting rail. Worn weather seals and blocked drainage paths in the sill are the usual causes. Sashes that feel loose or rattle are a useful clue for a surveyor, as they often point to wear or poor alignment. If you're thinking about upvc double glazed sash windows, take a look at our vertical sliding sash windows.

Problems Above or Around the Window

Sometimes the window is innocent. Blocked gutters, cracked render, damaged brickwork or roofline faults can send water down inside the wall and out around the frame. Replacing the window won't fix this, so it's worth checking above the window before you blame the frame.

Where Is the Water Coming From and What Can You Do Now?

The position of the water tells you where to look, and a few careful steps today can limit the damage while you find the cause.

Where the Water Appears Tells You Where to Look

Top of the frame: Check the gutters, render and lintel above the window.

Sides of the frame: Look at the mastic and the joint between the frame and the wall.

The sill: Think drainage slots, gaskets or condensation.

Between the panes: This points to a failed sealed unit.

Only in strong wind and rain: Small gaps in the mastic or gaskets are the likely cause.

What to Do Right Now

Start by drying the area so you can see where fresh water appears. Take dated photos, which help if you need a surveyor or a guarantee claim later. Gently clear the drainage slots with a soft brush, check the gutters, and re-seal only small, clean, dry mastic gaps.

There are a few things to avoid:

• Don't block or seal the drainage slots.
• Don't seal over damp or rotten material.
• Don't keep patching a leak that returns.

Not Sure Where the Water Is Getting In?

Book a free survey, and we'll find the source, then give you a clear, no-obligation quote for repair or replacement.

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Should You Repair or Replace a Leaking Window?

Whether to repair or replace comes down to the cause. If the fault is small and the frame is sound, repair is usually enough. If the window itself has failed, replacement is the better long-term answer.

When Is Repair Enough?

Repair usually works for blocked slots, small mastic failures, worn gaskets and a single failed sealed unit in an otherwise sound frame. A homeowner can clear drainage slots, wipe down seals and re-seal small, clean, dry gaps. Replacing gaskets, renewing larger areas of mastic or swapping a sealed unit should be left to a professional.

When Should You Replace Instead?

Replacement makes more sense when the frames are warped or damaged, the sashes no longer close properly, several units have failed, or the leak returns after proper repairs. New windows can also help reduce draughts and condensation, so you may notice the benefit beyond the leak itself.

Does Installation Make a Difference?

It does. Good double glazed windows installation starts with a proper survey, includes careful sealing, and comes with a written guarantee. These matter as much as the product. It's also worth being clear that new windows won't fix a leak caused by the wall or roofline above them, which is why finding the true source comes first.

Does the Manufacturer Make a Difference?

Yes. The profile, weld quality, guarantee and aftercare all come from the manufacturer, and they're hard to judge by eye. When you're comparing windows double glazed upvc suppliers, look at who builds the window and what support sits behind it. Our window frames buying guide sets out the questions worth asking.

Double Glazed Windows from Klassic Trade Frames

Klassic Trade Frames makes made-to-measure double glazed windows in-house in Birmingham as a REHAU-authorised fabricator. As one of the uPVC window manufacturers in Birmingham, we supply homeowners, landlords, developers and trade customers with a product guarantee behind our work. If a survey shows replacement is the right answer, we can supply new windows built to suit your property.

What Customers Say

M Nandray ★★★★★ a year ago

Absolutely fantastic company. I had my bi-folding doors and all windows from Kalsi Group, and the work is brilliant. I've had no issues at all; the windows and bi-folding doors are of high quality, and the fitters that they recommended, Atlas Glazing, were fantastic. Cannot fault them at all and would highly recommend. The finishing was clean and tidy and of high quality.

Harveen K ★★★★★ 6 months ago

Klassic Trade Frames recently supplied and fitted all windows and doors for my new house. We were very impressed with the products and customer service throughout. Would definitely recommend their services :). Thank you!

Ready to Stop the Leaks for Good?

Get your free, no-obligation quote for new uPVC double glazed windows, including full installation, from Klassic Trade Frames today.

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Conclusion

Water coming through your windows is rarely a reason to panic, and the cause is often simpler than it looks. Work out whether you're dealing with a leak, condensation or a misted unit, clear the drainage slots, check the gaskets and mastic, and look above the window before you blame the frame. Small faults can often be fixed, but a leak that keeps returning is a sign to call in a professional.

If a survey shows replacement is the right answer, a well-made window and careful installation make all the difference. Whether you're comparing uPVC double glazed windows or looking for a local manufacturer, choose one that stands behind the product with a proper guarantee.

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