Double-glazed windows are a staple of modern homes, offering better insulation, noise reduction, and energy efficiency compared to single-pane alternatives. But when condensation starts to appear, many homeowners are left confused—or even alarmed. Is it a sign of a fault? Will it cause damage? And why does it happen in the first place? At DM Windows & Doors, we know these concerns are common and often misunderstood. The answer isn’t always straightforward. Fogging can occur for several reasons, and not all of them are cause for concern. Understanding where the condensation is forming—and why—is the first step to figuring out what’s going on with your windows. Let’s break down the most common causes, starting with the most frequently misunderstood.
Condensation on the Inside of the Glass
If you’re noticing moisture forming on the inside of your windows—particularly in the mornings or during colder months—you’re not alone. This is the most common type of window condensation, and thankfully, it usually has nothing to do with the window’s quality or installation.
What causes it?
This kind of fogging is caused by warm, moist air inside your home coming into contact with the cold surface of the inner glass pane. When the warm air hits that colder surface, the moisture in the air condenses into water droplets. It’s a simple result of everyday life—showers, cooking, kettles, and drying clothes indoors all contribute to higher indoor humidity. If you live in a modern, well-insulated home, you’re more likely to see this type of condensation, as well-sealed windows and doors trap air inside, along with any excess moisture.
How to reduce it
Luckily, this issue is often easy to manage. Improving ventilation is key. Extractor fans in kitchens and bathrooms help remove moist air at the source. Opening windows regularly—even for just ten minutes a day—can make a surprising difference. Dehumidifiers are also useful, especially in winter or in homes with limited airflow. Think of this kind of condensation more as a sign that your home needs to “breathe” a bit more, rather than a sign that your windows are faulty.
Condensation on the Outside of the Glass
Spotting mist on the outside of your double-glazed windows, especially first thing in the morning, might make you think something’s wrong. Ironically, it’s a sign that your windows are doing their job extremely well.
Why it happens
External condensation occurs when the outer pane of the window becomes cooler than the outside air, typically overnight. If the air is humid and the skies are clear (which allows heat to escape), the cooler glass causes the moisture in the air to condense on the outside surface. This is more likely to happen in spring and autumn, or after still, clear nights. In technical terms, your window is preventing the warmth from inside your house from reaching the outer glass, which keeps it cold—and that thermal barrier is exactly what double glazing is supposed to provide.
Is it a problem?
Not at all. It’s often a sign that your windows are highly energy-efficient, particularly if they use low-emissivity (low-E) glass. The condensation will typically disappear as the morning sun rises and the glass warms up. It doesn’t pose a threat to your property, and it doesn’t mean there’s a fault with the unit. If the foggy appearance bothers you for aesthetic reasons, there are coatings available that can reduce external condensation, but they’re usually not necessary.
Condensation Between the Panes
If you’re seeing fog or mist trapped between the two panes of glass in your double-glazed windows, it’s a different story altogether. Unlike internal or external condensation, this is a clear sign that something has gone wrong—specifically, a failure in the sealed unit. Double glazing works by creating an airtight gap between two panes of glass, usually filled with an inert gas like argon, which helps insulate your home by keeping the heat in and the cold out. When the seal around that unit breaks down, moisture from the air seeps in and becomes trapped between the panes. Once this happens, the unit loses its insulating properties, and the fogging becomes permanent.
What causes seal failure?
Double-glazed units can fail for several reasons. The most common is age—after 10 to 20 years, seals naturally degrade. However, manufacturing defects, poor installation, or extreme weather can speed up the process. For instance, windows facing strong sunlight can experience expansion and contraction, stressing the seals. Fogging may also worsen in damp or humid weather, as increased moisture makes it more visible. Unfortunately, it won’t clear up on its own, and no ventilation can fix it once the seal has failed.
Can it be fixed?
The good news is that you don’t always need to replace the entire window. In many cases, the affected unit (the sealed glass part) can be swapped out without touching the surrounding frame, which is quicker and more cost-effective, especially if the frames are still in good condition. However, if multiple windows are fogging up or the frames are old and worn, it might make more financial sense to replace the whole lot. Some homeowners also choose to upgrade to more energy-efficient glazing options, which can improve comfort and reduce heating bills.
Preventing Future Fogging
Once you’ve dealt with a failed unit, the last thing you want is for it to happen again. While no window lasts forever, there are simple steps you can take to extend the life of your double glazing and reduce the risk of fogging in the future.
Regular maintenance and early checks
Just like anything else in your home, windows benefit from routine care. Wipe down frames and glass regularly to prevent moisture build-up, and check for signs of wear around the seals. Tiny cracks or splits might seem minor, but they can let in moisture over time. It’s also worth paying attention to how your windows feel—if a once-cozy room starts feeling draughty or you notice cold spots near the glass, it could indicate an issue with the insulation, even before visible fogging appears.
Choose quality and professional installation
If you’re replacing existing units, the quality of both the materials and the installation plays a big role in long-term performance. Make sure you’re using a reputable installer who understands how to properly seal and fit double-glazed units. Look for features like warm edge spacers, argon gas fills, and low-E glass if energy efficiency is a priority.
If your double-glazed windows are fogging up, understanding where the condensation is forming and why is key to addressing the issue. Internal and external condensation are usually harmless and often linked to humidity or weather conditions, while moisture between the panes is a sign of seal failure that requires repair or replacement. By staying aware of these differences, carrying out simple maintenance, and investing in quality installation when needed, you can keep your windows clear, energy-efficient, and in good condition for years to come.
