Condensation on double glazed windows is one of the most common issues homeowners face. While it can be worrying, in most cases, it’s actually a sign that your windows are working perfectly! Understanding where the moisture is building up is the key to diagnosing the issue.

To help you, we’ve broken down the three distinct types of condensation. Stop reading for a second and check your uPVC windows to see which of the following conditions you are currently experiencing:

• Windows steamed up from the outside (Exterior Condensation)
• Windows steaming up on the inside (Interior Condensation)
• Condensation built up between the two glass panes (Sealed Unit Failure)

Once you’ve identified the issue, scroll down for our quick diagnosis and fix.

1. Windows Steamed Up From the Outside (Exterior Condensation)

Diagnosis: Your Windows Are Working Perfectly!

If you see moisture on the outside of your double glazing, this is the most common—and easiest—question to answer. It is a key indicator that your double glazing is performing perfectly and is highly energy efficient.

Why it happens: Modern, high-efficiency double glazing is designed to be such a good insulator that it prevents the warm air inside your house from escaping and heating the outer pane of glass. When the external temperature drops (typically overnight or on still, cold mornings), the outer pane stays cold. If the air outside is humid, the moisture condenses on the cold surface. It’s essentially the same as dew forming on grass.

How to fix it: You don’t need to fix anything! This type of condensation will naturally disappear as the sun rises and warms the glass. It is a positive sign that your uPVC windows are retaining heat and saving you money on energy bills.

2. Windows Steaming Up On the Inside (Interior Condensation)

Diagnosis: You Have Too Much Moisture & Poor Ventilation

If condensation and moisture are collecting on the inside of your windows, there is nothing wrong with your double glazing unit itself. This simply means there is excess humidity and poor ventilation within your home.

Why it happens: Everyday activities—such as cooking, showering, taking a bath, drying clothes indoors, or even breathing—release warm, moist air. When this humid air hits the coldest surface in the room (the glass), it cools rapidly and turns into water droplets.

How to fix it: The simple and immediate solution is to increase the airflow and ventilation:

• Open a window slightly for a few minutes each morning to allow hot, moist air to escape.
• Use extractor fans in bathrooms and kitchens for longer periods after generating steam.
• Ensure any trickle vents (small slots at the top of the window frame) are open.
• Avoid drying clothes on radiators inside if possible, or use a dehumidifier.

3. Condensation Built Up Between the Two Glass Panes (Sealed Unit Failure)

Diagnosis: Your Double Glazing Seal Has Failed (Call a Professional)

If you see condensation, moisture, or a fog-like haze between the two glass panels, this is the only condition that indicates your double glazing unit may be compromised. This is a cause for concern as the unit is no longer working as intended.

Why it happens: A double glazed unit is a sealed unit containing an inert gas (like Argon) that provides the superior insulation. If the perimeter seal fails due to age, natural wear, or physical damage, the insulating gas escapes, and moisture-laden air from outside gets trapped inside. When the temperature fluctuates, this trapped moisture condenses, meaning you are effectively left with the performance of single-glazed windows.

How to fix it: There is no reliable DIY fix. The unit requires professional intervention. The solution is to replace the glass unit itself (called the Insulated Glass Unit or IGU) within the existing uPVC frame. You do not necessarily need a whole new window.

How MoWindows Can Help

In most cases, if you’re experiencing condensation build-up, you probably don’t need to worry. Your double glazing is either working fine (outside steam) or you simply need to increase the airflow in your home (inside steam).

However, if you find condensation between your double glazing panes, this means the insulating seal has failed and it’s time to act. It’s best to get this seen to and fixed as soon as possible by double glazing professionals.

By getting in touch with Matthew Oliver Windows and Doors, we will come out to assess the situation and provide a fast, professional solution to repair or replace your glass unit.

Don’t compromise your home’s energy efficiency.

👉 Contact us today for a free, no-obligation quote or call us on [INSERT PHONE NUMBER HERE].

Posted in: Windows | Tags: double glazing, uPVC windows, condensation, window repair, failed seal