The Ultimate Guide to Double Glazing Condensation

It remains one of the most common questions we receive at our Andover showroom: “I bought new windows to stop condensation, so why do I still have it?” Discover the practical truth behind wet windows.

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Condensation naturally occurs when warm, moist air meets a cold surface (known as the Dew Point).

1. Internal Condensation (Inside the Room)

If the physical water droplets sit directly on the side of the glass you can touch from the inside of your bedroom or kitchen, this is generally not a window fault. Instead, it indicates a high-humidity ventilation issue within your property.

A typical family produces around 14 litres of water vapour a day simply by breathing, cooking, and showering. Because modern uPVC windows are highly airtight, they successfully trap all your central heating inside—but they also trap the moisture. When that moist, warm air hits the cold glass, it turns into water.

2. External Condensation (Outside the House)

If you wake up on a crisp Autumn morning and discover heavy mist on the outside of your windows (facing out into the garden), do not panic! You do not need to replace your windows.

This phenomenon generally only happens on highly energy-efficient windows (utilising Low-E glass). It proves that your inner pane of glass is keeping the expensive heat securely inside your house, meaning the outer pane remains completely freezing cold. Because the outer glass is cold, morning dew naturally forms upon it.

3. Misted Double Glazing (Between the Panes)

If you see water droplets, persistent mist, or fog trapped directly between the two sheets of glass (where you cannot wipe it off from the inside or the outside), your sealed glass unit has failed.

This means the black rubber perimeter seal has naturally perished over time. The insulating Argon gas has permanently escaped, and moist external air has been sucked into the cavity. Your window has completely lost its thermal efficiency.

4. Secondary Glazing Condensation

If you live in a heritage property or a conservation area, you might have secondary glazing installed behind your primary single-glazed timber windows. Condensation can sometimes form in the cavity between the two distinct window units.

This happens when warm, moist room air bypasses the seals of the inner secondary unit and gets trapped against the freezing cold primary glass. The solution requires ensuring the secondary unit is perfectly sealed against the room, while allowing the primary external window to vent slightly to the outside.

➤ Read our complete guide to Secondary Glazing Condensation here.

5. Frequently Asked Questions