Why Is There Condensation on the Outside of My New Windows? (And Why It’s Good News)

It’s a crisp autumn morning. You pull back the curtains to admire the view through your brand-new, energy-efficient windows… only to find they are covered in mist on the outside.

Panic sets in. Are they faulty? Have the seals failed already? Why did my old, draughty windows never do this?

The short answer is: Don’t panic. Condensation on the outside of your windows is actually the ultimate proof that your new windows are working perfectly. Here is the science behind why it happens, explained simply.

The “Gin & Tonic” Analogy: Understanding Dew Point

To understand why your windows are wet, imagine sitting outside on a warm summer evening with an ice-cold Gin and Tonic.

Within minutes, the outside of the glass becomes wet. Water droplets form and run down the side.

Is the glass leaking? No.

Is the glass faulty? No.

What you are seeing is the Dew Point in action. The ice in your drink has made the glass very cold. When the warm, moist air outside hits that cold surface, it can no longer hold onto its moisture, so it turns from a vapour into liquid water.

How This Applies to Your Windows

Your new windows are behaving exactly like that glass of Gin and Tonic.

Scenario A: Your OLD Windows (Inefficient)

In the past, your old double glazing (or single glazing) was poor at keeping heat in. Heat from your radiators escaped through the glass, warming up the outer pane. Because the outer pane was warm, the morning dew didn’t settle on it. You had a clear view, but you were paying for it with higher heating bills.

Scenario B: Your NEW Windows (Highly Efficient)

Your new A-rated windows are fantastic insulators. They are doing exactly what you paid for: keeping the heat inside your room.

Because the heat can’t escape through the inner pane, the outer pane stays cold—essentially the same temperature as the garden. On a cold morning, when the air is damp, that cold outer glass acts just like the G&T glass. The moisture in the air hits it and forms condensation.

The “Square of Mist” Mystery

You might notice something strange about the pattern of the condensation. It rarely covers the entire pane evenly.

• Internal Condensation: Usually starts around the edges of the glass. This is because the perimeter is the coldest point due to the “spacer bar” inside the unit acting as a thermal bridge, conducting cold from the frame.
• External Condensation: Usually forms a square or rectangle in the middle of the glass. This is because the middle of the pane is the furthest point from the frame. The frame (and spacer bar) leaks a tiny amount of heat, keeping the edges slightly warmer than the centre.

So, if you see a clear border around a foggy square in the middle of your window, that centre point is the coldest part of the glass, and that is where the dew point has been hit.

When Will It Happen?

You won’t see this every day. The phenomenon requires a specific set of weather conditions, usually seen in Spring and Autumn:

• The night must be clear (no cloud cover).
• The temperature must drop significantly overnight.
• There must be high humidity (moisture) in the air.
• There is little to no wind.

Just like dew on the grass or your car windscreen, it will naturally evaporate as soon as the sun rises or the air temperature increases.

Frequently Asked Questions (FAQs)