Many New Zealanders are familiar with windows fogging up on the inside on cold days – a phenomenon typically associated with the poor thermal performance of single-glazed windows and aluminium joinery without a thermal break separating the aluminium on the outside from the one on the inside.

But what often surprises our customers is when their new, high-performance double- or triple-glazed windows fog up on the outside during colder mornings. While it might seem concerning at first, this is actually a sign that your windows are working exactly as they should.

So Why Does External Condensation Occur?

External condensation is a natural result of efficient window technology. It typically occurs on windows that feature Argon-filled double or triple glazing and Low-E (low-emissivity) coatings. Two conditions must be present:

• The outer glass surface is colder than the surrounding outdoor air.
• The outdoor air is saturated with moisture (has high humidity).

When the saturated air meets the colder outer glass, some of the moisture in the air condenses on it. This effect can be aggravated in areas with high humidity levels, such as places near water, when in the early morning hours, the air is warming up faster than the outside glass, thus increasing the temperature difference between the cold glass and the air.

It’s Actually a Good Sign

Here’s the key takeaway: external condensation means your windows are doing their job. They’re so well-insulated that heat isn’t escaping through the glass, keeping the interior warm. Because the outside pane remains cold, moisture condenses there instead.

Once the sun rises and the glass warms up to match the air temperature. The condensation quickly disappears and you will be left with a crystal-clear view from your warm, healthy and energy-efficient home.

Want to Learn More?

Interested in upgrading to high-performance windows or learning more about condensation? Contact us today for expert advice and a free quote on our range of European double- and triple-glazed window systems.