Will Secondary Glazing Stop Condensation on Old Sash Windows?

Are you tired of waking up to wet sash windows every winter morning? Discover why single glazing sweats, the simple science of the “Dew Point,” and exactly how installing secondary glazing creates a thermal barrier to stop condensation for good.

Mopping up daily condensation is a tedious chore. While electric dehumidifiers certainly help manage indoor humidity, they are often just a temporary patch. To stop condensation on sash windows without replacing them, you need to address the root cause: cold glass.

1. Why Do My Single Glazed Sash Windows Sweat?

Interior condensation isn’t caused by your windows leaking rainwater; it is simply a matter of physics. It occurs when warm, moist air inside your home comes into contact with a cold surface.

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The sealed air gap acts as a thermal buffer, keeping the inner glass pane warm and dry against the interior room.

When warm interior air meets the cold single pane of an exterior sash window, it cools down quickly. Because cool air cannot hold as much moisture as warm air, it releases that excess water onto the nearest surface—your window glass. This threshold is known as the Dew Point.

Original 19th-century sash windows are poor thermal insulators. On a freezing day outside, the glass facing into your room is also cold, making it a natural magnet for indoor moisture.

2. How Secondary Glazing Solves the Condensation Problem

You cannot change the weather outside, and general household activities will always create some moisture. The variable you can change is the surface temperature of the glass facing the room.

Secondary glazing involves installing a separate pane of glass on the inside of your existing primary window. This creates a sealed air cavity that acts as a reliable thermal buffer.

• The Original Outer Window: Continues to take the brunt of the cold UK weather.
• The New Air Gap: Traps a layer of insulating air (often referred to as ‘dead air’).
• The New Inner Window (Secondary): Remains warm because it is separated from the cold outside air by the thermal cavity.

Because the new inner pane remains closer to room temperature, the interior air doesn’t cool down enough to hit the Dew Point. No cold surface means no condensation.

3. Heat Retention: U-Values Clearly Compared

Stopping morning condensation is excellent, but saving money on your winter heating bills is an added benefit. We measure thermal insulation using “U-values”—the lower the number, the better the insulation.

4. The Essential Role of Low-E (Low Emissivity) Glass

If your goal is maximum heat retention alongside condensation prevention, we recommend specifying Low-E (Low Emissivity) Glass such as Pilkington K or Saint Gobain Planitherm for your secondary pane.

This glass features an invisible, microscopic metal oxide coating on one side that acts like a thermal mirror:

1. It allows natural heat from the winter sun to enter your room.
2. It reflects the heat generated by your central heating back into the room, rather than letting it escape through the glass.

By combining the thermal air gap of secondary glazing with Low-E glass, you can achieve overall thermal performance that rivals modern double glazing, all without altering your original historic windows.

5. Frequently Asked Questions