Will Secondary Glazing Stop Condensation on Old Sash Windows?

It’s a familiar winter morning ritual for owners of period properties in Hampshire: waking up, grabbing a towel, and wiping down the puddles of water on the sash window sills.

It’s tedious, it ruins your timber frames, and if left unchecked, it leads to unsightly black mould. Dehumidifiers help, but they are a temporary patch. To stop condensation permanently on sash windows without ripping them out, you need to fix the root cause: Cold Glass.

1. Why Do My Sash Windows Sweat? (The Science)

Condensation isn’t a problem with your windows leaking water in; it is a problem with physics. It occurs when warm, moist air inside your home comes into contact with a cold surface.

When warm air hits the cold single pane of a sash window, it cools down rapidly. Cool air cannot hold as much moisture as warm air, so it releases that water onto the nearest surface—your glass. This is known as the Dew Point.

Original sash windows are terrible insulators. On a freezing day outside, the glass inside your room is also freezing. This makes it a magnet for moisture.

2. How Secondary Glazing Stops It

You cannot change the weather outside, and you need to breathe inside (creating moisture). The only variable you can change is the temperature of the glass.

Secondary glazing installs a second pane of glass on the inside of your existing sash window. This creates a sealed air gap that acts as a thermal buffer.

• The Outer Window: Still gets cold, taking the brunt of the weather.
• The Air Gap: Traps a layer of insulating air (or dead air).
• The Inner Window (Secondary): Stays warm because it is separated from the cold outside air.

Because the inner pane (the one touching your room air) is now warm, the air doesn’t cool down enough to hit the Dew Point. No cold surface = No condensation.

3. Heat Retention: U-Values Compared

Stopping condensation is great, but saving money on heating is better. We measure insulation using “U-values” (the lower the number, the better the insulation).

4. The Role of Low-E Glass

If your goal is heat retention, we highly recommend specifying Low-E (Low Emissivity) Glass such as Pilkington K or Saint Gobain Planitherm.

This glass has a microscopic metal coating on one side. It acts like a thermal mirror:

1. It lets heat from the sun (solar gain) enter your room.
2. It reflects the heat generated by your radiators back *into* the room, rather than letting it escape through the glass.

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

5. Frequently Asked Questions