Does secondary glazing stop condensation?

On the room-side pane, yes. The secondary glass runs at 15-17°C in winter rather than the 4-7°C of single glazing, which is above the 11-13°C dew point of typical room air, so water does not form.

Do I still need to ventilate the room?

Yes. Secondary glazing removes the cold surface where moisture condenses but does not remove the moisture itself, so we review the room ventilation strategy as part of the free survey.

Secondary Glazing for Condensation Control

Raise the room-side glass above dew point and the water has nowhere to form.

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4-7°C

Single-glazed surface in winter

15-17°C

With secondary glazing

11-13°C

Typical room dew point

£420

From, per window

Condensation forms when moist internal air meets a surface below its dew point. On a January morning a single-glazed pane sits at 4-7°C while a normally humid room has a dew point around 11-13°C, so water forms on the glass every night without fail.

The consequences are worse than the puddle on the cill. Persistent moisture rots timber sashes and cills, lifts paint, and feeds black mould on the reveal — repair costs that far exceed the price of preventing them.

Why the room-side pane stays dry

The secondary pane is exposed to room air on one side and the insulating cavity on the other, so it runs far warmer than the primary glass — typically 15-17°C in winter. That is comfortably above the dew point of normal domestic air, so condensation does not form on it.

Meanwhile the original pane, now buffered from room moisture by a sealed secondary unit, sees far less humid air, so its condensation reduces sharply too.

Room-side surface raised from 4-7°C to 15-17°C
Dew point of a typical room: 11-13°C
Primary pane shielded from room moisture by the seal
Timber cills and sashes protected from repeated wetting

Condensation inside the cavity

Occasionally moisture appears between the two panes. That is a diagnostic signal, not a product fault: it means humid room air is still reaching the cavity through an imperfect seal, or moisture is entering from outside through a failed primary window.

The fix is either to improve the secondary seal or to repair the primary window — and our installers check the seal line at handover precisely to prevent this.

Protecting original joinery

For owners of listed and period properties this is often the real driver. Repeated wetting is the primary cause of decay in original sashes and cills, and eliminating it materially extends the life of joinery that is expensive and sometimes impossible to replace like for like.

Condensation control specification

Room-side surface temperature: 15-17°C in winter (from 4-7°C)
Glass: 4mm float minimum; laminate where noise also matters
Seals: Full-perimeter EPDM — critical for cavity dryness
Cavity: 20-100mm optimal for this purpose
Ventilation: Room ventilation strategy reviewed at survey
Joinery protection: Eliminates repeated wetting of sashes and cills

Pricing guide

Specification	Price	Notes
Condensation-led specification, 4mm	from £420	No acoustic priority
Combined with acoustic, 6.4mm	from £480	Most common
Low-E coating upgrade	+£60	Warmer surface, per window
Free survey	Free	Includes humidity and ventilation review

All prices supplied and fitted in London, inclusive of survey and 25-year warranty. See the full secondary glazing price list.

Case study: Chronic condensation in a Greenwich flat

Greenwich, SE10

Problem: Daily condensation on four single-glazed sashes with mould on the reveals and early rot in two cills.

Solution: Four sealed secondary units with low-E glass on a condensation-led specification.

Result: No condensation recorded on the room-side pane through the following winter; mould treatment held, cills stabilised.

Frequently asked questions

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