Condensation Between the Panes: Causes and Fixes
Condensation on the room side of secondary glazing means the room is humid. Condensation in the cavity, between the old glass and the new pane, means something else entirely, and it is worth diagnosing properly because the fix depends on which way the moisture is travelling.
What the cavity is supposed to do
The cavity in a secondary glazing system is not a sealed unit. It is a deliberately unsealed air space, and it is designed to be slightly vented to the outside rather than to the room. That sounds counterintuitive, but it is the correct arrangement: any moisture that finds its way in should be able to escape outward through the original window's natural leakage, while the new inner frame is sealed tightly against room air.
Why moisture appears there
Cavity condensation almost always means humid room air is getting into the cavity and meeting the cold original glass. That happens when the inner seal is imperfect, when a slider is left slightly open, when the frame was fitted to an uneven reveal without packing and sealing, or when the original window has been made too airtight so nothing can dry outward. Occasionally it means water is entering from outside through failed pointing or a defective cill.
- Inner seal leaking room air into the cavity
- Panel left ajar in cold weather
- Original window overly sealed, no outward drying
- External water ingress through cill or pointing
The counterintuitive fix
The usual remedy is to tighten the inner seal and loosen the outer one. Sealing the secondary frame properly to the reveal, and leaving the original window's natural leakage paths alone, lets the cavity dry outward while stopping room moisture entering. Owners who have carefully draught proofed the original sash as well sometimes create the problem, because the cavity then has no route out at all.
Checking for external ingress
If the moisture appears after rain rather than after cold nights, and if it pools at the bottom of the cavity rather than misting the glass evenly, suspect water rather than vapour. Look at the external cill fall, the pointing around the frame, any failed mastic, and whether the drainage slots in the original window are blocked with paint. Fix the building defect first; no glazing change will out run a leak.
Reducing the moisture load in the room
Everything that lowers room humidity also lowers cavity risk. Cover pans while cooking, run the extract fan during and after showers, vent the tumble dryer outside, avoid drying washing on radiators, and keep a background heat level in rooms that would otherwise sit cold. Trickle vents in the secondary frame provide controlled background ventilation without opening anything.
Getting access to clean it
Whatever the cause, you need to be able to reach the cavity faces. Lift out panels come out entirely. Hinged units swing clear. Sliders can usually be lifted from their tracks. When specifying a system, ask how you will clean the inside faces, because a system that cannot be opened will accumulate dust and any occasional condensation marks with no way to deal with them.
When to call us back
If your system is ours and the cavity is misting regularly, that is a seal or adjustment issue and it is what the aftercare visit exists for. Frames settle, buildings move, and a compression seal occasionally needs adjusting after the first winter. It is a short visit and it usually resolves the problem completely.
Frequently asked questions
Not usually. Secondary glazing cavities are not sealed units, so misting points to seal adjustment, ventilation or an external leak rather than a failed product.
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