Condensation Between Secondary Glazing: Causes and Fixes
Why misting appears between your secondary pane and the original window, and the simple airflow rule that stops it.
Last reviewed October 2026 · Written by London Secondary Glazing
Why it forms
Condensation appears when warm, moist air touches a surface colder than its dew point. In a cavity, that surface is the original single pane.
Tight inside
The secondary unit should be the better-sealed layer, so damp room air cannot get into the cavity in the first place.
Loose outside
The original window should breathe slightly, letting any moisture in the cavity escape to the outside air.
Diagnose the problem by where the water is
Where the condensation sits tells you exactly which way the moisture is moving.
| What you see | Likely cause | Fix |
|---|---|---|
| Misting on the inside face of the original window | Warm, damp room air is leaking into the cavity past the secondary unit | Improve the secondary unit's seals and keep its sashes closed when the room is humid |
| Water pooling on the sill inside the cavity | Cavity sealed tight on both sides, so moisture cannot escape | Allow the outer window a little ventilation, such as a small gap or trickle vent |
| Misting on the room side of the secondary pane | High room humidity, often from cooking, drying clothes or bathrooms | Use extractor fans, open windows briefly each day, aim for 40 - 60% humidity |
| Mould on the reveal or frame | Persistent condensation that is not drying out | Clean with a fungicidal wash, then fix the airflow problem behind it |
A note on period sash windows
Original box sashes are rarely airtight, which works in your favour here. The small gaps around the meeting rails and staff beads let the cavity vent naturally. Problems usually start when the outer sash has been heavily draught-proofed or painted shut and the secondary unit is a loose fit.
If you are fitting secondary glazing to sash windows, ask your installer how the seals are arranged. Compression seals on the secondary frame and a breathable outer window give the best long-term result. On listed buildings this also avoids any damp damage to historic joinery.
Frequently asked questions
Condensation that will not go away?
We can inspect existing secondary glazing, including systems fitted by other companies, and recommend a fix.
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