Misted Double Glazing and Condensation Explained

Why sealed units mist up

A double-glazed unit relies on a sealed air gap between two panes, edged with a spacer bar containing a desiccant that absorbs any residual moisture at manufacture. When that seal fails, usually due to age, thermal movement, or a manufacturing fault, humid air can get into the gap. Once inside, it condenses on the inner glass surfaces where you cannot wipe it away, producing a permanent haze or fogging that clears and returns with temperature changes.

This is a mechanical seal failure rather than anything to do with how you use the room, and it cannot be fixed by better ventilation once it has happened.

Interpane condensation versus surface condensation

It helps to be clear about the three different things that can look similar from across a room.

• Internal condensation: mist on the room-side face of the inner pane, caused by warm moist room air meeting a cold glass surface. This is a ventilation and humidity issue, not a window fault.
• External condensation: dew forming on the outside face of the glass, usually on cool clear mornings. This is actually a sign of good insulation, since the outer pane is staying cold rather than heat leaking through it.
• Interpane condensation: mist trapped between the two panes that cannot be reached to wipe. This means the seal has failed and the unit needs replacing.

Ventilation and trickle vents

Modern homes are built and retrofitted to be increasingly airtight, which is good for heating bills but means moisture from cooking, showering, drying clothes and simply breathing has fewer places to escape. Trickle vents, the small slotted vents usually built into the top of a window frame, provide continuous background ventilation without needing a window open.

If your replacement windows are noticeably more airtight than your old ones, and you start seeing more surface condensation than before, this is often the actual cause, and it is worth checking that trickle vents are fitted, open, and not blocked by curtains or blinds.

Can you replace just the glass?

In most cases, yes. If the frame is sound and only the sealed unit has failed, a new double-glazed unit can be cut to size and fitted into the existing frame, which is considerably cheaper and quicker than replacing the whole window. This is usually the right approach for a single or small number of misted units in an otherwise good frame.

It is not always possible on very old or bespoke frame profiles where a matching unit size or bead depth cannot easily be sourced, in which case whole-window replacement becomes the more practical option.

Mould and damp connected to condensation

Persistent surface condensation that is not addressed can lead to black mould developing on frames, reveals and nearby plaster, particularly in bathrooms and bedrooms where humidity is naturally higher. This is a health and property issue as well as a cosmetic one, and tackling the ventilation problem early is far cheaper than dealing with mould remediation and repainting later.

Practical fixes for internal condensation

Most surface condensation problems respond well to straightforward changes in how a room is ventilated and heated, rather than requiring new windows at all.

• Keep trickle vents open, especially overnight and in bathrooms and kitchens
• Use extractor fans when cooking or showering and leave them running a few minutes afterwards
• Avoid drying washing on radiators in poorly ventilated rooms
• Maintain a reasonably consistent background heat rather than heating a cold room sharply
• Wipe away morning condensation promptly to stop it soaking into frames or sills

When to call for a survey

If mist is trapped between the panes and won't clear, that is a straightforward unit replacement. If several windows across the house are streaming with condensation despite reasonable ventilation habits, it is worth having the whole property looked at, since it may point to a broader ventilation or damp issue rather than the windows themselves. On the Wirral, older solid-wall properties without cavity insulation are particularly prone to this, and new, more airtight windows can make an existing ventilation shortfall more noticeable rather than causing it.

Common questions

Does misting between the panes mean the whole window needs replacing?

Not usually. If the frame is in good condition, only the sealed glazed unit typically needs replacing, which is cheaper and quicker than a full window replacement.

Why has condensation got worse since I had new windows fitted?

New windows are usually more airtight than what they replaced, which reduces incidental draughts that used to carry moisture away. Checking trickle vents are open and using extractor fans usually resolves it.

Is condensation on the outside of my windows a fault?

No, external condensation on cool mornings is normal and actually indicates the glass is insulating well, keeping the outer pane cold rather than letting heat through.

Can I stop condensation without opening windows in cold weather?

Trickle vents provide background ventilation without needing to open the window, and combined with extractor fan use, they usually reduce condensation significantly.