Condensation Between the Panes? How St Clement Homeowners Can Tell a Tired Unit From a Failed One

Condensation on the inside of a window is one of those everyday things most of us ignore until it starts to look permanent. In St Clement, where homes see their fair share of damp mornings, coastal air and bright, low winter sun, it is worth knowing the difference between a bit of surface mist and a sealed unit that has given up.

The good news is that a misted window is rarely an emergency. Most of the time it is a maintenance prompt, not a crisis. The trick is working out where the water is sitting and what that means for your wallet.

First, work out where the water is

There are two very different types of condensation to look for.

The first is on the room-side surface of the glass. You will see it on winter mornings, especially in kitchens, bathrooms and bedrooms. It clears when the room warms up or when you open a window. That is normal. It usually means the air in the room is carrying more moisture than the glass can cope with, and the fix is better ventilation, not new glazing.

The second is inside the sealed unit itself, between the two panes. You might see a patch of mist near one corner, a milky haze across the middle, or actual water droplets running down the inside of the cavity. That is not normal condensation. It means the edge seal has failed and moisture has got into the gap.

A useful check is to run a finger across the glass. If the moisture is on the surface, it will smear. If it is trapped behind the glass, your finger will stay dry. You may also notice a chalky white residue or a tide mark near the spacer bar, which is another sign the unit has been letting moisture in.

Why sealed units fail in the first place

A double-glazing sealed unit is a simple sandwich: two panes of glass, a spacer bar around the edge, a bead of sealant and a drying agent inside the spacer. The drying agent, usually a desiccant, is there to absorb any residual moisture during manufacture. Once the edge seal is compromised, the desiccant eventually becomes saturated and can no longer keep the cavity clear.

That can happen for several reasons. Age is the big one. Seals harden and crack over time, especially on south-facing windows that get a lot of sun. Movement in the frame, poor drainage around the unit, aggressive cleaning chemicals or a pressure washer aimed too closely at the edges can all shorten the life of a seal. Sometimes it is simply a manufacturing fault that shows up years later.

It is worth saying this plainly: a failed seal is not automatically the installer’s fault, and it does not mean the whole window is rubbish. Sealed units are consumable components in the same way a brake pad is. They have a service life, and some last much longer than others.

Could it just be a cold window?

Not every cold-feeling window is a failed one. Modern low-e glass can feel cooler to the touch on the room side because it is designed to reflect heat back into the room rather than warm the glass itself. A cold pane, on its own, proves very little.

The clues to look for are visual. Surface condensation that clears with ventilation is normal. Mist or droplets trapped between the panes are not. A faint blue-grey haze that never goes away, especially after a warm day, is another sign the cavity has moisture in it.

If you are unsure, take a photograph on a bright day and another on a cold morning. A pattern that stays in the same place is usually inside the unit. A pattern that comes and goes with cooking, showers and weather is usually a ventilation issue.

When replacing the sealed unit is the sensible fix

If the frame is sound, the hinges and handles work, and the drainage slots in the frame are clear, you often only need a new sealed unit rather than a whole new window. That is good news for the budget.

As a rough guide for a standard opening window, replacing the sealed unit often lands somewhere between £120 and £300 fitted, depending on size, glass specification and access. A full new window can easily run into several hundred pounds or more, especially if you want a matching style, improved energy rating or a different frame material. Conservatory roof panels and large picture windows will sit at the higher end.

The job itself is usually straightforward. The old unit is removed, the frame is checked and cleaned, and the new unit is fitted with fresh beads and seals. It is not a job to tackle yourself. Sealed units are heavy, fragile and need to be installed with the correct packers and drainage so the new seal is not stressed straight away.

When a full window replacement starts to make more sense

There are times when replacing just the glass is false economy. If the frame is soft, cracked or visibly rotten, a new unit will not solve the underlying problem. The same applies if the window is draughty, difficult to close, has failed hinges or has drainage problems that keep causing trouble.

It is also worth thinking about the age of the window as a whole. If one unit in a set of matching windows has failed, the others may not be far behind. That does not mean you must replace everything at once. A phased approach is often sensible: deal with the worst window first, then budget for the rest over the next year or two.

If the existing windows are single glazed, very old, or have a poor energy rating, a full replacement may give you a better long-term result than patching up one unit at a time. But if the frames are in good order and the only issue is misting, a sealed unit replacement is usually the more proportionate choice.

Check the warranty before you reach for the cheque book

Sealed units often come with a guarantee against seal failure, commonly five to ten years, though some suppliers offer longer cover. Fitting labour may be covered separately, or not at all. Dig out the original paperwork if you have it, and check whether the guarantee is transferable if you bought the house with the windows already in place.

If the warranty is still valid, contact the original installer first. Have the installation date, window location and photographs ready. A good firm will inspect the unit and explain whether it is covered. If the guarantee has expired, get two or three quotes and ask whether the price includes disposal of the old unit, new beads and a workmanship guarantee.

One small warning: do not be tempted to drill a drainage hole into the sealed unit itself. Some window frames have drainage slots, and those are meant to be there. The sealed cavity is not. Drilling into it will not fix the mist and will almost certainly void any remaining guarantee.

A simple repair-or-replace checklist

• Surface condensation that clears with ventilation: improve airflow, use extractor fans and crack a window when cooking or showering.
• Mist or droplets between the panes: the sealed unit has failed and needs replacing.
• Frame soft, rotten or badly warped: consider a full window replacement.
• One failed unit in otherwise sound frames: replace the unit, not the window.
• Several units of the same age failing together: plan a phased replacement rather than an emergency spend.
• Warranty still in date: contact the original installer before paying anyone else.

A misted window is a nuisance, but it is also a fairly honest one. It tells you the seal has done its time. Once you know whether the water is on the surface or inside the cavity, the decision usually becomes much less daunting. And if you are still unsure, a local glazier should be able to tell you within a few minutes of looking.