Why Is There Condensation on the Inside of My Windows?

Condensation on the inside of your windows is warm, moist air from inside the house meeting a pane of glass that is colder than the air touching it. The moisture in that air turns back into water on the cold surface. It is almost always a ventilation and surface temperature problem rather than a fault in the glass itself, which is why wiping it away every morning never actually solves anything.

The honest nuance is that condensation is not automatically a sign you need new windows. From working with clients across Epsom, Ashtead and Hinchley Wood, we've found the same pattern repeats. A 1930s semi with original single glazing will run water down every pane on a cold morning, and so will a house with twenty year old double glazing that has lost its edge seals. But so will a brand new, perfectly good window in a bedroom where the door stays shut, the trickle vent is closed and there is washing drying on the radiator. Working out which of those three you have is the entire job.

What actually causes condensation on the inside of a window?

Three things have to line up. There has to be moisture in the air, there has to be a surface cold enough for that air to reach its dew point, and there has to be too little ventilation to carry the moisture away before it settles.

Glass is nearly always the coldest surface in a room, which is why it shows there first. That does not make the window the cause. It makes it the messenger. The same air is depositing moisture into cold corners, behind wardrobes on external walls and around window reveals, where you will not see it until mould appears.

How much moisture is a normal household producing?

Considerably more than most people expect. The Energy Saving Trust puts it at up to 100 litres of moisture a week for a typical household, varying with the size of the home, the number of people in it and how it is lived in. Cooking, showering, drying clothes indoors and simply breathing all add to the total.

That figure is worth sitting with, because it reframes the problem entirely. A hundred litres a week has to go somewhere. If it is not extracted at source or ventilated out, it will find the coldest surface in the house and settle on it.

• Drying washing indoors is the single biggest avoidable source. A full load holds a couple of litres of water, and on a clothes horse in a closed room every drop of it ends up in the air.
• Showering without running the extract fan adds a large volume of moisture in a few minutes, and an unvented bathroom pushes it straight onto the landing the moment the door opens.
• Cooking without the extractor running does the same in the kitchen, and boiling and steaming are far worse than frying for this.
• Unvented tumble dryers and racks over radiators add to the load continuously rather than in short bursts, which is what makes them so effective at causing a problem.

Is condensation a sign my windows have failed?

Sometimes, and it depends entirely on which side of the glass the water is sitting.

• Water on the room side of the glass is a ventilation and temperature issue. The window is doing its job of separating a warm room from a cold outside, and the glass is simply the first surface cold enough to show it.
• Water trapped between the two panes is a genuine failure. The seal around the edge of the unit has broken down and moist air has got into the sealed cavity. No amount of ventilation will change that, and it cannot be cleaned.
• Water on the outside face of the glass, usually first thing in the morning, is a good sign rather than a bad one. It means the outer pane is staying cold because the glass is holding the heat inside where you paid for it to be.

If the inner pane is cold enough to stream on a mild day, the U-value of the unit is worth checking, because a poor one leaves the inner face sitting close to outside temperature. Older units drift in performance over time as well as failing outright.

Why is it always the same two or three rooms?

Bedrooms and north-facing rooms, in our experience, almost without exception.

Bedrooms are the worst possible combination. Two adults asleep for eight hours put a surprising amount of moisture into a closed room, the heating is usually off overnight so the glass is at its coldest, and the door stays shut so nothing moves. North-facing rooms get no solar gain during the day at all, so the glass never warms up enough to dry out between one night and the next.

Certain kinds of property show it more than others. The interwar semis that make up so much of Epsom, Ashtead and Banstead often have solid or poorly filled cavity walls, so the whole external envelope runs cold and the glass is only the most visible part of it. Victorian and Edwardian houses where the chimney breasts have been blocked up have lost the background ventilation their original design quietly relied on, and nothing was put back in its place.

What actually fixes it?

Ventilation first, in almost every case, and it costs a great deal less than anything else on the list.

• Use the extract fans you already have, and check they are genuinely moving air. Approved Document F sets the rates at 30 litres per second in a kitchen where the fan sits next to the hob, or 60 litres per second where it does not, along with 15 litres per second in a bathroom, 30 in a utility room and 6 in a WC. A fan that will no longer hold a sheet of paper against its grille is achieving none of that.
• Open the trickle vents and leave them open. They are sized to provide continuous background ventilation, and closing them because a room feels draughty is the most common self-inflicted cause of condensation we come across.
• Shut the bathroom door while the fan runs rather than afterwards. Containing the moisture until it has actually been extracted stops it dispersing through the rest of the house and reappearing on a bedroom window.
• Dry washing outdoors wherever possible. If it has to come in, put it in one room with the door closed and the vent open rather than spreading the load across the house.
• Leave a gap behind furniture on external walls. Air needs to move across a cold surface, and a wardrobe pushed flat against an outside wall creates exactly the still, cold pocket where mould starts.

In our experience, running a bathroom extract fan with an overrun timer works better than opening the window after a shower, because the fan removes the moisture while it is still concentrated and airborne, whereas an open window mostly sheds heat and lets the damp air drift onto the landing before it has cleared. It is a small change, it costs very little, and it is usually the one that makes the visible difference.

Will new windows stop it?

They will change it, and they can improve it a great deal, but only if the ventilation goes in alongside them.

Modern sealed units with a low-emissivity coating keep the inner pane much closer to room temperature, so the glass stops being the coldest surface and the water has nowhere obvious to form. That is a real improvement and most people notice it within days.

There is a trap here that deserves saying plainly. Replacing draughty old windows with well-sealed new ones removes a great deal of accidental ventilation the house was quietly depending on. If the new windows go in with the trickle vents shut and no working extract fans, the moisture load has not changed while the escape routes have shrunk. Some people find the problem moves rather than disappears, showing up on the reveals, in the corners and behind furniture instead of on the glass.

This is precisely why background ventilation is now written into the rules. Approved Document F was updated in its 2021 edition, in force in England from 15 June 2022, and most replacement windows are expected to carry trickle vents as a result. It is not an upsell. It is the part that makes everything else work.

When is replacement genuinely the right answer?

When the frames or the units have actually failed, rather than when the glass is simply doing its job.

• Misting between the panes cannot be repaired and only worsens, because the desiccant inside the spacer bar is saturated and cannot recover.
• Water sitting on the frame rather than the glass usually points to failed gaskets or blocked drainage rather than a household ventilation problem.
• Timber frames that feel soft at the bottom corners have been wet for a long time and will not come back.
• Original single glazing will always run with water in a heated house, whatever you do about ventilation, because a single pane cannot be anything other than cold.

If two or three of those are showing at once, that is usually the point at which replacement stops being optional, and the wider signs that a window has reached the end of its working life tend to arrive together rather than one at a time.

So, should you worry about condensation on your windows?

Not usually, and not immediately. Condensation on the room side of the glass is telling you the house is producing more moisture than it is clearing, and in most homes that is solved with ventilation habits and working extract fans rather than a new set of windows. Deal with the moisture first and see what is left over.

Where it does point to something real is when the water is trapped between the panes, when the frames themselves are staying wet, or when the glazing is old enough that the inner pane can never be anything but cold. Those are worth having looked at properly.

Our team has over 30 years of combined experience fitting and replacing windows across Surrey, and we would far rather tell you that a working fan and an open trickle vent will sort it than sell you something you do not need. If you would like an honest opinion on which of the three you have, get in touch and we will tell you what we find.