Condensation
on windows: what to do?

Run your finger over the misted glass. If the mark wipes away under your finger, the mist is on the room side: by far the most common case, and the only one that depends on how the house is ventilated and heated. If nothing moves, neither inside nor when wiping outside, the damp is trapped between the two panes. If it wipes off from the outside, the condensation is on the outer side.

On the room side, a few drops at the bottom of a bedroom pane on waking, which disappear within the hour after opening up, are nothing to worry about. It becomes a real problem when the water runs down to form a puddle on the sill, when the mist lasts all day, when the window seals blacken, when the wood of the frame greys or splits, or when dark stains spread onto the reveal and the wall around it.

A hygrometer puts a number on what you see. ADEME recommends keeping indoor air humidity between 40 and 60%. A room that stays permanently above that does not lack a cloth, it lacks renewed air.

One last case to rule out: water that appears under the sill or in the reveal on rainy days, with no mist on the glass, is not condensation. It is water entering through the joinery or through the wall, a water infiltration that is treated quite differently.

The warm air in a room carries water vapour. When it touches a surface colder than its dew point, this vapour turns back into liquid. As the glass is often the coldest wall in the room, this is where excess humidity shows first: mist is a symptom, rarely the disease.

1. Too much vapour produced. A shower, an uncovered pan, laundry drying on a rack, two people sleeping with the door closed: all of this releases water into the air. Paraffin or gas heaters without a flue produce a lot too, notes the ARS Nouvelle-Aquitaine. The clue: the mist follows your routine. At its worst on waking in the bedrooms, just after the shower or during cooking.

2. Ventilation switched off, clogged or blocked. A VMC switched off because it is noisy, extraction vents covered in dust, air vents taped over to get rid of a draught: the damp has no way out. ADEME is categorical: an extraction vent or ventilation grille must never be blocked. The clue: hold a sheet of paper in front of the bathroom vent. If it does not stick, air is not leaving.

3. Air vents forgotten when the windows were changed. It is the sentence we hear most often: “it has been streaming since we changed the windows”. The old joinery let air through its defects. New windows are airtight; if they were ordered without air vents, the VMC draws from a sealed house and the damp stays inside. The principle set by the order of 24 March 1982 on the ventilation of dwellings is nonetheless simple: fresh air enters through the main rooms and leaves through the service rooms. The clue: look at the top of the frame of each bedroom and living room window. No grille, no air vent.

4. A barely heated room. The spare room, closed with the radiator off, or the bedroom kept cool for sleeping: its glass and walls are colder than the rest of the house, and the vapour produced elsewhere comes to settle there. The ARS recommends heating the home to limit cold zones, “seats of condensation”. The clue: the mist is always concentrated in the same room, the coolest one.

5. Single glazing or cold joinery. Single glazing stays almost at the outside temperature; in a Savoie winter, it easily becomes covered in water, even in a properly ventilated house. An old aluminium frame without a thermal break, or a poorly insulated junction between the frame and the wall, creates the same cold spot. The clue: the mist forms on the oldest window, or only around the edge of the glass and on the frame.

Blocking the air vents to gain a little warmth. You remove the air intake the ventilation needs, and the mist gets worse.

Leaving the window ajar for hours. Air renews poorly through a slit, and the wall around the opening sash cools down, which creates exactly the surface where water settles. Better to open wide for a few minutes.

Relying on a dehumidifier alone. It removes water from the air as long as it runs, but changes nothing about missing ventilation or cold glass: it is a complement, not a solution.

Cleaning or repainting the black stains on the reveal without treating the cause. They return the following winter. Cleaning mould on a wall, around windows as elsewhere, only comes after the damp has been dealt with.

Changing the windows without looking at the ventilation. New joinery without air vents can make condensation worse instead of removing it, by shifting the damp onto the walls.

Double glazing is a sealed unit: two panes separated by a layer of dry air or gas, and a seal all around. When this seal ages or gives way, humid air enters the cavity and condenses on the inner face of one of the panes. You cannot wipe it off, and neither airing nor a VMC will change anything.

The defect cannot be repaired: the glazing has to be replaced, or the whole window if the frame is itself worn. Replacing the glazing alone is done by keeping the frame; it makes sense on a recent window with a sound frame. Our quotes contain no line for glazing alone, so we cannot give you a price for this work. On an old window, whose frame is warped or whose wood is damaged, changing the complete joinery is often more logical: our prices for fitted windows are given further down.

If the window is recent, first find the date the works were accepted. During the following year, the completion guarantee (garantie de parfait achèvement) requires the company to repair the defects reported; after that, the guarantee that applies depends on the nature of the defect. Contact the company that fitted the window first, then the glass manufacturer.

A pane covered in mist on the outside in the morning often worries owners of new windows. Yet it is proof that the glazing insulates well. It lets so little heat through from the house that its outer face stays at the outside temperature. On a clear night, it cools further by radiating towards the sky, falls below the dew point of the outside air and becomes covered in water, exactly like the windscreen of a car parked outdoors.

The phenomenon is more visible near Lac du Bourget and Lac d'Annecy and in valley bottoms, where the morning air is often laden with humidity. It disappears as soon as the sun comes out or the air warms up. There is nothing to be done, and above all nothing to blame the window for.

The habits, spending nothing. Air each room for 5 to 10 minutes a day, with the window wide open, switching the radiator off in the meantime: this is ADEME's guidance, winter included. Open the bathroom after the shower rather than letting the vapour drift towards the bedrooms. Cover pans, run the cooker hood, dry laundry outside or in a ventilated room. Keep a steady temperature rather than switching off the heating in a bedroom completely.

Putting the ventilation back in working order. Dust the air vents and extraction vents, remove anything blocking them, check that the VMC unit is running. Some of the windows that stream are sorted out at this stage.

Adding air vents. On windows already fitted, a joiner can cut the slots in the frame and fix air vents in them, chosen to match the VMC in place: a hygro B, for example, goes with humidity-controlled air vents. France Rénov' recommends checking the ventilation with the tradesperson each time windows are replaced.

Installing or replacing the VMC. A house with no mechanical ventilation, or whose unit is out of order, will not get rid of its mist through everyday habits alone. The humidity-controlled single-flow is the one we fit most often in renovation; the dual-flow does away with air vents in the windows but needs a second set of ducts. The detail is in our guide to the price of a VMC.

Changing the windows. This is the right answer to single glazing and cold frames. Full removal allows the junction between the frame and the wall to be dealt with; renovation-fit installation, on the old frame, does not solve this thermal bridge, stresses France Rénov'. Air vents are specified from the order. Our guide to window replacement follows the project step by step.

Treating the cold wall around. When the mist spreads onto the reveal and the wall, the wall itself is too cold. The windows are then decided together with internal wall insulation or external insulation, because the position of the joinery within the thickness of the wall depends on it.

The amounts below are taken from our 2024-2026 quotes in Savoie and Haute-Savoie. They exclude VAT, are rounded, and do not refer to any client in particular. The window prices are those of our guide to the price of a fitted window, drawn from 25 of our quotes.

Two honest caveats. The air vent line comes from a single quote: take it as an order of magnitude, not a rate. And dual-flow remains rare on our projects, three houses only, whereas the hygro B single-flow comes in at a very stable price from one house to the next.

For the price including VAT, a humidity-controlled single-flow VMC, a dual-flow that meets the criteria of the order and a window that reaches the performance thresholds are subject to VAT at 5.5% in a home completed more than two years ago. Other improvement works are in principle subject to the 10% rate; the detail is in our guide to VAT on works.

Call someone in if the mist returns every morning despite daily airing, if it appeared after a change of windows, if mould re-forms around a window, or if the mist is between the panes. In a wet room, where vapour is strongest, our article on bathroom mould details what falls to the ventilation.

On the visit, we look at the three sides of the problem: the glazing and the seals, the air vents window by window, how the VMC works, the heating of each room. Our joiners fit the windows and air vents, our plumber-heating engineers install the ventilation, our plasterboard fitters and painters redo the reveals: our salaried craftsmen, each in their own speciality, on a single quote.

On the quote, check three things. The air vents: one line per window, with their type (self-regulating or humidity-controlled) and their compatibility with the VMC. The VMC: single or dual flow, hygro A or B, number of vents and rooms served. The windows: full removal or renovation-fit installation, treatment of the reveal and of the junction with the wall. A quote that changes the windows without a word about ventilation deserves a question.

If condensation is only one of the signs of a damp house, with flaking walls or a cellar smell, start with our guide damp walls: what to do?, which helps to separate the causes.