Condensation on the Inside of Windows: Why It Happens and How to Stop It

Short answer: Water on the inside of the glass is moist indoor air meeting a cold surface. Whether the windows or the ventilation are to blame depends on the glass. On single or older double glazing it happens even in a well-ventilated home, because the glass is simply too cold. On modern double or triple glazing it only happens when the air inside is very humid, which means too much moisture and too little ventilation. Cut moisture where it starts, extract it where it is made, keep the vents open and the heating steady. If modern windows still stream every morning, the ventilation can’t keep up.

Inside, between the panes or outside?

• Between the panes: the seal of the glass unit has failed. That is a job for a glazier; ventilation won’t help.
• On the outside: normal on energy-efficient glass on clear, cold mornings. It shows the glass keeps the heat in. Nothing to do.
• On the inside: the rest of this guide.

Why does condensation form on the inside of windows?

Warm air holds more water than cold air: at 20 °C up to about 17 g per cubic metre, at 5 °C only 6.8 g. When room air touches a surface colder than its dew point, the extra water turns into droplets. In a room at 20 °C and 50% humidity the dew point is 9.3 °C; at 60% it is 12 °C. Any surface colder than that gets wet.

The water comes from inside the house. A home produces about 3 to 8 litres a day, 1,000 to 2,800 litres a year, and one person breathes out about a litre a day.

• A shower or bath: up to about a litre.
• Cooking a meal: around half a litre to a litre.
• Clothes dried indoors: one wash can hold well over a litre of water.
• A new build or a renovation: plaster, screed and timber give off moisture for months.

Irish homes add two things: a mild, damp climate, and houses that become much tighter after new windows and insulation, often with the trickle vents taped over. The moisture stays inside.

Is it the windows or the ventilation?

It depends on how cold the glass gets. For a room at 20 °C and 0 °C outside, this is the indoor humidity at which the middle of the pane starts to get wet:

These figures are for the middle of the pane. The edges and the frames are colder, especially metal frames without a thermal break. At −5 °C outside the thresholds drop to 28%, 56%, 78% and 85%.

Replacing single glazing with double glazing without adding ventilation often moves the problem rather than solving it. The glass is no longer the coldest surface, so the water and the mould move to cold corners, reveals and the backs of wardrobes.

Why is it worst in the morning and in bedrooms?

Two people sleeping in a closed bedroom breathe out more than half a litre of water overnight. The heating is off, the room and the glass cool down, and the humidity climbs. By morning the glass is the coldest surface in a damp room.

Is condensation on windows a problem?

On its own, a little water on cold mornings is harmless. Every morning, it runs onto the frames and sills, rots timber and grows black mould on seals and reveals. Mould on timber starts once the air at the surface stays above about 75–80% humidity, and on plaster or concrete above about 90–95%. Damp and mould are linked to asthma and breathing problems. More on mould and how ventilation helps.

How to stop condensation on windows, step by step

1. Today, for free

• Put lids on pots and use the cooker hood or kitchen extract while cooking.
• Dry clothes outside, or in a room with the door shut and the extract running.
• Run the bathroom fan during the shower and for at least 15 minutes after, and keep the bathroom door shut.
• If your house has trickle vents in the windows or background vents in the walls, keep them open and clean.
• Keep the heating steady rather than on and off, and don’t let bedrooms get very cold.
• Keep furniture a little away from cold outside walls.
• Wipe the glass in the morning so water doesn’t reach the frame. It treats the symptom, not the cause.

2. Measure the humidity

A cheap hygrometer tells you more than any guess. In winter, aim for 40–60%. If it reads over 60–65% every morning, the moisture made in the house is not getting out.

3. Check the extract fans

In a home with background vents and extract fans, TGD F expects at least 60 l/s in the kitchen (30 l/s from a hood next to the hob), 30 l/s in a utility room, 15 l/s in a bathroom and 6 l/s in a WC. A fan that barely holds a sheet of tissue paper is not doing the job.

4. Whole-house ventilation

• cMEV in an existing home: one quiet unit extracts from the kitchen and bathrooms day and night and boosts when humidity rises; fresh air comes in through the window vents. About €2,200–3,000 installed.
• MVHR in a new or very airtight home: filtered, pre-warmed fresh air to every room and about 90% of the heat kept. From €6,200, priced by floor area.

With single glazing, start with the windows. Which system fits your house: home ventilation systems compared, what MVHR costs, or get a price for your floor area.

5. You already have MVHR or cMEV and the windows still get wet

• Check the filters: clogged filters cut the airflow. See MVHR filters.
• Make sure the unit isn’t left on a low or off setting, and that the summer bypass isn’t running in winter. See summer bypass.
• Many systems were never balanced, or were turned down to stop a noise. Airflow measurement and balancing starts at €270. See commissioning and airflow balancing.
• An overdue service: see ventilation servicing.

How to stop condensation on windows overnight

• Leave the bedroom door slightly open, and the trickle vent open if the room has one.
• Keep a low, steady heat instead of letting the room go cold.
• Don’t dry clothes in the bedroom.
• Leave MVHR or cMEV on its normal setting at night, never off.

What doesn’t work

• Moisture-absorbing tubs and crystals: they hold a few hundred millilitres, while a home makes several litres a day.
• Window vacuums: they collect the water but don’t stop it.
• A window open all day in winter: it cools the room and the glass. Five to ten minutes of airing after a shower or cooking is fine.
• A dehumidifier on its own: it helps while it runs, but it uses electricity and brings no fresh air. Steady heating and ventilation work better in the long run.

New windows and condensation

New windows stop the draughts that used to ventilate the house by accident. In a house with background vents and extract fans, or with cMEV, trickle vents should be fitted in the new frames, because that is where the fresh air comes in. With MVHR they are not needed: the fresh air comes in through its ducts. If new double or triple glazing streams with water, the windows are doing their job and the house needs more ventilation.

Frequently asked questions

Get help from Eco Vent

Call us on 057 851 1416; the line is answered straight away. Tell us how old the windows are, what ventilation the house has and what your humidity meter reads. With no mechanical ventilation, we price a system from your house plans. With a system already in place, we service or rebalance it.

Sources

• BSRIA Topic Guide TG 26/2024, Mould in Buildings
• NSAI S.R. 54:2014+A2:2022, Code of practice for the energy efficient retrofit of dwellings
• Technical Guidance Document F (2019)
• SEAI: ventilation systems