Why windows fog up in winter, and what to do

What causes condensation on windows?

Air can hold less water vapour as it cools. When indoor air touches a cold surface, it cools there, reaches its dew point and drops its moisture as droplets. In winter the window is usually the coldest surface in a room, so it shows the problem first. Cooking, showers, laundry, breathing, houseplants and humidifiers all add moisture, and a house with good air sealing keeps it in. Cold surfaces elsewhere, from thermal bridges to uninsulated corners, can collect moisture too, as described in thermal bridging.

Is it inside, outside or between the panes?

Where the moisture appears tells you what is happening.

• On the room side. Indoor humidity is too high for the temperature of the glass. This is the common winter case.
• On the outside. On clear, still mornings, very efficient glass can fog outdoors because its outer surface stays cold. It is harmless and shows the window is holding heat in.
• Between the panes. The seal has failed and humid air has entered the unit. The glass unit usually has to be replaced; wiping will not help.

What indoor humidity level is reasonable in winter?

Many building and health agencies suggest keeping indoor relative humidity roughly between 30 and 50 per cent, with the upper end falling as it gets colder outside. In severe cold, a level that felt fine in autumn may be too high. A small hygrometer costs little and shows the actual number. If the window is wet every morning, lower the reading rather than just wiping the glass. Persistent moisture risks mould, rotting sills and stained walls, so deal with it promptly.

How do you reduce condensation?

Start with the sources and the air.

• Use exhaust fans. Run the bathroom fan during and after showers and the kitchen hood while cooking, and vent them outdoors.
• Add controlled ventilation. A heat recovery ventilator brings in fresh air and removes humid air with little heat loss, see the HRV guide.
• Turn down humidifiers. Many houses do not need one in a well-sealed building.
• Move air. Open blinds and curtains, and keep furniture and drapes clear of the glass so that warm air reaches it.
• Dry clothes elsewhere. Indoor drying adds litres of water to the air.
• Use a dehumidifier. It helps in basements and damp periods.

Will new windows solve it?

Often they help, since the inside glass surface in a good double or triple unit with a low-e coating stays warmer, so condensation needs more humidity to form. They do not cure a humidity problem, however. A tighter house with new windows can show moisture elsewhere if ventilation is weak. See low-e windows and high performance glazing for what changes inside the unit. A blower door test, explained in our air tightness guide, shows how tight the house has become, and an energy adviser can recommend ventilation to match.

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