Why are my windows wet in the morning?

Wet windows are condensation: water vapour in the room air turning to liquid on a cold surface. It is the most common reason people look for a dehumidifier.

Why it happens

Air always carries some water vapour. When that air touches a surface colder than the air, such as a window pane overnight, it cools, can hold less vapour, and the excess becomes droplets. The more moisture in the room, and the colder the surface, the more water you see.

Where the moisture comes from

Ordinary living adds water to the air: breathing, cooking, showers and baths, kettles, and drying clothes indoors. Fife Council's condensation guide says two tenants can put up to 15 pints (about 8.5 litres) of water vapour into the air from everyday activities, and a household of five or six, 25 to 30 pints (about 14 to 17 litres). Flueless gas heaters, paraffin heaters and tumble dryers that vent indoors add more.

Why winter is worst

Outside walls and glazing are colder, we keep windows shut, and we dry washing indoors. Moisture has fewer ways out and more cold surfaces to land on.

What to try first

• Extract it at the source. Run bathroom and kitchen extractor fans, or open a window, during and for at least 15 minutes after cooking or showering.
• Keep doors closed to kitchens and bathrooms so steam does not spread to cold bedrooms.
• Vent tumble dryers outdoors, and keep lids on pans.
• Dry washing smartly. Avoid piling wet towels on radiators in a closed room.
• Ventilate daily. A short, wide open window clears moist air faster than leaving a window ajar all day. In winter this costs some heat, so it is a balance.
• Wipe windows in the morning so water does not soak into frames and sealant.
• Measure. A cheap hygrometer tells you if the room sits above 60%. See what humidity should my home be?

When a dehumidifier makes sense

If you have done the above and the room is still damp, or you cannot ventilate (a rented flat, laundry drying indoors, a room with no extractor), a dehumidifier removes the moisture instead of letting it escape. A compressor model suits a heated room, a desiccant model suits a cold one. See compressor vs desiccant, then use the sizer to find the capacity.

When it is something else

Wet patches that appear after rain, tide marks, or damp near plumbing are not condensation. Read when damp isn't a dehumidifier problem.

Source: Fife Council, Landlord Guide to Condensation Control, for the moisture figures and ventilation steps. Litre conversions are ours. This is general information, not building advice.