Why am I getting condensation in summer?

People are told, correctly, that condensation is a ventilation problem, and then find that opening everything up on a muggy August day makes their cellar or utility room wetter. Both facts are true, because the two seasons work in opposite directions. Knowing which one you are looking at tells you whether to open the window or shut it.

Step by step

1. Work out which surface is cold.Summer condensation forms where warm humid air touches something below the dew point. The usual suspects are cellar and basement walls, solid floors, unheated north-facing rooms, cold water pipes and cisterns, and the inside of a fridge or freezer left open. If the wet surface is cold to the touch on a warm day, that is your answer.
2. Check the direction against the winter pattern.Winter condensation appears on the coldest outward-facing surfaces — window glass, external wall corners, cold bridges — because warm damp indoor air meets them. Summer condensation appears on the coldest inward surfaces, often below ground, because warm damp outdoor air has been let in to meet them.
3. Stop ventilating with humid air.This is the counter-intuitive part. On a warm, muggy day, opening a cellar or a cold utility room to the outside brings in air holding far more moisture than the cold air inside can retain, and it condenses out on every cold surface. Keep such spaces shut during the humid part of the day.
4. Ventilate when the outside air is genuinely drier.Early morning and cooler, clearer days are when outdoor air can actually remove moisture from a cold space. A cheap hygrometer inside and a weather app showing outdoor humidity is enough to make the call.
5. Use a dehumidifier rather than an open window in a cellar.For a below-ground space the reliable answer in summer is a dehumidifier with the space closed, not ventilation. A compressor dehumidifier works well at summer temperatures — the desiccant type is the one that earns its place in cold conditions.
6. Deal with cold pipes and cisterns directly.Cold water pipes and toilet cisterns sweating in summer look exactly like a leak and are a common false alarm. Pipe insulation on the affected run, or an insulated cistern, stops it. Confirm it is condensation by drying it and watching whether it reappears evenly over the whole cold surface rather than dripping from one point.
7. Do not confuse it with a leak or with rising damp.Condensation appears evenly across a whole cold surface, is worst in muggy weather, and dries when the humidity falls. A leak is localised and persists regardless of weather. Rising damp has a tide line and does not come and go with the forecast.
8. Watch for mould before you see water.Mould grows at relative humidity well below the point where liquid water appears, so a cellar or wardrobe can go mouldy in a humid spell without ever looking wet. Musty smell is the early warning.

Tips

• A cheap hygrometer in a cellar or utility room is the single most useful thing you can add, because it turns a guess about whether to open up into a decision.
• Stone and tiled floors in an unheated north-facing room can go slippery-wet in a muggy spell. It is condensation, not a leak, and closing the room up plus a dehumidifier fixes it.
• Do not store cardboard boxes, photographs or fabric directly against a cellar wall or floor. Raise them on a rack so air can move behind them.
• A fridge or freezer in a garage or outbuilding will run wet in summer humidity, and the door seal is where it shows first.

Common mistakes

• Opening a cellar up on a muggy day to dry it out — Warm humid air entering a cold space condenses on every cold surface. It adds water rather than removing it — the opposite of the winter instinct.
• Assuming any damp patch in summer is a leak — Sweating pipes, cisterns and cold floors are extremely common in humid weather and produce convincing puddles. Check whether the whole cold surface is wet before lifting floorboards.
• Buying a desiccant dehumidifier for a summer cellar problem — Desiccant models earn their advantage in cold conditions. At summer temperatures a compressor model extracts more water per unit of electricity.

Questions people ask