Condensation, penetrating damp or rising damp? Telling them apart when a tenant reports mould

When a tenant reports mould, the first question is where the moisture is coming from. The answer decides the fix, who needs to be involved, and how quickly it can be put right. It is also the question most damp and mould disputes turn on.

There are three common sources, plus leaks.

Condensation

Condensation is the most common cause of damp and mould in UK homes. Warm, moist air from cooking, showers, drying clothes and breathing meets a cold surface and turns back into water.

Signs:

• Black spot mould in corners, around window frames, behind furniture against outside walls, and in bathrooms and kitchens.
• Water droplets on the inside of windows, especially in the morning.
• Worse in the colder months.
• No tide mark, and no salt deposits on the wall.

Condensation is often treated as purely a tenant lifestyle issue. How a home is used plays a part, but so do extractor fans that don't work, missing trickle vents, cold walls with little insulation, and heating that is expensive to run. Those sit with the landlord.

Penetrating damp

Water getting in from outside through a defect in the building.

Signs:

• A patch in one area that grows or darkens after rain.
• It can appear at any height, including upstairs walls and ceilings.
• There is usually a defect nearby: a blocked or leaking gutter or downpipe, cracked render, failed pointing, a slipped roof tile, or failed seals around a window.

The fix is the defect first, then drying out the wall.

Rising damp

Moisture from the ground drawn up through a wall.

Signs:

• Ground-floor walls only, usually no higher than about a metre.
• A horizontal tide mark.
• White, powdery salt deposits, and crumbling or blown plaster.
• Decaying skirting boards.

It usually points to a failed damp-proof course, or one that has been bridged, for example by a flower bed or path built up against the outside wall. Genuine rising damp is less common than it is often said to be. Get an independent survey before paying for a chemical damp-proof course.

Leaks

A leaking pipe, a failed shower seal or an overflowing tank. Usually a sudden, localised patch that doesn't follow the weather. Find and fix the leak first.

Where the weather comes in

Condensation depends on the weather as well as on how the home is used. BS 5250, the UK code of practice on moisture and condensation in buildings, sets out the mechanism: when indoor humidity stays above 70% for long periods, the surface of external walls can rise above 80% humidity, which is enough for mould to grow. Long spells of very humid weather make it harder for a property to get rid of the moisture produced inside it.

Damp/mould watch on Property Risk Alerts reads outdoor humidity for each property's location and emails you when it stays at 90% or above, or the air stays close to its dew point, for five days in a row. It uses weather data for the area. Nothing is installed in the property. It tells you when conditions favour condensation, so you can check ventilation and heating in vacant properties and put tenant reports in context.

What to record when a tenant reports mould

• The date of the report, and how it was made.
• Dated photos of the affected area, with something in shot for scale.
• Exactly where it is: which room, which wall, what height, and whether it is an external wall.
• Whether it is near a window, bathroom, kitchen or a known defect.
• What ventilation is in place and whether it works.
• What the weather was doing. A dated humidity record for the property's location helps separate a weather-driven condensation episode from a leak that has been there all year.
• What you did next, and when.

That record matters whoever turns out to be responsible. Damp and mould growth is one of the hazards assessed under the Housing Health and Safety Rating System, and local authorities can take action against private landlords where it is serious. For the timescales Awaab's Law is set to add, see Awaab's Law and private landlords.

For damp you can't trace to a clear cause, get a qualified surveyor in.

Add your properties and start a dated humidity record for each one.