Common Causes of Mould and Condensation in Rental Properties

Mould growth in a property is almost always a symptom of excess moisture. That moisture can come from condensation, penetrating damp, rising damp or a plumbing leak — and the cause determines who is responsible for addressing it. Getting this diagnosis right is important, because the wrong treatment will not solve the problem and may lead to disputes between tenants and landlords.

What Is Condensation?

Condensation occurs when warm, moisture-laden air comes into contact with a cold surface — a window pane, an external wall, or a corner behind furniture — and the moisture in the air converts to liquid. In a typical household, significant amounts of moisture are generated every day through cooking, bathing, showering, using washing machines and even breathing. In a well-ventilated property, this moisture escapes to the outside. In a poorly ventilated or inadequately heated property, it builds up and eventually deposits on cold surfaces.

Common Causes of Condensation Mould

Inadequate Ventilation

The most frequent cause of condensation mould in rental properties is insufficient ventilation. Modern properties with double glazing and draught-proofing are much better at retaining heat but can trap moisture if mechanical ventilation is not provided or used. Extractor fans in kitchens and bathrooms should run during cooking and bathing and for some time afterwards. Trickle vents on windows should remain open. Rooms should be aired regularly by opening windows, even briefly in cold weather.

Insufficient Heating

A property that is inadequately heated will have cold wall surfaces, particularly on external walls. These surfaces attract condensation. The relationship between heating and condensation is sometimes misunderstood — it is not simply about the amount of heat, but about maintaining consistent temperatures throughout the property, including rooms that are not in regular use. Cold spots in unused bedrooms are a common source of mould behind wardrobes.

Drying Laundry Indoors

Drying clothes on radiators or airers inside a property significantly increases the moisture content of indoor air. In a property without adequate ventilation, this moisture has nowhere to go. If tenants must dry laundry indoors — which is understandable in a property without outdoor space — keeping a window open in the same room and running any available ventilation reduces the impact.

Blocked or Absent Extractor Fans

Extractor fans in bathrooms and kitchens are specifically designed to remove moisture-laden air. If they are faulty, blocked or missing, moisture accumulates rapidly. Landlords are responsible for ensuring extractor fans are installed and in working order. Tenants are responsible for using them. If a fan is not working, this should be reported to the landlord promptly. Our handyman service can replace or install extractor fans as part of a void-period or maintenance visit.

Other Types of Damp

Not all mould is caused by condensation. Two other forms of damp are common in older rental properties:

• Penetrating damp. Water entering through a defective roof, broken guttering, cracked rendering or failing pointing. This typically appears as a damp patch on walls or ceilings that worsens in wet weather. This is the landlord's responsibility to repair. Our plumbing and handyman services can assist with identifying and resolving sources of water ingress.
• Rising damp. Moisture from the ground rising through walls where the damp-proof course has failed or is absent. This is less common than condensation mould but requires specialist diagnosis. It appears as a tide-mark on lower walls and is not caused by tenant behaviour.

Identifying whether mould is caused by condensation or penetrating/rising damp matters because the treatment — and the responsibility for fixing it — is completely different.

Responsibility for Mould: Tenants vs Landlords

This is one of the most contentious areas in rental property management. The starting point is that:

• Mould caused by the tenant's lifestyle — inadequate ventilation, excessive moisture generation, failure to use extractor fans — may be considered the tenant's responsibility.
• Mould caused by structural defects — failed damp-proof course, leaking roof, inadequate insulation — is the landlord's responsibility.
• Mould caused by a combination of factors — poor insulation plus inadequate heating plus limited ventilation — may involve shared responsibility.

The Homes (Fitness for Human Habitation) Act 2018 strengthened tenants' rights regarding damp and mould, and landlords have a duty to address it regardless of the cause if it makes the property unfit for habitation. If you are a landlord dealing with a mould complaint, addressing the structural causes promptly — and advising tenants on ventilation practices — is both the correct approach and protects you from legal challenge.

What to Do About Existing Mould

Surface mould — the black spotting that appears on bathroom sealant, window frames and walls — can often be removed with appropriate antifungal treatments. However:

• Removing mould without addressing the underlying cause will result in it returning.
• Mould that has penetrated bathroom sealant generally requires the sealant to be stripped and replaced rather than treated.
• Walls with significant mould may need antifungal treatment, replastering in severe cases, and repainting with a moisture-resistant finish.

If you are preparing a property for new tenants after a mould problem, a thorough deep clean followed by appropriate redecoration — using the right paint for the room type — gives the best foundation. Read our guide on best paint finishes for rental properties for recommendations on moisture-resistant options.