Common Causes of Mould and Condensation in Rental Properties

Mould and condensation are among the most common — and most disputed — issues in rental properties. Tenants complain that the property is damp; landlords suspect the tenants are at fault. In reality, the causes are often a combination of both, and resolving the problem requires understanding where it is actually coming from.

How Condensation Forms

Condensation occurs when warm, moisture-laden air meets a cold surface. The air cools rapidly, can no longer hold the same amount of water vapour, and releases it as liquid water — condensation — on the surface. In a home, this most commonly occurs on cold external walls, single-glazed windows, and poorly insulated surfaces.

If condensation is allowed to persist, it creates conditions where mould can grow. Black mould — Cladosporium or Aspergillus — is the most common type found in residential properties and can cause health problems, particularly for people with respiratory conditions.

Causes on the Tenant's Side

Tenant behaviour significantly affects condensation levels within a property. Common contributing factors include:

• Drying clothes indoors: One of the largest sources of moisture vapour inside a home. A single load of wet laundry releases a substantial amount of moisture as it dries.
• Inadequate heating: Cold surfaces are more prone to condensation. If a tenant keeps the property very cold, the interior walls and windows remain cold enough to cause condensation even at relatively low humidity levels.
• Cooking without ventilation: Cooking produces significant moisture. Not using extractor fans, or covering the extractor, prevents this moisture from being expelled.
• Not ventilating bathrooms: Showers and baths produce large amounts of steam. Closing the bathroom door, not running the extractor fan, or not opening a window allows this moisture to spread through the property.

Causes on the Landlord's Side

The structure and condition of the property also play a significant role. Landlords have a responsibility to ensure the property is free from hazards under the Homes (Fitness for Human Habitation) Act 2018, and persistent damp caused by structural deficiencies falls within this obligation. Common property-side causes include:

• Inadequate or blocked ventilation: Extractor fans that don't work, trickle vents that have been sealed, or a complete absence of ventilation in kitchens and bathrooms
• Poor or absent wall insulation: Cold external walls cause condensation even with normal tenant behaviour
• Single glazing: Single-glazed windows are significantly colder than double-glazed units and are a frequent site of condensation and mould growth around the frame
• Blocked gutters: Overflowing gutters saturate the external wall, leading to penetrating damp rather than condensation. Annual gutter cleaning is essential maintenance.
• Penetrating damp through walls or roof: Cracked render, failed pointing, or a leaking roof allows water to enter the wall structure, causing damp that presents as mould on interior surfaces

Telling Condensation Damp from Structural Damp

Condensation damp typically appears on cold surfaces — corners of rooms at external walls, around window frames, and on north-facing walls. Structural or penetrating damp typically appears in a pattern related to the external defect — below a leaking gutter, near a cracked render section, or around a failing roof joint.

If in doubt, a damp survey or a careful external inspection will usually identify the source. Do not simply treat the internal mould without identifying and addressing the underlying cause — it will return.

Addressing Mould in a Rental Property

1. Identify the cause: is it condensation, penetrating damp, or rising damp?
2. Address the root cause — improve ventilation, repair the external defect, or improve insulation
3. Treat and remove existing mould growth with appropriate products
4. Repaint affected areas with a moisture-resistant paint if appropriate
5. Provide tenants with guidance on reducing condensation through ventilation and heating

For properties requiring redecoration after mould treatment, our painting and decorating service uses appropriate moisture-resistant finishes. For structural issues such as damaged gutters or roof defects, our handyman and maintenance service can assess and resolve the problem.

Get Help with Your Property

If you're dealing with persistent mould or condensation in a rental property, Multrex Maintenance can help identify the cause and carry out the necessary works. Contact us to arrange an assessment or request a free quote.