Damp on internal walls is a common problem in UK homes, particularly in older properties but also in newer builds. It can be worrying to discover patches of moisture, peeling wallpaper, or mould growth indoors, and many homeowners are unsure what the root cause might be. Understanding why damp occurs is the first step towards dealing with it effectively.
This article explains the main causes of damp on internal walls in clear, practical terms. It is written for beginners and aims to help you recognise different types of damp, understand how they develop, and know what signs to look out for.
What Is Damp?
Damp refers to unwanted moisture that enters or forms within a building. When this moisture affects internal walls, it can damage plaster, decoration, and even the structure of the property if left untreated.
Damp is not a single problem with one cause. Instead, it usually falls into one of three main categories:
- Rising damp
- Penetrating damp
- Condensation
Each type has different causes, symptoms, and solutions.
Rising Damp
What Is Rising Damp?
Rising damp occurs when moisture from the ground travels upwards through walls by capillary action. This happens when there is no effective barrier to stop water moving up from the soil into the masonry.
Most modern homes have a damp-proof course (DPC), which is a horizontal barrier designed to prevent this movement of moisture. Rising damp is more likely if the DPC is missing, damaged, or bridged.
Common Causes of Rising Damp
- Absence of a damp-proof course (common in properties built before the late 19th century)
- A damaged or deteriorated damp-proof course
- External ground levels raised above the damp-proof course
- Debris in cavity walls allowing moisture to bypass the barrier
Typical Signs of Rising Damp
- Damp patches on walls up to around one metre from the floor
- Tide marks or staining on plaster
- Crumbling or powdery plaster
- Rotting skirting boards
- White, powdery salts on wall surfaces
Simple Example
If soil, paving, or flower beds outside are higher than the damp-proof course, moisture can bypass the barrier and soak into the internal wall at ground level.
Penetrating Damp
What Is Penetrating Damp?
Penetrating damp occurs when water enters the building through external walls, roofs, or openings and then travels through to internal walls. Unlike rising damp, it can appear at any height on a wall.
This type of damp is often linked to defects in the building fabric.
Common Causes of Penetrating Damp
- Cracked or damaged brickwork
- Faulty or blocked guttering and downpipes
- Leaking roofs or flashing
- Broken or missing roof tiles
- Defective window or door seals
- Porous masonry absorbing rainwater
Typical Signs of Penetrating Damp
- Localised damp patches that worsen after rainfall
- Peeling paint or bubbling plaster
- Dark patches on walls or ceilings
- Damp around windows or external walls
- Mould growth in isolated areas
Simple Example
A leaking gutter allows rainwater to run down an external wall. Over time, the wall becomes saturated, and moisture appears on the internal surface.
Condensation
What Is Condensation?
Condensation occurs when warm, moist air comes into contact with a colder surface, such as an internal wall. The moisture in the air cools and turns into water droplets.
This is the most common cause of damp in UK homes and is often mistaken for rising or penetrating damp.
Common Causes of Condensation
- Poor ventilation
- High levels of indoor humidity
- Inadequate heating
- Cold internal wall surfaces
- Drying clothes indoors without ventilation
Typical Signs of Condensation
- Water droplets on walls or windows
- Black mould growth, especially in corners or behind furniture
- Damp patches that appear and disappear
- Musty smells
- Mould on ceilings and window reveals
Rooms Most Affected
- Bathrooms
- Kitchens
- Bedrooms
- Utility rooms
Simple Example
Cooking without using an extractor fan releases moisture into the air. If the walls are cold, this moisture condenses on them, leading to damp and mould.
Other Causes of Damp on Internal Walls
Leaking Internal Plumbing
Leaks from pipes, radiators, or appliances can cause damp patches on internal walls. These are often hidden and may worsen gradually over time.
Signs include:
- Localised damp areas
- Staining or blistering paint
- Persistent damp even in dry weather
- Increased water bills
Poor Insulation
Walls that are poorly insulated are colder, making them more prone to condensation. Solid walls, common in older properties, are particularly susceptible if not properly insulated.
Bridged Damp-Proof Course
Even if a damp-proof course is present, it can be rendered ineffective if:
- External render covers it
- Internal plaster extends below it
- External ground levels are too high
This allows moisture to bypass the barrier and affect internal walls.
Structural Defects
Issues such as cracked foundations, damaged render, or defective cavity wall insulation can also contribute to damp problems indoors.
How to Identify the Cause of Damp
Identifying the correct cause is essential, as treatments vary depending on the type of damp.
Questions to Ask
- Does the damp appear after rainfall?
- Is it concentrated near the floor or higher up?
- Is there mould growth?
- Is the room poorly ventilated?
- Are there any visible leaks or defects?
Why Misdiagnosis Is Common
Condensation is often mistaken for rising damp, leading to unnecessary work. Similarly, treating the symptoms rather than the cause can allow the problem to persist.
Why Damp on Internal Walls Should Not Be Ignored
Leaving damp untreated can lead to several problems:
- Damage to plaster and decoration
- Growth of mould and mildew
- Unpleasant odours
- Timber decay
- Potential health issues, particularly for people with respiratory conditions
Early identification and appropriate action can prevent more serious damage.
Frequently Asked Questions
Is damp always a sign of a serious structural problem?
No. Many cases of damp, especially condensation, are related to ventilation and lifestyle factors rather than structural defects.
Can damp appear on internal walls only?
Yes. Condensation and internal leaks often affect internal walls, even when external walls appear dry.
Why does damp keep coming back after repainting?
Painting over damp does not address the underlying cause. Moisture will continue to penetrate or condense, eventually breaking through the paint.
Is mould always caused by damp?
Mould needs moisture to grow, so it is almost always linked to damp conditions, particularly condensation.
Can central heating reduce damp?
Consistent, moderate heating can help reduce condensation by keeping surfaces warmer, but it must be combined with proper ventilation.
How long does it take for damp to dry out?
This depends on the cause and severity. Condensation issues may improve quickly once ventilation is improved, while structural damp can take months to fully dry after repairs.
Conclusion
Damp on internal walls can have several causes, including rising damp, penetrating damp, and condensation. Each type behaves differently and presents its own set of signs. Understanding these differences makes it easier to identify the source of the problem and take appropriate action.
Many damp issues are preventable with good maintenance, adequate ventilation, and early attention to warning signs. By recognising the cause rather than just the symptoms, homeowners can protect their property and maintain a healthier indoor environment.
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