How do I know if I have rising damp or condensation?

Rising damp typically shows as tide marks or white salts (efflorescence) low on a ground floor external wall, usually below 1 metre. Condensation appears as black mould on cold surfaces - corners, window reveals and north-facing walls - and is worst in winter and in rooms with poor ventilation.

Can I treat damp myself?

Condensation is often DIY-fixable through improved ventilation, heating and insulation. Penetrating damp from a specific source like a leaking gutter is DIY-able if you can access it safely. Rising damp and interstitial damp almost always need a qualified specialist.

How much does it cost to fix damp in the UK?

Condensation fixes range from £50 DIY (draught-proofing, extractor fan) to £800 for a whole-house ventilation system. Rising damp treatment typically costs £800-£3,000 including DPC injection and replastering. Penetrating damp repairs range from £300 to £2,000 depending on the source. All figures include VAT.

Is black mould dangerous?

Black mould (Cladosporium, Aspergillus or Stachybotrys) can cause or worsen respiratory problems, allergies and asthma - particularly in children, the elderly and those with existing conditions. Under the Homes (Fitness for Human Habitation) Act 2018, landlords are legally required to address serious damp and mould. Do not ignore it.

What is the white powder on my walls?

White powder or crystals on masonry walls is called efflorescence - mineral salts drawn to the surface by moisture moving through the wall. It is a reliable indicator of water ingress, most commonly associated with rising damp or penetrating damp rather than condensation.

What is interstitial damp?

Interstitial damp (also called interstitial condensation) occurs when warm moist air migrates into a wall, roof or floor structure and condenses inside the construction - not on a visible surface. It is common in modern insulated buildings and can cause hidden timber rot and mould inside walls. Diagnosis usually requires a specialist survey or hygrothermal modelling.

How severe is my damp problem?

Our diagnosis tool rates severity as mild, moderate or serious based on the number of symptoms you report, whether they are constant or weather-triggered, the presence of structural symptoms like crumbling plaster, the room affected, and the age of your property. A serious rating means you should seek a professional survey without delay.

Damp & Mould Diagnosis Tool UK - Find the Cause and Fix It

Is it condensation, rising damp, penetrating damp or interstitial damp?

Damp and mould in UK homes is one of the most misdiagnosed - and therefore mis-treated - problems in home maintenance. There are four distinct types, each with a completely different cause, fix and cost. Treating rising damp with anti-condensation paint, or blaming condensation when you actually have a leaking gutter, wastes money and leaves the problem to get worse.

This tool asks you about the location of the problem (which room, which floor, which wall), the symptoms you can see (black mould spots, tide marks, white salts, wet patches), the timing (worse in winter or after rain?), and basic property details (age and wall type). It then scores your answers against the diagnostic profile for condensation, rising damp, penetrating damp and interstitial damp - and tells you which is most likely, what it will cost to fix, and whether you can tackle it yourself.

✅Takes 2 minutes

📋4 types diagnosed

💰Fix costs included

🔧DIY vs specialist verdict

1Location

2Symptoms

3Timing

4Property

Where is the damp or mould?

Which room?

🛌Bedroom

🛁Bathroom

🍳Kitchen

🛋Living Room

🚪Cellar / Basement

🏠Other Room

Which floor?

🏠Ground floor

🪜Upper floor

🪟Top floor / loft

Which wall?

🌌External wallfaces outside

🏠Internal wallnot touching outside

💡Ceiling

📐Cornerwall meets wall or ceiling

Select one option from each group

What can you see? Tick all that apply

Black or grey mould spotsFuzzy patches, often in clusters. Common on silicone, grout, walls and window reveals. Tide marks or stainingHorizontal brown or yellow lines, typically low on the wall. White powder or crystalsChalky white deposits on brickwork or plaster (efflorescence). Wet or damp patchesPatches that appear wet or feel damp to the touch. Peeling paint or wallpaperPaint bubbling or wallpaper lifting away from the wall surface. Condensation on windowsWater droplets or persistent condensation on the inside of glass. Musty or earthy smellPersistent damp smell even when the room appears dry. Damp below 1 metre onlyProblem confined to the lower portion of the wall - does not go higher. Plaster crumbling or softPlaster breaks away easily or feels soft when pressed.

When is it worst?

❄Winter / cold weather

🌧During or after rain

📅All year round

❓Not sure

How is the room ventilated?

💨Extractor fanworking fan in room

🚶Windows onlyno extractor

🚫Poor / nonewindows rarely opened

Tell us a little about the property

Approximate age

Pre-1800 Georgian & earlier

1800-1919 Victorian / Edwardian

1919-1945 Inter-war

1945-1980 Post-war

1980-2000 Modern estate

2000+ New build

Wall construction

🧱Solid brickwall is one solid thickness

⬜Cavity walltwo layers with a gap

❓Not sure

Most likely cause

Estimated fix cost

Can you DIY it?

Likely causes

Recommended next steps

Know Your Enemy

The Four Types of Damp

Getting the diagnosis right matters - each type has a completely different cause, fix and cost. Treating rising damp as condensation (or vice versa) wastes money and leaves the problem unsolved.

Condensation

The most common type - accounts for ~70% of damp problems

What it is: Warm, moist air hitting cold surfaces and turning to water. The mould feeds on this moisture.

Signs: Black mould in corners, on window reveals and cold walls. Worse in winter. Condensation on windows. Often in bathrooms and bedrooms.

Fix: Improve ventilation (extractor fans, trickle vents), reduce moisture sources (dry clothes outside where possible), increase background heat. In severe cases, a whole-house mechanical ventilation system.

Cost: £50-£800 depending on approach. Often partially or fully DIY-able.

Rising Damp

Groundwater travelling up through masonry

What it is: Water from the ground being drawn up through porous bricks and mortar by capillary action. Usually where the damp proof course (DPC) has failed or was never installed.

Signs: Tide marks and white salts (efflorescence) on lower walls, typically below 1 metre. Crumbling plaster. Mainly in pre-1919 properties.

Fix: DPC injection, replastering with salt-resistant render, addressing external ground levels. Needs a specialist - DIY treatment products have a poor track record.

Cost: £800-£3,000 per affected wall. Get multiple quotes and be wary of high-pressure salespeople.

Penetrating Damp

Water entering through the building fabric

What it is: Rainwater or groundwater getting through the walls, roof or around windows and doors due to a defect in the building fabric.

Signs: Damp patches that appear or worsen during or after rain. May appear anywhere on the wall or ceiling, not just at the base. Localised rather than spread along the full wall.

Fix: Find and fix the source - repointing, replacing flashing, repairing gutters/downpipes, re-sealing windows. Once fixed, allow to dry before redecoration.

Cost: £300-£2,000 depending on the source. Gutter repairs are DIY-able; roof and flashing work usually needs a roofer.

Interstitial Damp

Moisture condensing within the wall structure

What it is: Water vapour moving through the wall from the warm interior side and condensing within the structure where it hits the dew point. Common where insulation has been added without proper consideration of vapour control.

Signs: Damp appearing inside a wall without an obvious external source. Often appears after insulation was retrofitted. Can cause hidden rot and structural damage.

Fix: Vapour control layers, breathable insulation systems or specialist assessment. Needs a surveyor or building physicist - not a DIY job.

Cost: £1,000-£5,000+ depending on extent. Often discovered during other renovation work.

UK Damp & Mould Diagnosis Tool