Damp in buildings: how to tell condensation, penetrating damp and rising damp apart

The three mechanisms compared

Condensation

Air holds moisture, and the warmer it is the more it holds. When that air meets a surface at or below its dew point, the moisture it can no longer carry deposits on the surface as liquid water. That is surface condensation, and it is the most common cause of damp in occupied buildings.

Why it appears where it appears

Condensation collects on the coldest surfaces. In most buildings those are:

• Thermal bridges, where heat escapes faster than through the surrounding fabric: window reveals, lintels, the junction between wall and floor slab, and the corners where two external walls meet
• Behind furniture placed against an external wall, where air cannot circulate and the surface runs colder
• Unheated rooms in an otherwise warm house, particularly bedrooms with the door closed and the radiator off
• Single glazing and thin window frames, which are usually the coldest surface in the room

Where the moisture comes from

Every occupied building generates water vapour continuously. The main sources, roughly in order of how much they contribute:

• Drying laundry indoors, especially on radiators or an airer
• Unvented tumble dryers, which discharge that water directly into the room unless they are condensing or ducted outside
• Cooking, particularly boiling and steaming without a lid or extract running
• Showering and bathing
• The occupants themselves, through respiration and perspiration, day and night
• Flueless bottled gas or paraffin heaters, which produce roughly as much water vapour as fuel burned. These are a frequent hidden cause

Why it got worse after you improved the building

This surprises people, and it is one of the most common patterns we are called to. Replacing draughty windows or sealing gaps reduces uncontrolled ventilation, so moisture that used to leak away now stays in the building. Meanwhile the new glazing is warmer than the old, so the glass stops being the coldest surface in the room. The condensation does not stop; it moves to the next coldest surface, which is usually a wall or a corner, where it is far more damaging than water on glass.

Interstitial condensation

Interstitial condensation forms inside the construction rather than on its surface. It matters because it is invisible. By the time it shows, decay is usually well advanced.

The most common way to create it is to insulate a solid wall on the inside without addressing vapour control and ventilation. Internal insulation keeps the room warmer, which is the point, but it also leaves the masonry behind it colder than it was before. The dew point moves out of the plaster and into the wall build-up. Moisture that previously condensed visibly on the wall face now condenses within the construction, where nobody can see it and nothing dries it.

This is not an argument against insulation. Externally applied insulation generally moves the dew point outward, away from the structure, and improves matters. The problem is not insulating; it is insulating on the wrong side, or insulating without providing the ventilation the building now needs.

Penetrating damp

Penetrating damp is water entering the building through the envelope. Unlike condensation, it has an external source, and that source can almost always be found by looking at the outside face of the same wall.

Usual causes

• Blocked, leaking or overflowing gutters and downpipes. The most common cause and the cheapest to fix. Look for staining running vertically down the wall
• Defective pointing in the mortar joints, allowing wind-driven rain into the masonry
• Cracked or blown render, which is worse than no render at all: it lets water in and then holds it there
• Failed sealant or flashing around windows, at roof abutments, and around penetrations such as extract vents and pipes
• High external ground levels piled above the damp proof course, so water bypasses it entirely. Frequently caused by a new patio, path or raised planter
• Cavity bridging, where mortar droppings or debris in the cavity form a path for water to cross to the inner leaf
• Defective roof coverings, slipped tiles, or failed valley and parapet details

How to confirm it

Penetrating damp tracks the weather. If the patch darkens during rain and dries back afterwards, or if it only appears when rain is driven from one particular direction, that is strong evidence. Photograph the outside of the wall while it is actually raining. The source is usually visible when it is happening and invisible an hour later.

Rising damp, and why it is over-diagnosed

Rising damp is ground moisture drawn upward through porous masonry by capillary action, in the same way liquid climbs a paper towel. For it to occur, three things must be true at once: the masonry must be porous, there must be a source of ground moisture, and the damp proof course must be absent, failed or bridged.

Height is self-limiting. Capillary rise is balanced against evaporation from the wall face, which is why rising damp rarely exceeds about one metre and why it produces a horizontal tide mark rather than a diffuse spread.

The salt problem, and why meters mislead

Ground water carries dissolved salts, chiefly nitrates and chlorides. As moisture evaporates from the wall face the water leaves but the salts stay, accumulating in the plaster. Those salts are hygroscopic: they absorb moisture from the air. This has two consequences that matter enormously for diagnosis.

• The wall can continue to read and feel damp long after the original source has been dealt with, because the salts are now drawing moisture from the room air
• Electrical moisture meters of the type commonly used in damp inspections measure electrical conductivity, not moisture. Salt contamination raises conductivity and produces a high reading on a wall that is not wet. A high meter reading is not proof of rising damp

Genuine rising damp does exist and does need addressing. But before accepting the diagnosis, establish whether a damp proof course is present, whether external ground levels or internal plaster bridge it, and whether the pattern and seasonality actually fit.

Coastal and Mediterranean buildings

Most published damp guidance assumes a northern European climate. In Gibraltar and comparable coastal Mediterranean locations the weighting is different, and applying UK assumptions produces wrong answers.

• High ambient humidity year round means the margin between air temperature and dew point is often small. Surfaces do not need to be very cold to reach it
• Mild, wet winters shift the condensation season and make it less obvious than a hard frost would
• Airborne marine salt deposits on and into the fabric. Because those salts are hygroscopic, buildings near the sea can show persistent damp readings and staining with no liquid water source at all
• Summer condensation from air conditioning. This is the reverse of the northern European pattern and is frequently missed. Cooling a room in humid conditions creates surfaces below the dew point of the surrounding air, so condensation forms on cooled surfaces and inside constructions during the hottest months
• Chloride attack on reinforced concrete. Salt penetrating concrete causes the embedded steel to corrode and expand, breaking the concrete away

Five common misdiagnoses

1. An injected damp proof course specified for condensation. The most frequent error. A chemical DPC addresses capillary rise. It has no effect whatsoever on moisture condensing out of room air, so the problem returns, usually within a season.
2. A high moisture meter reading treated as proof of rising damp. Conductivity meters respond to salts as well as water. On a salt-contaminated wall they read high when the wall is dry.
3. Tanking applied to a wall with penetrating damp. This conceals the ingress rather than stopping it. Water continues to enter the masonry and moves elsewhere, often appearing above the tanked area or in the adjoining room.
4. Replastering without removing the cause. New plaster on a wall that is still being wetted, or still full of hygroscopic salts, fails again. The replaster is often the most expensive part of the job and the first to be re-done.
5. Anti-mould paint used as the remedy. It suppresses the visible growth for a while. It does nothing about the surface temperature or the moisture load that caused it, and the mould returns once the coating is exhausted.

What to try first, at no cost

If the pattern points to condensation, and in most cases it will, these cost nothing and resolve a meaningful proportion of problems. Try them for a full month before spending money on anything.

1. Stop drying laundry indoors, or confine it to one room with the window open and the door closed. This alone fixes some cases outright
2. Check your extract fans actually move air. Hold a sheet of tissue to the grille. If it does not hold, the fan is not working, whatever noise it makes. Run extracts during and for a period after showering and cooking
3. Open trickle vents and stop blocking airbricks. They were fitted for a reason
4. Move furniture a few centimetres off external walls so air can circulate behind it
5. Heat low and continuous rather than in short bursts. Intermittent heating produces cold surfaces at exactly the times moisture peaks
6. Do not leave individual rooms unheated with the door shut. The classic mould location is a cold spare bedroom in an otherwise warm house
7. Put lids on pans, and avoid flueless bottled gas or paraffin heaters entirely

Keep a note of what you changed and when. If the problem improves, that is diagnostic in itself, and it is useful evidence if you do end up needing a survey.

When you need a survey rather than a guide

Get a building survey if any of the following apply:

• You have tried the free measures for a month and the problem persists
• There is structural involvement: decayed timber, blown plaster over a wide area, movement or cracking
• You are buying, and a survey has already flagged damp
• You have been quoted for damp proofing and want the diagnosis verified before committing
• The damp appeared after building work and responsibility is in question
• Concrete is spalling with steel exposed
• You need evidence that will stand up in a dispute, claim or negotiation

A survey adds what a guide and photographs cannot: moisture readings at depth as well as at the surface, thermal imaging to locate cold bridging precisely, and a professional opinion that carries liability.