Damp in walls: telling the origin apart before you repair

A damp stain on a wall looks like any other damp stain on a wall. That resemblance is the whole problem: there are five distinct origins, they are repaired in five distinct ways, and the repair that is right for one is money wasted on the other four.

The default repair — scrape back, apply a waterproofing product, skim and paint — works in one of the five cases. In two of them it does nothing. And in the other two it makes matters worse, because it seals the wall and shuts the water inside.

What follows is how they are told apart, which is the only thing you need to know before hiring anybody.

The five origins

It is worth looking at the drawing twice, because the whole argument is in it: all five look much alike from inside the room, and they are five completely different routes. One rises, one comes in, two form on the spot without anything having entered, and one escapes from a pipe.

Rising damp: the one that climbs

Water in the ground rises through the pores of the material, against gravity, until the height it reaches balances what evaporates off the surface. That is why it always stops at a similar height along the whole wall, and why the upper edge is so characteristic: it is a line, not a gradient.

The signature that gives it away is not the water, it is the salt. Ground water carries dissolved salts; as it evaporates at the surface, the salt stays behind and crystallises. That produces the white powder, the skim coat that bulges and comes away in plates, and the paint that lifts along the same band however many times it is repainted.

The real repair means cutting off the rise — a horizontal barrier at the base of the wall — or draining and ventilating the foot of the wall, or both. It is building work, not a product. And before that you have to settle where the ground water is coming from, which is very often a garden watered against the façade, a pavement with no fall, or a broken downpipe discharging at the foot of the wall.

Penetrating damp: the one that comes in

It is the only one of the five that correlates with rain, and that correlation is the diagnosis. It appears or intensifies when it rains, and it dries when the rain stops.

Two qualifications that throw people off. The first: the rain that gets in is not the heavy rain, it is the rain that comes with wind, because the wind drives the water against the façade and pushes it through defects that water falling vertically would never find. A façade can hold for years and then let water in with the first slanting downpour.

The second: the point of entry is almost never above the stain. The water comes in through a crack, travels inside the wall or the cavity, and comes out wherever it finds a way. The points it comes in by are almost always the same: junctions of window and wall, sills with no drip groove or falling the wrong way, cracks in the façade, movement joints, and the fixings of balustrades and canopies.

If the water is coming from above rather than from the façade, the problem is a different one and is dealt with separately in water getting in through the roof.

Surface condensation: the one that forms on the spot

This is the one most often repaired wrongly, because no water is coming in from anywhere. The water was already in the air of the room.

Warm air holds more vapour than cold air. When that air touches a surface cold enough, the vapour that no longer fits turns liquid on it. It is what happens to a glass of cold water on a hot day, and it is exactly what happens to the inner face of a façade wall on a cold night.

All its signs follow from that. It appears on the coldest surfaces — outside corners, the junction of wall and slab, the wall behind the wardrobe — and it appears at night. It spreads evenly, with no centre. And it almost always brings black mould, which is its surest signature: mould needs sustained surface moisture, not liquid water running.

Three things produce it, and not one of them is fixed with a product:

1. 1How much vapour is produced indoors. Cooking, showering, drying clothes inside, an open tank or an aquarium, and above all how many people live there. A person at rest gives off vapour continuously; a family in a closed flat, a great deal.
2. 2How much ventilation there is. Vapour only leaves if the air is changed. The sealed window, the bathroom with no extractor fan and the entirely reasonable habit of shutting everything up for security or against the cold are the commonest cause of all.
3. 3How cold the surface is. A thermal bridge — a concrete beam or column crossing the envelope, a lintel, the junction of slab and façade — is always colder than the wall around it, and condenses first. That is why mould tends to draw the structure of the building.

What does work acts on all three causes: ventilate — open the windows properly, an extractor fan in bathroom and kitchen, no drying clothes indoors —, warm or insulate the cold surface so that it stops being the point where the vapour condenses, and pull the furniture off the wall so the air can move. The first of those is free and settles a good share of the cases.

Interstitial condensation: the one you cannot see

It is the same physics, with one difference that makes it dangerous: it happens inside the thickness of the wall, not on its face. The vapour crosses the material, meets a plane cold enough inside the section, and condenses there.

At first there is no stain at all. The water builds up where nobody sees it, and does three things: it wets the insulation, which once wet stops insulating and chills the section further, so it condenses more still; it wets any metal components inside; and it degrades the material from within. By the time something finally shows, it has been at work a long while.

Suspicion is almost always raised by elimination: damp that is wide and diffuse, with no correlation to rain, no rising damp band, no leak, in a wall with insulation or a cavity. And it is confirmed by opening up, which is the uncomfortable part: there is no non-destructive way to see inside a wall with certainty.

The consequence has to be faced: in Colombia, condensation inside the wall is not regulated. There is no limit value to demand and no requirement to invoke. Whatever is done about it is the professional judgement of whoever designs, and if it matters to you, you have to ask for it in the contract — because nobody is obliged to deal with it.

Plumbing leak: the one that does not depend on the weather

It is the easiest to rule out and the one ruled out too late most often. The sign is utterly simple: it is damp all year round, rain or no rain, cold or warm. It has no season.

It may be in the drinking water supply, which is under pressure and usually gives a stain that grows steadily, or in the waste pipework, which is not under pressure and shows up intermittently, tied to use of the fitting above. A persistent smell tips the balance a long way towards the second.

The tests that decide

Everything above can be settled, in the great majority of cases, without instruments. These are the tests in order of cost, and the first one closes the most arguments.

1. 1The sheet test. Tape a sheet of clear plastic to the wall over the damp area, sealing all four edges well. Wait a day or two and look at which side the water appeared on. If it condensed on the face of the plastic looking into the room, it is condensation. If the water is under the plastic, between plastic and wall, it is coming from inside the wall: rising damp, penetrating damp or a leak. It is the cleanest cut in the whole diagnosis and it costs nothing.
2. 2The calendar test. Over two weeks, note down when it gets worse: with rain, in the early hours, after a shower, always the same. Correlation with rain points to penetrating damp; with night and cold, to condensation; with nothing at all, to a leak.
3. 3The water meter test. The one with the stopcock closed. It rules out a pressurised leak in an afternoon.
4. 4The height and the edge. Measure how far up the damp reaches and look at whether the upper edge is a line or a gradient. A sharp horizontal line at a constant height is rising damp, with hardly any argument left.
5. 5Looking for the salt. Scrape the white powder gently. Efflorescence goes with water arriving from the floor or the ground, not with condensation.
6. 6Surface and room moisture meters, if the picture needs sharpening. They measure moisture in the material and in the air, and they let you compare the same wall at several points and at different hours. They help to narrow things down; on their own they do not give the origin.
7. 7Thermography, for large buildings or when thermal bridges or the run of a leak have to be located. It sees differences of temperature, not water, and it has to be read by somebody who knows how.

When damp stops being a matter of finishes

Sustained damp does not stay in the paint. In reinforced concrete it eventually reaches the reinforcement, and steel as it corrodes takes up more volume than it had, so it bursts the concrete covering it from within. That damage has a recognisable signature: a long straight crack, parallel to the bar, often with a rust stain. It is described, along with the other four, in cracks in walls.

That is the point at which the conversation stops being about stains. If there is spalling as well, reinforcement in view, deformation or chronic damp affecting several flats, what is needed is no longer a builder but a formal diagnosis: when a building needs a pathology study deals with exactly that boundary.

Who is answerable

Two fronts, and it pays not to mix them.

If the building is new, the argument is with the builder, and it has a wrinkle almost nobody knows about: none of the three sources of warranty — the Civil Code, the consumer statute and the safe housing statute — contains the words humedad or filtración. Damp and water ingress are simply not named in them. The Civil Code speaks of the building perishing or threatening ruin; the consumer statute separates the stability of the works, at ten years, from the finishes, at one. The full table, with what can and cannot be argued, is in water getting in through the roof.

If it is propiedad horizontal — the Colombian regime under which a building is split into privately owned units and property held in common — the first thing is to know whose wall it is. Article 3 of Ley 675 de 2001 makes essential common property, among other things, "the façades and the roofs or slabs serving as covering at any level". A façade wall that condenses or lets water in belongs to the community of owners, even though the stain is inside your flat. An internal partition is yours.

What the general meeting can authorise and what it cannot, and by what majority, is in altering a flat in propiedad horizontal.

What not to do

• —Painting over it. It stops nothing, erases the evidence of how far it reached and makes it impossible to know whether it grew.
• —Waterproofing a wall with rising damp from the inside. The water goes on climbing and comes out higher up, or bursts the render from behind.
• —Waterproofing a wall that is condensing. The water is not coming from the wall. It is the most sold repair on this list and the most useless.
• —Using anti-damp paint as a diagnosis. It is a finish, not a remedy. It can be reasonable once the cause is settled; never instead of settling it.
• —Sealing the windows and the ventilation grilles to keep the cold out. It is the fastest way to turn a house into a condensation chamber.
• —Repairing the first origin you find and closing the case. Two of them at once is very common.
• —Closing the wall up without drying it. Wet insulation stays wet, and a wall closed under false pretences shows itself again months later, by then with damage.

In short

Before hiring anybody, settle three questions. Does it get worse with rain, at night, or with nothing at all? Is the top edge of the stain a sharp line or a gradient? And which side of the plastic does the water appear on? Those three answers separate most cases.

With rain and at the height of a defect, it is penetrating damp. A low band with a sharp edge and salts, rising damp. At night, even, in corners and with black mould, condensation: ventilation and temperature, not a waterproofing product. With no relation to anything and damp all year, look for a leak and start at the meter.

And if you cannot explain the path of the water from start to finish, do not repair yet. Every blind repair erases evidence and makes the next diagnosis more expensive.