Cold-spot condensation
A Socratic walk-through of cold-spot condensation — reasoned out one step at a time, not lectured.
The question we started with
THE QUESTION #Why does mould appear on one cold corner of a room that is heated throughout?
A room is heated evenly and used normally. Black mould appears in one corner — usually behind furniture, often at the junction of two outside walls, or where a wall meets the ceiling. The rest of the room, exposed to exactly the same air, stays clean.
The householder is told they have a damp problem, and often that they should ventilate more or heat more. But the air in that corner is the same air as everywhere else, carrying the same amount of water vapour. Whatever is different about the corner is not the air. It is worth finding out what, because the diagnosis determines whether any of the usual advice will work.
Reasoning it through
REASONING #Start with what mould needs. Not liquid water pooling, and not a leak — black mould on a wall surface grows on a damp surface, and it will establish itself at surface humidities well below saturation given time. So the question becomes: why is one patch of wall wetter than the rest?
Air holds water vapour, and the maximum it can hold rises steeply with temperature. Warm air in a heated room can carry a good deal; cool that same air and its capacity falls, until at some temperature it is holding all it can. That temperature is the dew point of the air, and it is a property of the air's moisture content, not of any surface.
Now the crucial step. The air in the room is at one temperature and one dew point throughout, near enough. But the surfaces bounding the room are not all at the same temperature. A wall loses heat outward, so its inner face sits below room temperature — a little below in a well-insulated wall, considerably below where insulation is missing or bypassed. And when air touches a surface colder than its dew point, water condenses out onto that surface. Not everywhere: exactly there.
So the mechanism is a threshold comparison, and it has two terms. Raise the moisture in the air and the dew point rises toward the surface temperature. Lower the surface temperature and it falls toward the dew point. Mould appears wherever those two cross, which is at the coldest surface in the room — and only there, which is why the patch is sharply bounded.
Now ask why corners in particular. Two effects combine. Geometrically, an external corner has more outside surface collecting cold than inside surface being warmed by the room, so it runs colder than the flat wall either side. Structurally, corners, lintels, floor edges and window reveals are where insulation is most often interrupted by something conductive — a concrete slab edge, a steel lintel, a mortar bridge — creating a path that shortcuts the insulation. Either produces a locally cold patch on the inside.
And that explains the furniture. A wardrobe against an outside wall blocks the room's warm air from washing over that surface, so the surface behind it runs colder still, while the air trapped behind carries the room's moisture. It is the coldest place in the room and the least ventilated at once.
Notice what has not been invoked: rising damp, a leak, or a failure of the wall to breathe. Nothing wet is coming through the wall. The water was in the room's air, and the wall merely provided the cold surface for it to leave the air onto.
The analogy
THE ANALOGY #Think of a cold drink on a summer table. The glass beads with water within a minute. Nobody concludes the glass is leaking, and nobody suggests the drink is seeping through — the water plainly came out of the room.
Now put two glasses on the table, one straight from the fridge and one at room temperature. Only one beads. Same air, same humidity, same table. The difference is entirely in the surface temperature, and the wet one is not defective; it is simply the only one below the dew point.
The glass warms up and the beads evaporate within the hour, whereas a thermally bridged wall stays cold all winter, so the wetting is continuous rather than transient — which is what turns a physical curiosity into a biological problem, since mould needs persistence rather than a single wetting.
Clarifying the model
THE MODEL #"Ventilate more" and "heat more" both work, but on different terms, and neither is guaranteed. Ventilation removes moisture-laden air and lowers the dew point, attacking one side of the comparison. Heating raises surface temperatures somewhat, attacking the other. But heating also raises the air's capacity to hold moisture without removing any, so in a house where moisture is being generated faster than it is removed, heating alone can leave the dew point rising alongside the surfaces. And if the cold spot is a genuine thermal bridge, raising room temperature moves that surface far less than it moves the rest of the room — the bridge is, by construction, coupled to outside. That is why the corner is often the last thing to respond to more heating.
This is condensation damp, and distinguishing it matters. Penetrating damp tracks rain and a defect — cracked render, blocked gutter, failed seal. Rising damp shows a tide line at low level. Condensation appears on the coldest surfaces, is worst in winter and in rooms generating moisture, and appears high and in corners as often as low. Treating one as another is the standard expensive mistake — a chemical damp-proof course does nothing for a cold corner.
The tenant-blame framing is worth naming, because the physics does not support it. Condensation is frequently attributed entirely to occupant behaviour: drying washing indoors, not opening windows, blocking vents. Those genuinely raise the moisture term, and they are real contributors. But the surface-temperature term is a property of the building, and a wall with an uninsulated bridge will condense at moisture levels that ordinary living produces unavoidably. Both parties to that argument tend to state their side as the whole cause. The honest position is that it is a threshold crossing with two terms, and either can be the one that is out of the ordinary in a given case.
The falsification test. If the mechanism is surface temperature falling below the air's dew point, then measuring the surface temperature at the mould patch and the room's dew point should show the first below the second — and the patch should map onto a cold area visible in a thermal image, with no correlation to rainfall. If the affected patch were no colder than the clean wall beside it, condensation would be the wrong diagnosis and something bringing water from elsewhere would have to be found.
A picture of it
THE PICTURE #How to readThe bars are inside surface temperatures at points around one room; the flat line is the dew point of the room's air, which is the same everywhere because it is a property of the air rather than of any surface. Read the crossing: wherever a bar falls below the line, that surface is wetting. The values illustrate the argument rather than measuring a particular room. Note that raising the heating lifts the left-hand bars readily and the right-hand ones barely, because those are coupled to outside — and that the line rises too if moisture is added.
What became clearer
WHAT CLEARED #Nothing is wrong with the air in that corner, and usually nothing is coming through the wall. The corner is simply the coldest surface in the room, and the coldest surface is where the room's own moisture leaves the air first. Mould is the visible record of a threshold being crossed in one place and not others — which is why the patch has a sharp edge, why it sits behind the wardrobe, and why the fix has to move one of the two terms rather than treat the wall as diseased.
Where to go next
ONWARD #- How thermal bridging is designed out at slab edges and window reveals, and why older buildings are full of it.
- Why mould can establish at surface humidities well below full condensation.
- How penetrating and rising damp are told apart from this, and why the remedies do not transfer.