Cape Town Has No Central Heating, and That Changes Everything
Walk into a Cape Town house in July with black mould in the corners of a bedroom, damp patches behind the wardrobe and streaming windows, and the instinct is that water is getting in.
Usually it is not. The water was already inside.
A family of four puts a substantial amount of moisture into the air every day, through breathing, cooking, showering, washing and drying laundry indoors. In a climate with central heating, that air stays warm enough to hold the moisture and ventilation carries it out. In a Cape Town winter, the house is heated for a few hours with a portable heater, windows stay shut against the rain and wind, and the air cools overnight against uninsulated external walls.
Warm moist air meeting a cold wall gives up its moisture onto that wall. That is condensation, and it is the most common form of damp in this city by a wide margin.
It is identifiable. Black spot mould, worst in external corners, on window reveals, behind furniture against external walls, and in the least-heated rooms. Worse on cold mornings. Worse when the house has been shut up. It does not correlate with rainfall the way real penetrating damp does.
It is also the cheapest to fix, which is exactly why it is rarely the diagnosis offered.
When It Is Genuinely Penetrating Damp
Penetrating damp is water crossing the wall horizontally from outside, and it always has a specific source.
The weather-facing wall. Cape Town winter rain comes on north-westerly fronts at 50 to 65kph, so the north and west elevations take a beating that the sheltered sides never see. Penetrating damp that appears on one elevation only, during and after fronts, is a strong signal.
Cracked or blown render. Render that has lost its bond lets water in through the crack and then holds it against the masonry behind, unable to evaporate.
Failed pointing in older masonry, particularly on exposed elevations.
Defective gutters and downpipes. Water discharging down a wall for a whole winter saturates it thoroughly. The most common single cause we find, and a gutter repair rather than a damp treatment.
Sills, copings and thresholds without a proper drip detail, letting water run back onto the wall face.
Retaining walls and below-ground structure, where the damp comes laterally from saturated soil rather than from rain.
Salt damp. Where the wall has been wet long enough to concentrate salts in the plaster, those salts hold moisture from the air by themselves, and treating the ingress alone will not dry the surface.
Which Wall, and Which Season
The elevation and the timing usually settle the diagnosis between them.
North and west elevations, damp during and after rain: penetrating. Those faces take the winter fronts head on.
South-facing rooms, damp on cold mornings, black mould in corners: condensation. South elevations get least sun, stay coldest, and are where condensation shows first.
Damp that is worse in August than in February: could be either, since Cape Town rain and its cold arrive together. This is where moisture readings and pattern matter more than the calendar.
Damp in a room with a bathroom or kitchen next to it, or where laundry is dried: condensation, until proven otherwise.
