Damp, Condensation and Mould in Caspian Villas: Where the Problem Starts

A few times a year we get this call: “The villa is barely three years old, but the bedroom corner has gone black.” And the owner’s first guess is almost always a leak — a burst pipe, a failed roof membrane, water rising through the slab.

Sometimes it is. But in most of the cases we have seen, nothing leaked at all. The water sitting on that wall did not come from outside. It came out of the air in that room. And until the cause changes, it will return every time you repaint.

Start with one number

The Mazandaran Meteorological Office records the following for the Babolsar synoptic station over a 66-year period (1952–2018):

Source: Mazandaran Meteorological Office — Babolsar climate

Hold on to that first figure: 80%. This entire article is about it.

What condensation is, and why it rewrites the rules here

Air holds a limited amount of water vapour, and that limit depends on temperature: warmer air holds more. When humid air meets a cold surface it cools, its capacity drops, and the surplus vapour turns into liquid water. The temperature at which this happens is the dew point.

It is what you see on a glass of iced water. The water did not seep through the glass; it condensed out of the room.

Now the numbers. Dew point can be calculated precisely, using the Magnus formula that meteorologists use:

Read the first row again. An ordinary summer day in Babolsar: 30 °C, 78% humidity. The dew point is 25.7 °C.

Which means every surface in that house below 25.7 °C is getting wet.

So: what do you set the air conditioner to? 24? 23? Then the wall behind it, the window glass, the cold-water pipe inside the wall, and the chilled stone floor are all below the dew point. All of them are collecting water.

That is not a defect. That is physics working exactly as designed. The job of architecture is to stop any surface from getting that cold.

Winter: same mechanism, opposite direction

In winter warm and cold swap places, but nothing else changes. Indoors is warm and humid — breathing, cooking, showering and drying clothes all produce vapour. Outdoors is cold. Whichever part of the envelope is colder than the rest is where water lands first:

Calculated with the Magnus formula; reference: Alduchov & Eskridge, Journal of Applied Meteorology, 1996

An ordinary winter night in Babol: 22 °C indoors, 60% humidity. Anything reaching 13.9 °C starts to sweat. Next question: which part of your wall is at 13 °C that night?

The answer: thermal bridges

Here is the root of it. Across a properly insulated wall, the internal surface temperature is fairly uniform. But wherever a conductive material crosses the insulation, it creates a shortcut for heat, and the inner surface there stays locally cold. That is a thermal bridge.

In typical construction on the Caspian coast, they sit here:

• Concrete columns and beams carried through to the inner face. Concrete conducts heat several times faster than clay block. The corner with a column in it is the coldest point of the room — mark where mould appears and it usually traces the column line exactly.
• External corners. Two faces losing heat outside, fed by one small corner inside. Corners always run several degrees colder than the middle of a wall.
• Cantilevered balconies. A slab that starts inside and runs uninterrupted to the outside is a cooling fin, in the literal engineering sense.
• Lintels and window reveals. The thinnest point of the insulation, at the busiest point for vapour.
• The wall-to-foundation junction. Where most low-level staining begins.

Part 19 of the Iranian National Building Regulations devotes a chapter to exactly this: classifying thermal bridges and the methods for eliminating or reducing them. It is not an optional refinement; it is national code.

Why this is more than a stain

In 2009 the World Health Organization published its first guidelines on indoor dampness and mould, the product of a two-year review by 36 experts. The headline finding:

Occupants of damp or mouldy buildings are at up to 75% greater risk of respiratory symptoms and asthma.

The same report estimates that 10–50% of indoor environments in Europe, North America, Australia, India and Japan are affected by dampness. This is neither rare nor marginal.

Source: WHO Guidelines for Indoor Air Quality: Dampness and Mould (2009)

And its central recommendation: remove the persistent dampness itself rather than killing the mould that grew from it. Mould is the symptom.

What to do in design

1. Treat insulation as continuous, not as patches

Insulation covering 95% of the envelope but stopping at the columns turns that remaining 5% into the collection point for every problem. It must run unbroken around the whole building — over columns, around balcony slabs, behind lintels. Draw the insulation line on a wall section: if you have to lift the pencil, that is where the trouble is.

2. Capture vapour where it is produced

Kitchens and bathrooms generate most of a home’s vapour. An extract fan that genuinely discharges outdoors — not into a suspended ceiling — removes it before it spreads. This is the cheapest and most effective item on this list.

3. Plan so air can move

At 80% humidity, natural ventilation is not a luxury; it is a requirement. Openings on two opposite faces, drawing air through the house, pull moisture out of the building itself. That decision is made at plan stage and is close to irreversible afterwards.

4. Take the building’s separation from the ground seriously

Babolsar receives 896 mm of rain a year and the water table along the coast is high. Rising damp from the foundation is one of the most common sources of low-level staining, and one of the hardest to fix after construction.

5. Choose facade materials by water absorption

A facade that stays clean for years in a dry climate can stain within two winters in Sarkhorud. Here the selection criterion is absorption rate and wet–dry cycling behaviour, not catalogue appearance.

A note on sequence

Almost everything above is a design-stage decision. The insulation path, the openings, the envelope build-up and floor levels all become expensive to change once the frame is up. That is why, in our villa design and construction process, these decisions come before final material selection.

And if you have not bought land yet, part of this is solved there: ground level relative to the road, distance from water, prevailing wind. That is what the pre-purchase land checklist covers.

The fundamental difference between this climate and Tehran’s comes from exactly the same place; we put the numbers side by side in villa design north vs Tehran.

In short

If one sentence survives: on the Caspian coast, mould is not a cleaning problem. It is a surface-temperature problem.

As long as a corner of your house reaches 13 °C on a winter night, that corner will collect water — whatever anti-mould paint you apply. The fix is to raise that surface temperature: continuous insulation, no thermal bridges, and ventilation that removes vapour before it settles.

None of this is expensive. It simply has to be decided at the right time.

If you are designing a villa in Babol, Babolsar or Sarkhorud and want these decisions made correctly from the start, get in touch.