Moisture & Condensation

Warm air can hold more water vapour than cold air - roughly, its capacity doubles for every 10-11 degC of warming. Relative humidity (RH) is how full the air is relative to that capacity: 50% RH means the air holds half the vapour it could at that temperature. The dew point is the temperature you would have to cool that air to for it to reach 100% RH - saturated - so any further cooling forces water out as liquid. Air at 27 degC and 70% RH (a typical humid Indian afternoon) has a dew point around 21 degC: touch it to any surface below 21 degC and water beads out.

This is the whole mechanism, and it is why a cold drink 'sweats', why a mirror fogs after a shower, and why the cold-water pipe in a humid plant room drips. The relationships between temperature, humidity and dew point are captured on the psychrometric chart, the moisture equivalent of the tools in Module 1 - and hygrothermal simulation tools trace exactly these quantities through a building. For design, hold one sentence: condensation happens wherever a surface is colder than the dew point of the air touching it. Everything else in this lesson is where, in a building, that happens.

The first place cold-meets-moist is on visible interior surfaces. Surface condensation forms when a room's inner face - a window pane, a cold wall corner, the chilly spot at a thermal bridge (lesson 3.2) - falls below the room air's dew point. Single-glazed windows streaming in winter are the obvious case; less obvious is the corner behind a wardrobe, where still air, a cold external corner and a thermal bridge combine to drop the surface temperature just far enough.

You rarely need liquid water to get the real damage. Mould germinates at around 80% RH at the surface - well before visible droplets - so a persistently cold, humid surface grows mould even when it never actually drips. That is a health issue (spores, allergens, asthma triggers) as much as a finish issue. The cure is physics, not paint: raise the surface temperature (better glazing, kill the thermal bridge, add insulation so the inner face stays warm), lower the indoor humidity (ventilate moisture at source - kitchens, bathrooms), and keep air moving so no cold, still pocket forms. Anti-mould paint treats the symptom; a warm, dry surface removes the cause.

A quick way to feel the numbers: warmer air holds more moisture, so as a surface cools, the local relative humidity right at that surface climbs even though the room's average RH has not changed. A room at 24 degC and 55% RH is comfortable, but a window pane at 12 degC forces the air touching it up toward saturation - so mould can bloom on the cold reveal while the room feels perfectly dry. This is why the fix is almost always to lift the surface temperature (glazing, insulation, killing the bridge) rather than to chase the room humidity down alone. The coldest surface in the room is where the trouble starts, every time - which conveniently is also where a simulation or an infrared camera will point you first.

More insidious is interstitial condensation - water forming inside the wall build-up, out of sight, where you find it only once timber has rotted, steel has rusted or insulation has slumped and lost its value. The mechanism combines two flows. Heat conducts from warm side to cold side, so the temperature falls across the wall (lesson 3.1). Simultaneously, water vapour diffuses from the warm, humid side toward the cold, dry side, pushed by the difference in vapour pressure. Plot the actual temperature through the wall and the dew-point temperature set by the vapour, and wherever the real temperature dips below the dew-point line, vapour condenses inside the material.

The figure shows exactly this: two lines crossing inside the outer, colder layers, with a shaded risk zone. This is why the position of the insulation matters as much as its amount. Insulation on the cold (outer) side keeps the whole structure behind it warm - above dew point - and dry; insulation on the warm (inner) side leaves the masonry behind it cold, pushing the dew-point crossing into the structure. The classic protections are to keep the structure warm and to control vapour: a vapour control layer (a retarder or barrier) on the warm side slows vapour before it reaches the cold zone, while keeping the cold side relatively open so any moisture that does get in can escape - 'tight inside, open outside' for a cold climate. Get the layering wrong and you build a wall that farms water in its own core.

Almost every vapour-control rule in the textbooks was written for cold climates, where the inside is warm and humid and the outside is cold and dry - so vapour drives outward and the barrier belongs on the inside (warm) face. Copy that detail into warm-humid, air-conditioned India and you can cause the very problem you meant to prevent. In Chennai, Mumbai or Kochi in summer the outside is hot and very humid while the air-conditioned inside is cooler and drier - so the vapour drive reverses, pushing moisture from outside inward toward the cool interior. Now the cold surface is the inner face, and a vapour barrier placed on the inside (the cold-climate habit) sits exactly where vapour condenses against it, trapping water in the wall.

The figure contrasts the two drives. The warm-humid consequences are real and common: condensation on the back of internal finishes, damp behind vinyl wallpaper or foil-faced insulation applied to the inside, and mould blooming where a chilled interior meets humid-driven vapour. Sound warm-humid practice is different in kind: keep humid outdoor air out of the assembly, put any low-permeability layer toward the outside if used at all, favour assemblies that can dry inward toward the conditioned space, and design for the surfaces that will actually run cold - chilled ductwork, AC-cooled walls. This is the sharpest example in the whole module of why a rule of thumb is dangerous and a hygrothermal simulation against the local climate earns its place: the same wall detail that keeps a wall dry in Shimla can wet it in Chennai.

Two levels of analysis serve most design. The classic hand method is the Glaser (dew-point) method: plot temperature and dew-point (or vapour-pressure) profiles through the assembly under steady winter conditions and check whether they cross - a quick, code-adjacent screen for interstitial risk, and a good way to compare two build-ups. Its limits matter: it is steady-state, ignores a material's ability to store and redistribute moisture, ignores rain and air leakage, and was built for cold climates - so it is a screening tool, not a verdict, and it is weakest exactly in the warm-humid, reversing-drive case.

For anything serious, transient hygrothermal simulation (tools such as WUFI) steps heat and moisture through the wall hour by hour against a real weather file, accounting for storage, drying and driving rain - the moisture counterpart to the dynamic thermal modelling of lesson 3.3. As always in this course, the model is decision-support: it tells you which detail dries and which stays wet, not a guaranteed outcome, and real failures also come from workmanship, air leakage carrying far more moisture than diffusion, and rain the model never saw. The durable design habits are simple and physics-led: keep interior surfaces warm and above dew point, keep indoor humidity down with source ventilation, get the insulation and any vapour layer on the correct side for your climate, and let assemblies dry in the direction their climate allows. Do that and moisture stops being the failure that outlives every energy saving you made.