Heat guide
Dew Point, Humidity and Condensation
Dew point is the temperature to which air must be cooled, at approximately constant pressure and water-vapour content, for saturation to occur.
Relative humidity depends on temperature
Relative humidity compares the actual water-vapour pressure with the saturation vapour pressure at the same temperature. Because saturation pressure changes strongly with temperature, relative humidity can change even when the amount of water vapour stays the same.
Condensation begins at sufficiently cold surfaces
If humid air contacts a surface below its dew-point temperature, the air immediately next to that surface can become saturated and water can condense.
Cold bridges raise condensation risk
Thermal bridges can create colder interior surface temperatures than nearby insulated areas. This is one reason thermal-bridge control matters for moisture as well as energy.
A dew-point calculation is not a mould diagnosis
Mould growth depends on time, material, surface humidity, temperature and biological conditions. A dew-point result identifies a condensation condition, not a medical or building-health diagnosis.
Approximate formulas have limits
Many web calculators use empirical dew-point approximations that work well over common indoor ranges. Accuracy declines outside the range for which the approximation was developed.
Dew point does not by itself describe mould risk
Condensation occurs when a surface is cold enough for adjacent air to reach saturation, but biological growth depends on more than whether liquid water appears. Surface relative humidity, material sensitivity, duration, temperature and nutrient availability all matter. A surface can therefore experience elevated moisture risk without visible droplets.
Dew-point calculations are useful for identifying obvious condensation conditions, but building-moisture assessment often needs a broader view of drying potential, leakage, vapour transport and time-dependent surface humidity.
Dew point tracks moisture content more directly than relative humidity
If air is heated without adding or removing water vapour and pressure changes are small, its relative humidity falls because saturation vapour pressure rises, while dew point remains approximately unchanged. This is why a warm room can have low relative humidity even though the absolute moisture content has not changed.
Cooling that same air raises relative humidity until saturation is reached at the dew point.
A surface can condense water while room air remains unsaturated
Room air might be far below 100% relative humidity, yet a sufficiently cold window or pipe can cool the thin layer of adjacent air to saturation. Condensation therefore depends on local surface temperature as well as room-average temperature and humidity.
Thermal bridges, chilled pipes and poorly insulated glazing are common examples where this local condition matters.
Interstitial condensation requires a different analysis
Condensation can also occur inside a multilayer construction when temperature and vapour-pressure profiles interact unfavourably. A room-air dew-point comparison with the interior surface cannot determine that risk by itself.
Building-envelope moisture analysis may require vapour diffusion, air leakage, material moisture storage, climate exposure and transient simulation.
Dew point is often a better moisture metric for comparing air masses
Relative humidity depends strongly on temperature, so two rooms at different temperatures can have the same relative humidity while containing different amounts of water vapour. Dew point tracks the vapour content more directly and therefore gives a clearer indication of how close a cold surface may be to condensation.
This is especially useful when comparing outdoor air, conditioned indoor air and air that has simply been heated or cooled without moisture being added or removed.
Condensation can occur transiently even if annual averages look safe
Short cold snaps, night-sky cooling, intermittent occupancy and sudden moisture generation can create temporary condensation conditions that annual-average temperature and humidity data do not reveal. Bathrooms, kitchens and cold glazing are familiar examples.
A robust assessment therefore considers the timing and duration of cold-surface and high-moisture events rather than relying only on one steady design point.