Relative Humidity, Dew Point and Why Walls Get Wet

The calculator uses the room temperature and relative humidity to estimate the dew point. Adjust both readings to see how changes within the room affect the temperature at which condensation can begin.

The closer the dew point is to the air temperature, the greater the risk that ordinary cold surfaces will develop moisture. A high dew point means that a wall does not need to be particularly cold before condensation occurs.

For a meaningful assessment, temperature and humidity should be considered alongside actual surface temperatures. A hygrometer reading taken from the centre of a room cannot show how cold an external corner, window reveal or concealed wall surface may be.

Relative humidity describes how close the air is to saturation at its current temperature. When indoor humidity rises, the dew point also rises and condensation can begin on surfaces that are only slightly cooler than the room.

This is why the same external wall may remain dry under one set of conditions but develop condensation after cooking, showering, drying clothes indoors or reducing the amount of ventilation.

Room temperature also affects the relative humidity reading. Heating the air generally lowers its relative humidity, but it does not remove the water vapour already present. If the room cools again without that moisture being removed, the relative humidity will rise.

Stable heating can help keep walls and other surfaces warmer, while suitable ventilation allows excess moisture to leave the property. The relationship between moisture production, temperature, ventilation and surface temperature determines whether condensation develops.

In practical terms, warmer air can contain more water vapour before it reaches saturation. As that air cools against a window, external wall or other cold surface, its ability to retain the moisture reduces.

Once the air beside the surface reaches its dew point, some of the water vapour changes into liquid condensation. This is why moisture often appears on the coldest parts of a room rather than close to where the vapour was originally produced.

Air movement also matters. Furniture positioned tightly against an external wall can prevent warmth from reaching the surface and restrict circulation. The wall behind it may become considerably colder than the rest of the room, creating ideal conditions for condensation and mould.

The table shows the approximate dew point for different combinations of room temperature and relative humidity. If a wall, window or other surface falls to or below the stated temperature, condensation may begin forming.

The figures demonstrate why a small increase in indoor humidity can make a significant difference. At a room temperature of 20°C and relative humidity of 50%, the dew point is approximately 9.3°C. At 80% humidity, it rises to approximately 16.4°C.

Condensation commonly affects the coldest and least ventilated parts of a property. The pattern can provide useful clues, but it should not be assumed that every damp wall or mould problem has the same cause.

Possible signs include:

A damp survey considers the temperature, relative humidity and moisture conditions within the property, together with the location and pattern of any mould or surface water.

The surveyor may assess ventilation, heating patterns, cold surfaces, external walls, window reveals and areas where air movement is restricted. Relevant building defects are also considered so that penetrating damp, plumbing leaks and other possible moisture sources are not overlooked.

Professional hygrometers and moisture meters can help build a clearer picture, but individual readings are interpreted alongside the property construction and visible evidence. Where conditions vary over time, longer-term environmental monitoring may be recommended.

Following the inspection, you will receive a written report explaining the likely cause of the moisture, the evidence supporting that conclusion and any further investigation or action considered necessary.