Water is indispensable to life, yet within the enclosed environment of the home it can present persistent challenges. Damp and mould, common problems in the UK housing stock, are not simply matters of poor housekeeping or faulty construction, but the predictable outcome of physical processes that govern how water behaves in air. An understanding of these processes is essential to managing indoor moisture effectively.
Water exists in three states: solid, liquid, and gas. At temperatures below 0°C it freezes into ice; above 100°C it becomes vapour. Between these limits, however, water may exist simultaneously as liquid and gas. Crucially, water does not need to boil in order to enter the air. Evaporation occurs at any temperature above freezing, as surface molecules acquire enough kinetic energy to escape into the surrounding atmosphere. This slow but continuous process explains how puddles disappear on cool days and why moisture accumulates indoors even in the absence of visible steam.
The rate at which evaporation occurs depends on several factors. Higher temperatures increase molecular motion, accelerating the process. A larger surface area, such as water spread thinly across tiles or fabric, allows more molecules to escape at once. Lower air pressure also facilitates evaporation. These variables are constantly at play inside homes, particularly in kitchens, bathrooms, and utility rooms.
Once airborne, water vapour remains invisible until conditions favour condensation. When warm, moisture-laden air cools, the vapour condenses into microscopic droplets that may appear as mist or settle on colder surfaces. What is commonly referred to as “steam” is often not vapour at all, but condensed water droplets made visible by a temperature differential. A familiar illustration is the visibility of one’s breath on a cold winter morning, compared with its invisibility on a warm summer’s day. The moisture content of the breath is essentially the same; only the surrounding air temperature differs.
This principle has direct relevance indoors. A poorly insulated bathroom may quickly fill with visible condensation during a shower, while a well-heated, insulated space may show little outward sign of steam. In both cases, however, the amount of moisture released into the air is comparable. The absence of visible condensation does not imply the absence of moisture.
It is this accumulation of airborne water that underlies damp and mould. When humid air repeatedly condenses on cool surfaces such as walls, ceilings & window frames, it creates a build-up of dampness in which mould spores can thrive. Effective control therefore depends not on masking symptoms, but on managing evaporation, condensation, and air movement together.
Ventilation plays a central role. Extractor fans remove moisture-laden air before it can settle, but their effectiveness depends on how and when they operate. Maintaining adequate indoor temperatures is equally important, as warmer surfaces are less prone to condensation. Everyday practices, such as covering pans while cooking, avoiding indoor clothes-drying without suitable ventilation, and wiping down wet surfaces, can also reduce the moisture burden. These measures are most effective when combined with consistent airflow through the dwelling as a whole.
A frequently misunderstood component of domestic ventilation is the humidistat-controlled extractor fan. Such fans are often criticised as unreliable or ineffective, when in reality the problem usually lies in incorrect expectations or inappropriate setup. A humidistat fan is not a dehumidifier; it does not remove more moisture than a conventional extractor operating for the same duration. Its distinguishing feature is simply that it activates automatically when relative humidity rises above a set threshold.
This automatic triggering can be advantageous, particularly in spaces where lights are not always switched on during occupancy, such as bathrooms with good natural light or utility rooms. The adjustable sensitivity of the humidistat is not intended to allow occupants to select a preferred humidity level, but to accommodate variations in background humidity associated with geography and climate.
Ambient humidity in the UK can vary widely. Autumn mornings in the Home Counties frequently begin with relative humidity above 90 per cent, while 2025's unusually dry spring conditions saw values fall below 30 per cent. In other parts of the world, such as countries on the equator, markedly different baselines apply. These external conditions influence how a humidistat-controlled fan behaves once installed.
If, for example, a fan is set to activate at 80 per cent humidity while a bathroom window is open on a damp morning, the system may enter a counterproductive cycle. Moist external air is drawn in to replace the extracted air, preventing humidity levels from falling below the trigger point. At the same time, the influx of colder air cools surfaces, encouraging further condensation. By contrast, with windows closed and internal temperatures maintained, the fan is more likely to reduce humidity efficiently once the source, such as a running shower, is removed, and switch off automatically accordingly rather than continuing to run.
In most UK homes, a trigger setting of around 80 per cent relative humidity provides a reasonable balance across the year, though coastal locations, northern regions, and properties with existing damp may require adjustment. In such cases, continuous low-level ventilation systems such as dMEV units, particularly those incorporating humidistats, can be instrumental in removing moisture that re-enters the air as a building dries. We have added some suitable options at the end of the article that we recommend.
Damp, then, is not an inevitable feature of domestic life, nor is it solved by a single device or habit. It is the outcome of well-understood physical processes acting within enclosed spaces. Managing it successfully requires aligning ventilation, temperature control, and everyday behaviour with the underlying science of water in air, a modest investment in understanding that can yield substantial improvements in both building health and human comfort.