Water is an essential part of our lives, but inside the home, it can become a problem when it contributes to damp and mould. Understanding the science behind water and its three states - solid (ice), liquid (water), and gas (vapour) - is the first step to controlling moisture in your home.
Water in Its Three States
- 0°C or below → Ice
- 100°C and higher → Vapour
- 0°C–100°C → Water & Vapour
It’s important to note that water doesn’t need to reach the boiling point to turn into vapour. Evaporation can occur at any temperature above the freezing point. This is a slow process in which water molecules at the surface gain sufficient kinetic energy to escape into the air.
- Evaporation: Gradual escape of water molecules from a liquid’s surface.
- Boiling: Rapid conversion of water to vapour at a specific temperature when the water’s vapour pressure equals the surrounding atmospheric pressure.
Factors Affecting Evaporation
- Temperature – Higher temperatures increase the kinetic energy, and therefore the movement of water molecules, speeding up evaporation.
- Surface area – Liquid water spread over a larger surface area evaporates faster.
- Air pressure – Lower atmospheric pressure allows water to evaporate more easily.
Even when you don’t see steam, water is constantly evaporating—this is how puddles disappear and clouds form.
Understanding Steam and Condensation
Steam is water in its transitional phase from liquid to gas, or vice versa.
When water vapour cools, molecules bind together to form droplets, which become heavier and fall as liquid water or appear as visible mist. The visibility of steam depends on the surrounding air temperature, not the actual moisture content.
For example:
- Stand outside on a hot summer’s day, take in a deep breath and blow out. What do you see? Nothing.
- Do the exact same thing in the dead of Winter, and this time you will see your breath billowing away from you. More technically, you will see the water vapour condensing back into liquid water droplets as the cold air reacts with your warm breath.
Same breath, same volume of moisture, different appearance due to the environment.
The same principle applies in bathrooms and wetrooms. A well-insulated, heated bathroom may produce less visible steam during a shower, but the volume of moisture in the air is the same as in a colder, poorly insulated bathroom.
Why This Matters for Damp and Mould
Moisture accumulation is the primary cause of damp and mould growth. Understanding evaporation, condensation, and airflow allows you to control moisture before it becomes a problem.
Simple Home Habits to Reduce Damp
- Ventilate – Use extractor fans in bathrooms and kitchens to remove moisture. Keep windows closed where possible to keep colder air out.
- Control temperature – Keep rooms warm and insulate; this raises the temperature of the surfaces and reduces the speed of condensation forming on them.
- Reduce humidity at the source – Cover pots when cooking, dry clothes outside or in a ventilated room, and lower the temperature of your showers where applicable.
- Increase air circulation – Install or upgrade extractor fans to Humidistat* fans, dMEV continuous-running fans, or install a PIV unit to improve overall air quality and airflow in the home.
- Wipe down wet surfaces – Shower screens, tiles, and mirrors can trap water that eventually contributes to mould.
By understanding how water behaves and adjusting your habits accordingly, you can significantly reduce the risk of damp and mould in your home.
*Humidistat Fans
Humidistat fans are largely misunderstood. They are simply extractor fans that can be automatically triggered by rises in humidity. They are often described as unreliable, frustrating to use or simply faulty. This is usually due to a lack of understanding of how they work or incorrect expectations of the unit, and therefore, they have not been installed or set up correctly.
Essentially, a Humidistat fan is no different to any other extractor fan, except that it can be automatically triggered by humidity. A common misconception is that they are dehumidifiers and will work to lower the humidity in a room beyond that which a non-humidistat fan can do, but this is not the case. The humidity sensor is simply another way to automatically trigger the fan, meaning that they can be installed in bathrooms, utility rooms and kitchens without having to be attached to the light circuit and run simply when humidity is present to remove the moisture. Handy in en-suite bathrooms, where a trip to the bathroom in the middle of the night would otherwise trigger the unit with the light switch and the running noise could disturb a sleeping occupant in the bedroom. Or in a utility room where clothes are drying, or perhaps a hot air dryer is running, raising the humidity in the unoccupied room.
Most humidistat fans can have the sensitivity of this humidity sensor adjusted to allow you to control the fan’s running. This is not to allow the user to decide on the level of humidity they want their room to reach. Humidistat fans have this adjustment so that you can set up the unit to behave depending on where you live geographically.
This morning, as I write this, the current ambient humidity in the home counties, according to the Met Office, is 94%. This is not uncommon for this time of year (early October), and yes, as the day progresses, that humidity percentage will lower. In contrast, back in April this year, a record low of only 25% was recorded in Hampstead, North-West London.
In contrast, today's humidity in Dubai is 45%
Where you live and install your fan will greatly affect how the humidistat behaves. As an example, if this morning I set my humidistat to 90% humidity as the trigger point to activate the unit in my bathroom and have my window open, two things will happen. Firstly, the running shower will trigger the fan into operation. The room will require new air to be pulled in to replace the exiting air. This will come from the open window, supplying the room with colder air that has a humidity content of 94%. We now have a vicious cycle, where the fan will continue to run until the ambient humidity of the air being pulled in through the open window has dropped below 90%. As this colder air is being pulled into the room, it is also condensing the water vapour in the air faster from the running shower, and also lowers the temperature of the surfaces, speeding up the condensation of vapour coming into contact with them.
However, suppose the window is shut and the internal temperature is maintained. In that case, it is likely the fan will reduce the humidity below 90% once the shower is turned off, and less condensation will have formed. The return of air into the bathroom will come from the rest of the house, past the bathroom door, and is likely to be warmer than the external air and possibly hold less humidity. But it must be noted that, when there is high ambient humidity, this can still cause humidistat fans to run continuously until the humidity drops naturally.
For the most part in the UK, setting a humidistat fan to around the 80% mark is generally a good place to start for a balanced year-round approach. There are, of course, exceptions; coastal areas and the far north can struggle with higher ambient humidity, and houses that already have damp issues will develop higher internal humidity levels when heating is applied. This last one is crucial, however. Allowing a fan to run to remove the re-evaporated moisture from a damp problem is critical to removing the water from the building, and in these situations, a dMEV continuous-running fan with a humidistat is highly recommended.
For guidance on the best way to set up a humidistat fan, please see our article in the link below:
Setting up and adjusting humidistat fans
For information on dMEVs, see the article in the link below:
What does dMEV mean, and should you consider installing one in your home?
For information on PIV units, see the article in the link below:
What is Positive Input Ventilation | Extractor Fan World