Types of Damp: Condensation and Mould Growth in Your Home
Updated: May 17
Condensation Causes Black Mould
We all do things on a daily basis that create water vapour. We cook, shower and dry wet clothes to name just a few. The creation of water vapour coupled with poor ventilation causes condensation. The water vapour condenses into water droplets on all your surfaces rather than being ventilated out of the house. Condensation is by far the biggest cause of black mould growth in homes across the UK.
If water vapour comes into contact with a cold surface (like an uninsulated wall or ceiling or a cold window) which is below the dew point it will form condensation - water droplets on a surface instead of a mist. If the condensation is not dealt with it will soak into all organic materials causing them to become damp. It's then just a matter of time until mould begins to grow.
What is Dew Point?
The Met Office defines dewpoint as follows:
The temperature at which condensation occurs is called the Dew point and is dependent upon the humidity and pressure of the air.
Dew point is the temperature at which condensation occurs. Or, in other words, the temperature at which water vapour forms into water droplets on your surfaces. This is key. Mould feeds on moisture and needs it to grow. Dew point is the key to understanding if you're creating condensation in your home. When your home has reached dew point, it means the environment has now become one in which water begins to condense on your surfaces. In other words - mould food.
However, it's not as simple as keeping your home above a certain temperature to avoid condensation and mould. There are three variables that affect condensation: temperature, air pressure and humidity.
If you understand the dew point of your home, you know when to either turn the heating on or open some windows and turn on extractor fans. How can you ensure your home doesn't reach dew point? We'll go through some examples, assuming air pressure is constant as this is more difficult to manipulate.
Humidity and Dew Point
The moisture content of air is measured as relative humidity. Relative humidity is measured as a percentage. When relative humidity reaches 100% it is the point at which the air reaches saturation point at its current temperature. At 100%, water droplets, or dew, begin to condense on surfaces from the moisture in the air (providing the surface is the same temperature or cooler than the air).
The biggest influencing factor on household humidity levels is human behaviour. Cooking, showering and drying clothes cause water vapour to be released into your air, significantly increasing humidity levels. Many homes across the UK aren't equipped with sufficient means of ventilation.
Temperature and Dew Point
As stated above, if you never did anything in the home to create moisture, maintaining warmth would be enough to prevent condensation and mould. However, daily tasks like cooking, showering and drying clothes, create significant levels of water vapour. This means the air will reach 100% relative humidity very quickly, regardless of temperature. Condensation will begin to form on any surfaces at the same or lower temperature.
Condensation is an ever-increasing problem in today’s homes. Modern windows, doors and bricked-up fireplaces dramatically reduce the heat that is lost from our properties, which is a good thing in principle, but they also prevent air that has a high moisture content from escaping the home and being replaced by new air.
Calculating the Dew Point of Your Home
Dew point is the combination of air temperature, surface temperature and relative humidity which cause airborne water to condense. The dew point is never fixed and will increase or decrease depending on what the relative humidity value of the air near the surface is.
Let's look at an example:
If air temperature is 21ºC and relative humidity is 70% condensation would occur on surfaces at 15ºC or less.
If air temperature is 21ºC and relative humidity is 90% condensation would occur on surfaces at 19ºC or less.
Most people tend to keep their homes around 21ºC and the typical relative humidity levels in the UK vary between 76 - 88%.
The table below illustrates the relationship between relative humidity, the room temperature and the dew point; the dew point temperatures can be seen in the grey section and are provided in Degrees Celsius.
We appreciate calculating your home's dew point isn't going to be for everyone. But in understanding the dew point and the principles which underpin it, everyone can make some simple changes to their home to reduce the likelihood and frequency that the dew point will be reached. That is; keeping the air and surfaces warm, reducing humidity and ventilating water vapour.
How Can I Reduce Humidity Levels in My Home?
You can control relative humidity in your home in a number of ways related to the behaviours that cause moisture. The use of extractor fans in kitchens and bathrooms help immediately expel water vapour into the outdoor air. Shutting the doors to these rooms whilst the extractor fans work also helps.
Ventilation is also key. Trickle vents in windows work well, but a more sophisticated option is a heat-recovery ventilation unit. These replace the air in your home by taking the stale, damp air outside, then bring fresh air back in via a separate grille, passing it back over the heat exchanger to be warmed. It is also possible to buy central extract systems which connect all of the wet areas in your home to a central fan before discharging the stale, moist air outside.
Another ventilation option is a positive input ventilation (PIV) system that works by gently supplying fresh, filtered air into the property from a unit installed in the loft area and a distribution diffuser mounted in the ceiling. The continual supply and gentle positive pressure result in the air being continually diluted, displaced and replaced to create a healthier indoor air quality.
Add insulation so that internal walls are kept at a temperature above the dew point of the air inside. Internal wall insulation is best when it is not an option to add insulation to the exterior of your property.
The recommended relative humidity for internal air is 40-50% and maintaining this has a positive effect on both the condition of our homes and our health (the latter is addressed later on this page).
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