First, let’s examine what condensation actually is. There is always moisture present in the air, with levels varying according to temperature and climate. The activities of building inhabitants, weather conditions and the construction of the property all influence how much moisture is created, how long it remains and how much damage it does. Once water vapour is airborne, the key mechanism by which condensation is created is the heating and cooling of air.
When air heats, its ability to hold moisture increases. As it cools, its maximum water content declines proportionally – this moisture in the air then has to go somewhere. This usually takes the form of condensation on surfaces in the property. This is measured in relative humidity (RH) – the amount of water vapour present in any given sample of air expressed as a percentage of the amount needed for saturation at the same temperature. This is essentially the amount of water in the air as a percentage of how much water the air can hold. Another key measure is Dewpoint Temperature – this is the relative temperature at which the air becomes saturated.
In most cases, this water comes to rest on surfaces in the home, usually congregating on surfaces that are the coolest – for example on and around windows. Condensation isn’t always visible, especially on surfaces that are wallpapered for example. It also doesn’t leave what’s called a ‘tidemark’ – a line around a room where water seems to have settled. This is usually a different problem – often water leaking into the property either from surrounding buildings or damaged water pipes.
There are four key factors that can affect condensation and mould in a property:
Traditional wisdom has always held that the activities of residents have an impact on humidity and therefore on mould development. In a normal 5-person household daily activities create approximately 15 litres of water which is absorbed into the air. For example, cooking on average adds 3 litres of water to the air a day, whilst each individual in the property adds 0.85 litres per day whilst awake and 0.3 litres per day asleep. Washing and drying clothes are the single greatest addition with 5.5 litres a day being added.
A report published by Sustainable Homes stated that occupant density was one of the largest factors in the increased prevalence of mould. With an increased number of people in the home, there is naturally a higher level of moisture being created through the average day-to-day activities of the residents. The property is also unlikely to have a different ventilation system from their lower density neighbours – meaning the “ventilation rates are likely to remain in proportion to the size of the home”.
According to the same Sustainable Homes study, the way a property was heated also had an enormous impact on condensation and mould issues. When a property was consistently under-heated, there was a significant increase in reported cases of condensation and mould, when compared to neighbouring properties that were being heated adequately.
There is also a direct correlation with a property’s energy efficiency rating with an increasing rate of condensation, damp and mould. A property with a significantly lower energy efficiency rating will have its heat levels drop faster. The property will therefore experience larger temperature fluctuations than one that is adequately insulated. In general, underheated and inefficient homes are much more likely to experience condensation and mould than those that are not due to their inability to remain warm.
Insulation of a property is another big factor in increased rates of condensation and mould. As mentioned above – a property that cannot hold heat well will struggle significantly more to keep its dew point temperature at a high enough level. A property that lacks appropriate insulation (or worse has insulation that is compromised but without the resident’s knowledge) will struggle to keep temperatures at an appropriate level.
In poorly insulated homes there is also a far greater likelihood of cold patches on its walls – which are the most likely area for condensation to occur. This is most common where insulation is either inconsistently applied or where it has failed in locations. A property that is appropriately insulted is less likely to suffer these spots and are therefore less likely to have significant mould outbreaks. Both thermally poor construction (in the cases of solid walls and unfilled cavities) and also failed insulation construction (in the cases of failed wall insulation and poorly installed insulation) contribute to this problem significantly.
Ventilation is the fourth and final major factor in condensation and mould development. Ventilation, when properly deployed, keeps moist air from remaining inside the house. It does this by venting moist air from within the property and pushing fresh air from outside into the property which will have a much lower relative humidity (and therefore dewpoint temperature).
An appropriately ventilated property allows for a much higher level of relative moisture generation in the property as the outside air that is pulled in is capable of absorbing far more water than existing air within the property. This is supported by a number of different studies that suggest well-ventilated properties have a significantly lower chance of mould.