It is important to note that the primary factor in the development of mould is the presence of moisture. However, the availability of food, oxygen, and temperature also plays a significant role. The likelihood of mould growth on a material can vary.
According to Approved Document F, if the relative humidity remains consistently high for extended periods, mould growth is likely to occur. The table below, sourced from Approved Document F, outlines the specific relative humidity levels needed for mould growth to take place. It also emphasises the importance of the duration of high humidity in the potential for mould growth.
| Indoor air relative humidity | |
| Moving average period | Room air relative humidity |
| 1 month | 65% |
| 1 week | 75% |
| 1 day | 85% |
An important consideration is that these measurements are significantly lower than the 100% relative humidity needed for surface condensation to form. Liquid water does not need to be present for mould to grow. Given the right conditions of a suitable surface and sufficiently high relative humidity levels, mould spores can germinate and lead to growth.
The UK’s growing population, high property values, housing shortages, and increased density have significant shifts in the way homes are occupied. These changes, along with other factors, have affected the internal environment of homes. For instance, the rapid increase in fuel costs has led to a push for greater fuel efficiency to meet international carbon reduction obligations. Government-backed programmes promoting insulation and draught proofing have altered the ability of some buildings to remove atmospheric moisture. Consequently, issues related to moisture, such as condensation and mould, are becoming more prevalent in UK homes, leading to more disputes between landlords and tenants regarding the causes of dampness.
Moisture in the air has always been a crucial concern for building surveyors. While the ventilation industry and building regulations have made significant progress in meeting the needs of new house constructions, occupants of older houses and flats still face challenges with moisture-related issues such as condensation and mould.
In the past, problems with condensation and mould growth were often dismissed as being the fault of the occupant. However, these issues can be complex and difficult to resolve by simply reducing moisture production. In many cases, adjusting heating and ventilation systems is a more practical solution than trying to decrease moisture levels if there is no obvious excessive source of moisture.
Achieving a balance between moisture production, insulation, heating, and ventilation, along with a good understanding of the science of air moisture and condensation, is essential for preventing condensation and mould growth. These problems are often multifaceted, and a comprehensive approach involving the entire building envelope is necessary to identify the root cause.
It is important to consider the building as a whole when addressing condensation and mould issues, rather than focusing solely on internal atmospheric conditions.
Insulation
The drive to reduce energy use and increase thermal performance in dwellings has, in part, been achieved by the control of draughts and the controlled release of moisture through the fabric of the building. In some situations, this has resulted in properties that are incapable of managing atmospheric moisture during periods of high moisture production. It therefore follows that the planned management of atmospheric moisture through positive or passive ventilation must be designed around the use and occupation style of the dwelling.
Any retrofit insulation measures should be noted during the external checks. Whilst the insulation helps to regulate internal temperature, the surveyor should ensure that the insulation is continuous and that no cold bridges have been formed as a result of discontinuity in the insulation. These are typically found around windows or at the base of walls where the insulation has been terminated above the level of the damp proof course.
Relative Humidity
The internal relative humidity is a crucial factor in the development of mould. The air typically contains water vapour in different amounts, with its capacity to hold moisture linked to temperature. Warmer air can hold more moisture. The level of moisture in the air is commonly measured as Relative Humidity (RH), which is a percentage of the maximum water vapour the air can hold at a specific temperature. When the air is saturated and cannot hold any more water vapour at the current temperature, it has a relative humidity of 100%.
If the air temperature decreases to the saturation point, it reaches a critical temperature where it cannot hold any more water, known as the dew point. At this temperature, water vapour is forced to condense into liquid water. This phenomenon explains why warm, moist air in contact with a cold wall or window causes condensation. Since relative humidity is influenced by temperature, caution should be exercised when using relative humidity alone, as fluctuations may be due to changes in temperature rather than an increase in moisture.
Using RH whilst surveying
Equipped with the information provided, it is possible to ascertain if condensation is present during a survey. However, due to time constraints and the changing levels of relative humidity, long-term monitoring is more reliable for determining if conditions are consistently conducive to mould growth.
By using a surface thermometer and hygrometer to measure wall temperature and relative humidity in a room, it is relatively straightforward to determine if the surface temperature is at or below the dew point, indicating potential condensation areas. Areas with restricted airflow, such as corners and behind furniture, are typically cooler and should be the first places to measure surface temperature. Taking readings from the wall’s centre may not accurately represent these cooler areas. Therefore, it is essential to have a hygrometer and surface thermometer as part of their diagnostic equipment when assessing defects in buildings.
Vapour pressure and moisture production
Vapour pressure, or equilibrium vapour pressure, is the pressure at which water vapour above water is in thermodynamic equilibrium with its liquid state. If the pressure is increased, the water vapour would condense.
In contrast to relative humidity, vapour pressure gives a measurable amount of moisture rather than a proportion and can help to determine levels of moisture production in relation to ventilation. This can be calculated by establishing the internal relative humidity and temperature, and then using a conversion factor to determine the vapour pressure. The same process is then carried out for external relative humidity and temperature.
By subtracting the external vapour pressure from the internal, a vapour pressure differential can be obtained. Due to moisture production from human activities such as showering and cooking during the winter months, indoor moisture levels are usually higher than external.
An average vapour pressure differential of 0.45kPa may be acceptable, although additional data may be needed for a more detailed assessment. Below 0.45kPa is typically considered “dry” or acceptable when ventilation balances moisture production, while above 0.6kPa is deemed “wet occupancy.”
It is important to note that relative humidity and vapour pressure data collected during a snapshot survey may be limited in its scope. Additionally, it is uncommon for a surveyor to be on-site at the optimal time when conditions for condensation and mould growth are present. Moreover, the use of purge ventilation, such as opening windows, is often done before an inspection, resulting in data that may not accurately reflect the occupants’ day-to-day living conditions.
Logging data over extended periods can provide more accurate information and help to create a more precise understanding of the typical moisture production within the property.
Ventilation
Ventilation is a crucial factor in managing humidity levels within a property. Inadequate ventilation can lead to indoor air quality problems such as condensation and mould growth. Conversely, excessive ventilation can cause heat loss and reduce energy efficiency.
Several recent studies have revealed that ducting is a significant factor in ventilation systems failing to meet the required standards, with flexible ducting being identified as particularly problematic.
The necessary amount of fresh air will vary depending on the specific dwelling and the number and activities of the occupants. It is essential to have a sufficient supply of fresh air to control pollutants and to safely and efficiently operate combustion appliances like gas fires.
Building Regs
The Means of Ventilation section in Approved Document F is often overlooked in building regulations compliance, despite its potential impact on occupant health. While strides have been made in improving ventilation requirements in modern housing, older properties may still have inadequate ventilation systems.
Properties built after 2010 are supposed to comply with Building Regulations and have proper ventilation in place. However, enforcement of these regulations is lacking, leading to potential issues in newer properties as well. When dealing with mould issues in post-2010 properties, it is important to check if the ventilation system has been installed and commissioned according to Approved Document F. Occupants should have been provided with a record of this information.
Determining the appropriate level of ventilation can be challenging, with only guidelines from Approved Document F available. It is recommended that building surveyors have a hygrometer on hand to assess the effectiveness of ventilation systems.
Heating
The heating system can have a significant impact on dampness issues and should be able to maintain consistent air and surface temperatures. This will help prevent problems related to excess moisture. It is preferable to have a steady background heat rather than intermittent heating, as this will help keep the building fabric at a higher ambient temperature. The building surveyor should make note of the type of heating system and the location of heating elements. During a quick survey, it may be challenging to identify problematic heating practices, but long-term monitoring will give a better understanding of the heating system.
Different heating methods can present various issues, but irregular use and certain types of heating can be major contributors to condensation and mould growth. Simply heating the air is unlikely to solve the problem, both due to cost and practicality. Condensation will occur if cold surfaces are not addressed. Therefore, any remedial action should focus on reducing moisture levels and eliminating cold surfaces.
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Key factors contributing to Portsmouth’s damp problem: