Before explaining that it is important to understand the terms used to describe humidity which are poorly understood.
The term “absolute humidity” is a measure of the amount of water vapor in the air expressed as a unit such as g/m3. The term “relative humidity” refers to the absolute humidity of the air at any given temperature expressed as a % of the maximum absolute humidity at that temperature. Relative humidity is a necessary metric because the maximum absolute humidity changes with temperature and the relationship is inverse. As the temperature falls the maximum amount of moisture the air can hold falls and vice versa. This means the relative humidity of the air increases as temperatures fall and decreases when temperature rise even if the moisture content of the air stays the same. We use the term “psychometrics” to describe the science of understanding and controlling the properties of moist air.
When it rains this results in huge amounts of new water vapor coming onto the air as the rainwater evaporates. This happens even during cold weather. This increases the absolute humidity of the local air and much of the new moisture will find it ways inside a property through natural ventilation openings (doors, windows, trickles, air bricks etc). Once inside the air is subject to temperature change so whilst there might be an increase in the absolute humidity of the air beyond internal levels (which are already subject to contribution from the occupancy’s normal living activities) this is often offset by the increased indoor air temperature compared to outside (especially in winter) which lowers relative humidity. However, rainfall also increases heat absorption and transfer from the walls to the rainwater (water absorbs heat 25 times faster than air) which results in a drop in temperatures on inner wall surfaces which increases relative humidity, especially in cold, poorly ventilated external wall surfaces, which can result in condensation during rainfall. When this phenomenon happens it is frequently misdiagnosed as rainwater ingress, rising damp or a water leak because it only happens during rainfall.
At Better Indoors we have an intimate understanding of the sciences of moist air. Our surveys always pinpoint the root causes of condensation damp and mould and our solutions always permanently address the root cause. Book a survey today.
