Misconceptions on Relative Humidity and Ventilation

A common misconception that we come across is how Relative Humidity (RH) relates to damp issues.


Relative Humidity is a sliding scale, which represents how much of the air is saturated with at the current temperature. As air heats up it can hold more water as it expands, meaning that the saturation level decrease so long as no new water vapour is added. An example of this is that if the RH is 100% at 1°C, when the air temperature is raised to 10°C the RH is 50%. This means that it can give a false indication unless fully understood.

We prefer to work in a more precise method called Absolute Humidity, this is how much water is in the air in mg mg/M³ of air, irrelevant of the temperature. Along with this more precise measurement, we implement a comparative data element through our AutoStat humidity controller. The AutoStat senses in 2-minute intervals, comparing this data back to the previous 2 hours’ worth of data. This comparison means that the unit is continually learning about your property, and not dependant on a set point to extract. The information is collected and stored on the data logger for up to 1 year, so it is possible to track your humidity levels, and see how the property is drying out. The Autostat will also respond to sudden spikes in humidity and reacts accordingly.

Through modern building practices, in both modern and older homes, we are increasing the insulation. This is leading to the air being able to hold more water and cause the damp issues seen with increasing frequency. The reason for this, is water in any state, will look for the area or state of least pressure; this leads to the water vapour in the air looking for either a lower pressure environment (the outside generally) or the coldest point to condense down on. As properties become more airtight, this leads to the cold spots being the only option for the water vapour. A snowball effect is then created, as when the water vapour condenses, it makes the surface and area colder, attracting more water vapour. This condensation then soaks into the fabric of the property, and cause issues such as damp spots, salt rising and black mould.

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