Dew Point Differential

What is Dew Point?

Dew point (TD) is a measure of how much moisture the air contains. In other words, it represents the temperature at which water vapour in the air condenses into liquid water. Dew points rise and fall in line with vapour pressure and can thus be considered a proxy for the amount of vapour in the air. Unlike vapour pressure, dew points are perhaps easier for the general public to understand and can be easily plotted on a standard temperature-relative humidity graph.

Formula for Calculating Dew Point (TD)

The standard dew point calculation is as follows:


Where TT is the temperature in degrees Celsius and RHRH is the relative humidity in percentage.

Importance of Dew Point Differentials

Dew point differentials are crucial for understanding the distribution of moisture in different zones within a property. To measure these differentials, one would need more than one hygrometer or, better still, a data logger. By comparing the dew points in various zones, it becomes easier to identify areas where excess moisture might be present.

Causes of Dew Point Differentials

  1. Excess Unventilated Vapour: Lack of ventilation can lead to increased moisture, thereby affecting the dew point.
  2. Leaks: Any form of water ingress can alter dew point levels in a particular zone.
  3. Condensation: Areas prone to condensation will have a different dew point compared to dryer zones.
  4. Hygroscopic Salts: The presence of hygroscopic salts can buffer relative humidity and, therefore, dew points. These salts absorb excess moisture when the air is humid and release moisture when it’s dry.

Why Does It Matter?

Understanding dew point differentials can help in identifying the root causes of unwanted moisture, thereby informing appropriate solutions. This understanding can be particularly helpful in diagnosing complex damp issues within properties.


Dew point differentials offer an accessible and effective method for monitoring moisture levels in various areas within a property. By using multiple hygrometers or data loggers, one can pinpoint areas of concern and take necessary actions to remediate the damp issues.

Understanding dew point differentials is not just for specialists; it’s valuable information for property owners, tenants, and maintenance staff alike. It can be a pivotal factor in maintaining a healthy, damp-free environment.

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