What Is Dew Point?

The dew point is the temperature at which the air becomes so saturated with moisture that it can no longer hold water vapour. At this point, the water vapour starts to condense into liquid - for example, in the form of dew on grass on an early summer morning or condensation on cold indoor surfaces.

Why is the dew point important?

Understanding the dew point is important in several areas, especially when it comes to ventilation, dehumidification and indoor climate in general. When the surface temperature of a space drops below the dew point, condensation forms, which over time can create a too humid climate with moisture damage, bad odour and microbial fouling as a penalty.

When warm, moist air comes into contact with a cold surface - such as an exterior wall, a window or a ventilation pipe - it cools down quickly. If the temperature of the surface then drops below the dew point, the air cannot retain the moisture, which instead turns into water droplets, or condensation.

A clear example is an outside-ventilated crawl space during spring and summer. When warm and humid outside air enters and meets cold surfaces such as the floor or joists, condensation can form, raising the relative humidity in the crawl space.

Common areas with condensation problems

• Basements and crawl spaces
• Cold walls in bathrooms
• Condensation on the inside of windows
• Uninsulated storage rooms or attics
• Holiday home
• Containers
• Boats or motorhomes in winter storage
• Etc.

How to reduce the risk of condensation

Below are some general measures you can take to reduce the risk of condensation - some are much simpler than others.

• Use a dehumidifier in enclosed spaces that are sensitive to condensation or have a high moisture content - such as bathrooms, crawl spaces, cold attics, storage rooms or garages. The space must be tight enough for the dehumidifier to work effectively and not 'dehumidify the world'.
• Improve ventilation so that moist air is ventilated away before it has time to accumulate.
• Insulate cold surfaces, such as external walls and roofs, to prevent temperatures from dropping below the dew point.
• Add heat to lower the relative humidity by increasing the air temperature.

Of the following, a dehumidifier is often the simplest and most cost-effective option.

How to calculate the dew point?

The dew point is determined by two factors:

• Air temperature in the space
• Relative humidity (RF or RH)

Using these values, you can either use a table or a digital hygrometer which shows the dew point directly. There are also various online dew point calculators for those who want to calculate it manually.

Example: If the temperature is 15°C and the relative humidity is 70 %, the dew point is around 9.57°C. This means that if a cold pipe or a concrete floor is colder than 9.57°C, condensation can form there.

By monitoring the dew point, you can prevent condensation and moisture problems in both small and large spaces - from bathrooms to crawl spaces and industrial premises.