Dew Point Definition for Home Owners

The Dew point definition according to the dictionary is -

“the atmospheric temperature (varying according to pressure and humidity) below which water droplets begin to condense and dew can form”.

Dew point is usually referred to as occurring at a specific temperature i.e. dew point (or condensation) occurs at XºC. But ,as usual, it’s not quite that simple, as there are several variables that affect dew point i.e. air pressure, temperature and humidity.

For the purposes of this explanation I’ll assume that the air pressure is constant, as a change in air pressure will alter the amount of vapour the air is capable of holding.

And realistically there isn’t a lot you can do about the air pressure anyway.

Dew Point Definition

What happens at dew point:

At dew point the amount of water vapour that is condensing to form dew or condensation equals the amount of water vapour being evaporated into the air.

Any cooler than this temperature (dew point) and the amount of condensing water starts to exceed the amount of water being evaporated and you start to see condensation forming of these cool surfaces.

So if someone was to say to you that the relative humidity was 100%, what they’re actually saying is, at the current air temperature and air pressure the air is completely saturated with water. 

So if that air came into contact with a surface that was cooler than the air temperature, dew would start to form on that surface.

Affect of Humidity of Dew Point:

As you’ll no doubt be aware the humidity level in the air is referred to as a percentage level e.g. 70%.

The lower the temperature, the higher the level of humidity. 

So as the temperature drops (assuming the air pressure remains constant) the relative humidity increases until it gets to 100%. At which point it condenses and forms dew. 

So when the humidity level gets to 100% it doesn’t matter what the air temperature is, as condensation will occur at any temperature once humidity levels reach 100% as the air can’t physically hold any more moisture.

Affect of Temperature on Dew Point:

Temperature is the second element to affect our dew point definition and has just as much effect on dew point as humidity does.

At higher temperatures the air can hold more moisture. 

So if your home is nice and warm the air in it will be able to hold more moisture before it starts to condense. However in your bathroom, when the shower’s in use, there’s so much steam that the air will get saturated (100% relative humidity) very quickly.

Once the air’s saturated, condensation will form on any surface that is at the same temperature or lower than the air temperature. 

But if your were to warm the air in the room up further (assuming air pressure remains constant) the air would then be capable of holding more moisture. Assuming of course the source of the water vapour (the shower) is now turned off and not adding to the level of water vapour in the air.

That’s why a house with little or no insulation will be more prone to condensation than a well insulated one. As the air inside it will reach saturation point earlier as the air temperature and wall and ceiling surfaces are colder than they would be in a well insulated home. 

Whereas a well insulated house will have warmer air and warmer surfaces meaning the air can hold more moisture before it reaches saturation point.

By way of an example:

Most people tend to keep their homes around 21ºC and the typical relative humidity levels in the UK and Ireland vary between 76 - 88% (depending on location, time of the year etc.).

  • If air temperature is 21ºC and relative humidity is 70%  condensation would occur on surfaces at 15ºC or less.

  • If air temperature is 21ºC and relative humidity is 80%  condensation would occur on surfaces at 17-18ºC or less.

  • If air temperature is 21ºC and relative humidity is 90%  condensation would occur on surfaces at 19ºC or less.

  • If air temperature is 21ºC and relative humidity is 100%  condensation would occur on surfaces at 21ºC or less.

So as you can see from the above examples if you want to avoid condensation occurring in your home you really need to try and keep surfaces above 17-18ºC and keep the relative humidity as low as possible.

Applying the Dew Point Definition to Your Home:

  1. By keeping air and surface temperatures higher means the air can hold more moisture before it condenses. Increasing insulation can help achieve this by keeping your home warmer for longer.

  2. By reducing the amount of water vapour in the air within your home you can reduce the risk of getting to 100% saturation. This can be achieved by increasing the controlled ventilation in your home and by avoiding adding unnecessary water vapour to the air by not drying clothes inside and using lids on saucepans etc.
Cavity Wall Insulation Helps Avoid Condensation


The full dew point definition can get very complicated which is why I’ve tried to explain it as simply as possible and all of the above can be simplified further by saying:

To reduce the risk of condensation in your home you need to reduce the amount of water vapour in the air and increase the air temperature so that it can hold more moisture before condensation occurs.

Or, in even simpler terms:

Increase heat and increase ventilation.

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