Dew Point Definition For Home Owners

1. Home
2. Condensation
3. Dew Point Definition

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.

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.

Effect of Humidity on 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.

Effect 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.

Conclusion:

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.

You might also like the following articles, or click here for the full site index.

• 

  Stop Bathroom Condensation Once and For All

  This is how I stopped my bathroom condensation, completely! - find out why bathroom condensation occurs and what I did to stop it once and for all.

• 

  How Condensation in the Attic can be Cured?

  Condensation in the attic can be a major problem. If left unchecked it can cause mould damp and rot. Here's how to stop it!

• 

  What Does Condensation Mean to Home Owners

  What does condensation mean to you, as a home owner? What it is, what causes it and what you can do to stop it?

• 

  How to Stop Black Mould

  Learn how to stop black mould in your home the right way.

• 

  Black Mould Removal - The Right Way!

  Getting black mould removal right sounds easy, but if you don't want it to come back there are a few things you really need to know.

• 

  Stopping Double Glazing Condensation

  Double glazing condensation is frustrating at best and damaging at worst, find out how to cure it with this simple guide.

• 

  Negative Pressure Ventilation in Your Home

  Can a negative pressure ventilation system help reduce condensation in your home and improve air quality. Get all the advantages and disadvantages before making your decision.

• 

  Humidity Sensing Fans For Bathrooms

  Humidity sensing fans are an essential upgrade from your standard bathroom extractor fan. Find out why I think all homes should have one.

• 

  How to Deal With Condensation in Your Home

  Learn how to deal with condensation in your home. Find out what causes it and what you can do to prevent it.

• 

  What is condensation in buildings?

  What is condensation in buildings? Find out why it occurs, what it is and how to get rid of it!

Share this page: What’s this?

FacebookXPinterestTumblrRedditWhatsAppMessenger

Enjoy this page? Share it with your friends. Here's how...

Would you prefer to share this page with others by linking to it?

1. Click on the HTML link code below.
2. Copy and paste it, adding a note of your own, into your blog, a Web page, forums, a blog comment, your Facebook account, or anywhere that someone would find this page valuable.