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Relative humidity: Witchcraft explained

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We are increasingly becoming aware of the term relative humidity but it is a subject that is often misunderstood. It can certainly appear like Witchcraft.

This article (attempts to!) explain why relative humidity changes depending on the temperature of the air. and a little information on what levels are healthy.

If we know how much moisture is in the air we can understand how it affects damp related problems inside a home. This can be measured with a low cost humidity meter.

High humidity levels also increase energy costs.

I will be publishing a number of other articles on damp and condensation in late December, so do register for updates if you haven't already.

I have also just published an article here with some really practical tools on this subject. They will help you suss if opening a window will improve indoor humidity or worsen depending on what the weather is outside.


Relative humidity is all about how much water the air can hold

All air holds water in it, invisibly, in the form of a gas called water vapour.

Relative humidity tells us how 'full' of water the air is. When air is full and cannot hold any more water this means it is 100% 'full' and has a relative humidity of 100%.

When air cannot hold any more water it can very easily start to dump its water in the form of water droplets.

  • Outdoors we see the air dumping water as large droplets (rain), or tiny droplets that float in the air (fog).

  • Indoors we see it as 'steamy' air over a hot bath where the warm moist air above the bath water is "full" of water.

How much water air can HOLD depends on the temperature of the air.

  • Warmer air can hold more moisture - In summer when it is hot and muggy we sometimes get sudden huge downpours and thunderstorms. This hot air holds a LOT of water.

  • Colder air can hold less moisture - This is why we so often see dew on grass after a cold night. Moist air has cooled so much at night that it can hold no more water and dumps it onto cold surfaces such as grass.


In the home

This works in similar ways inside a home.

Cooler air in a home cannot hold as much water as warm air can. Cool air is more easily 'full' and is more likely to dump the water out, especially if it is very close to a cold surface.

We see this as condensation on windows.

A typical example:

It’s 4pm, the air in a home is at 18C and its relative humidity is 75%. This means the air is holding 75% of the water that it possibly can.

The heating then comes on and within an hour the temperature in the home is now 20C.

Now the air in the home is warmer it is able to hold more water. So its relative humidity might falls to 66% for example.

The amount of water in the air has remained the same, but because the air is now warmer it 'is less full' than it was when it was a little cooler at 18C.

Perhaps it might help to think of relative humidity as 'relatively full of water'. The air at 20C is only 66% full whereas at 18C it was 75% full.

Now it is 4am and very cold outside and some of this 20C air is in contact with the inside surfaces of the window pane.

  • Because it is so cold outside, the insides of the window might be very cold, at 13C for example.

  • Damp air from the room near this cold glass will be colder than the rest of the room and won't be able to hold all of its water.

  • It will now easily dump its water onto the glass – we now see this as liquid water condensation on the window pane.

This is why moisture in cool air indoors is a problem. It is 'fuller' than it would be if it was warm. It is therefore more likely to dump its air on any cold surface that it can find.


A real example of the above

The pictures below show two identical humidity meters. One by the window in my hall. The other a few feet away in the hall. You can see it is colder by the window than in the other one. Because of this the air by the window is 'fuller' at 66%. The slightly warmer air a few feet away is less full.

The weather outside was quite mild. In colder weather the air by the windowpane glass would be much colder. The relative humidity will be much higher than 66%. If it gets too cold the air won't be able to hold the water it contains and it will dump it onto the glass as condensation.

It also means if you have a humidity meter, my general recommendation is not to leave it on the window ledge as it can easily give figures that are alarmingly high. Leaving them on a window sill will also mean they will be heavily influenced by any sun coming through the window, draughts, or by a radiator you might have below the window.



So, it is so important to either:

  • Find ways of reducing the amount of moisture going into the air (to avoid it getting so full) or

  • Replacing the air with drier air from outside (see my other content on this here) with ventilation.

  • Sucking the moisture out of the air using dehumidifier products.

If anyone knows how the above could be explained better to get in touch! I've consumed several paracetamols trying to write this blog!


Ideal humidity levels indoors

Firstly, if you have a humidity meter, don't leave it on a windowsill. This is usually much colder than the rest of the room. This will mean it will show a much higher humidity figure than if it was away from the window. It is best to monitor and make decisions on what you do (to ventilate for example) if you measure the room humidity rather than the windowsill humidity.

If you want to know more about humidity in the home this web site is very good.

The chart below is a good reference. There are many variations that are similar. Some people with certain medical conditions may have slightly different needs.


Geeky type stuff for those interested

Why does high humidity affect energy costs?

  1. Humid damp air is harder to heat. Your heating will have to work harder

  2. If heating is used to reduce damp or condensation issues this is obviously expensive

  3. Humid air is less comfortable in a home, especially cool humid air. It makes is feel chillier so more likely to turn the heating on or turn it up


Quick links to the other main articles on this subject.

Other supporting articles on damp:


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All content on the site should be treated as information and not advice. You should take professional advice where appropriate to different site articles.


Mark Thompson

Get Energy Savvy - simple practical home energy efficiency information

1,554 views1 comment

1 Comment

Jan 04, 2023

Great article Mark.

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