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Relative Humidity – Simply Explained with Useful FAQs

Relative Humidity

What is relative humidity in simple words?

Here is our definition of Relative Humidity (RH), explained – “the amount of water moisture / vapour in the air, expressed as a percentage, relative to what that air is capable of holding at a given temperature

Put even more simply, it is a measure of the amount of moisture in the air. The percentage and temperature part give the name to the ‘relative’ part of the above explanation is important because as temperature changes, the relative humidity changes (all other things being equal). Specifically, as temperature rises, relative humidity falls.

Also, the importance of the ‘percentage’ and ‘capable of holding’ parts combined specifically are that air has a capacity of moisture it can hold within. Here is an explanation using a sponge (instead of air) to help explain this practically…

Relative humidity example…

  • Imagine you had big sponge which was 1m x 1m x 1m (one meter cubed)
  • That same space, if it was 100% water would be 1000 litres
  • Now imagine the sponge at ‘maximum capacity’ could hold 600 litres
  • This would be the comment about ‘capable of holding’ from the definition
  • The sponge actually has 300 litres of water in it
  • That is 50% of its capacity so Relative Humidity = 50%
  • If it had 200 litres it would be Relative Humidity = 33.3%
  • If it was at the max of 600, Relative Humidity = 100%

How does temperature affect relative humidity?

  • As room temperature rises air can hold more water. So using our example above…
  • Lets say as we warm the sponge up (or the air in the real world) it’s capacity is more
  • So imagine our sponge can how hold 900 litres instead
  • It holding 300 litres would not be 50%, it would now be 33.3%
  • The opposite occurs to, as temperature falls, air’s capacity to hold water falls
  • That means as temperature falls, relative humidity rises with the same water in it
  • How? Well imagine it can now hold only 400 litres (capacity) instead of 600 litres
  • Now, the 300 litres in it represents 75% humidity.
  • So you can see in our three examples of normal, high and low temperature…
  • The absolute amount of water (absolute humidity!) of 300 litres is the same, but…
  • Relative humidity for each was 50%, then 33.3% and 75% respectively
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What is absolute humidity?

  • Absolute humidity is the absolute amount of water vapour in the air
  • This is usually expressed as Grams Per Kilo (GPK) of air
  • Grams of moisture / water that is present in one kilo of air

Both absolute humidity and relative humidity have a role to play in understanding the moisture conditions in the air, including with water leaks (which can impact that too).

Why is relative humidity important?

There are a number of reasons that relative humidity is important, including:

  • It is a good measure of the level of moisture / water vapour in the air
  • This in turn can affect lots of things in our everyday life, depending on the level
  • If humidity is high, we can find it harder to keep our bodies cool
  • The same applies to animals too
  • Even our hair is affected becoming more frizzy with humidity!
  • In the past people even used human hair to measure humidity
  • We can ‘feel’ humidity in the air, like when a storm is coming (it feels heavy)
  • As you might expect it is used in weather forecasting too
  • As we are sure you have seen, high humidity can be uncomfortable
  • It can not only affect us as humans but also the buildings we live in
  • It can affect things like condensation on windows, mould etc
  • These in turn can have health implications, including air quality
  • High humidity can be a possible indication of a leak in your property
  • Humidity in a property will be higher after a flood for example

Needless to say as experts in finding water leaks and water damage restoration, we take measurements of relative humidity on a regular basis in our work. For example, when we are drying a property after a water leak or flood, we want to make sure that we are getting the (raised from the incident) relative humidity levels back to normal, safe and stabilised levels.

We have a really useful guide with examples of how this is a measurement used on many of the professional damp meter readings that we use in our work.

Other FAQs about relative humidity…

What are dehumidifiers?

Dehumidifiers are devices (usually electric powered) that reduce the humidity in the rooms they are placed, by removing moisture from the air. They are usually either refrigerant dehumidifiers or desiccant dehumidifiers. We have more about how different types of dehumidifier work on our water damage page. We have another article about silica gel, found in desiccant dehumidifiers.

What is the difference between humidity and relative humidity?

Sometimes people use those different terms to refer to the same thing. That is that when people say humidity, they actually are referring to relative humidity which we explained above. Another measure of humidity is absolute humidity which we also explained.

How is relative humidity calculated?

There are two answers to this really. The simple one is in our definition and explanation at the top of this page. That is – “the amount of water moisture / vapour in the air, expressed as a percentage, relative to what that air is capable of holding at a given temperature”. However the more complicated answer is that it requires a number of other fairly complex measurements, which is why hygrometers are used. This Wikipedia page on humidity gives more detail if you are interested!

What is a comfortable relative humidity?

This can vary on the person but, usually, relative humidity between 30% and 50% is considered by many to be ‘comfortable’ and when people are putting HVAC (Heating Ventilation and Air Conditioning) systems in to control the air in their property, that’s typically the range they target. PIV units can help with this too.

What is the maximum humidity possible?

The maximum level of humidity is 100%. Think of it like a bucket, because of how the measurement is defined (as a percentage of what it is capable of holding) you cannot fill it to more than 100%.

What is a high relative humidity?

Humidity above 50% is often considered as high relative humidity for people, with anything above 70% being very high humidity.

What does 100% humidity feel like?

At 100% the air is saturated with water – the air cannot hold any more water vapour in it. And so, it can be very uncomfortable, sometimes depending on the temperature. Think of it like fog on a cold day or a sauna on a hot day.

What is a relative humidity meter?

A relative humidity meter is simply a device used to measure the humidity in the air. They can be used in various situations, including in the home or in industry. They are also used in moisture meters that leak detection specialists use.

What is the ideal humidity in a house?

The ideal, comfortable and safe level of humidity in a house is usually between 30% and 50% relative humidity. Where in that range people prefer may be down to personal preference but 40% is considered by many as a good level to aim for. We look at this in more detail in our article about humidity in houses.

How to lower humidity in my house?

There are many ways to lower humidity in the house including using dehumidifiers, air conditioning, having good ventilation and extractors (especially in bathrooms and kitchens), drying clothes outdoors if possible, avoiding certain plants that can affect it and of course, making sure you do not have a water leak which can increase moisture and humidity levels.

What humidity does mould grow at?

This can vary but mould generally grows more aggressively in the 65% to 85% range in a property, it is more common in areas of high moisture and condensation including bathrooms especially. We have a full guide to mould.

What is dew point?

Dew point is most commonly explained as the temperature at which the air is cooled to in order for it to become saturated with moisture / water vapour. It plays a big part in condensation and is why you get condensation on a cold window, condensation on pipes or surface in a bathroom – which is why you see it more in winter than summer. Infrared thermometers can help with this.

What has an effect on relative humidity?

Relative humidity can be affected by many things, especially in the home. Including the general climate / weather conditions (especially if windows are open or not), the temperature, the presence of things adding moisture to the air – this includes showers, baths, cooking, kettles, laundry and of course, water leaks. It will also be affected by items put in place to control humidity (or that can impact it) like dehumidifiers, air conditioning, ventilation, heating etc.

Is there more humidity from a bath or shower?

We have done an experiment on this and written an article comparing bath vs shower condensation which gives some very interesting results!

The existence of humidity can cause condensation in loft spaces too, which is common above bathrooms and shower cubicles, especially if there is a fault with extractor fans. We have a guide to where bathroom water leaks most often occur.

Water Leak in Bathroom
Water Leak in Bathroom – leak in floor, wall or ceiling