Myths and mis-information regarding Condensation

This blog has been divided up into two sections:


  1. The Quick Answer
  2. The more in detail science involved.


The Quick Answer

Condensation is the vapour form of H2O condensing in and on cooler substances becoming moisture.

Air is in contact with everything in a room including all the surfaces: walls, floors, ceilings, doors and windows. If the air has a high level of humidity, termed Relative Humidity (RH) it will make everything damp as the air temperature cools down.


Turning the heating up so the air can hold more vapour - yes it is true, the warmer the air is the more vapour it can hold up to a point (saturation level). What happens when the heating is turned off though? Plenty of condensation forms.


There is ONLY one way to reduce condensation. That is to remove the H2O vapour from the air. There are two types of electric powered dehumidifiers:


  1. Condensing dehumidifiers
  2. Desiccant dehumidifiers


There are also non-electric dehumidifiers. Basically a container with holes in and a hygroscopic substance such as silica gel, or crystals. It is the same thing as in those small bags that are in the packing of expensive leather bags and boot boxes, cameras and other higher value items.


The products will be stored ready for dispatch in warehouses, shipping containers and store rooms. It is very unlikely any of them will be heated to around 22°C. More likely they will be cold. Cold - no air flow - humidity in the air when packed = a small amount of possible condensation. Metal can tarnish, surface oxidise. A small amount of humidity can enable mould to grow on leathers.


Are the box dehumifiers any good? Yes, partly. They must be dried out fully to be able to absorb more moisture.



How do people dry them out? They put them in the oven whist it is still hot after baking. Where does all the water go? Back into the air again. It is a bit like trying to empty a bath full of water with a teaspoon. Fill a glass, and when full empty it back into the bath.


The BEST method of reducing condensation is to use an electric dehumidifier. The machines that will hold about 2ltrs of water are the minimum to be effective. The smaller machines don't have the capacity to effectively reduce condensation in a typical house. The small machines may be okay for say a bedsit or single room.


The objective is to remove as much humidity from the air as possible down to about 40%RH. That means there is about 40% of the air is H2O. Most machines have a sensor that turns the machine off when it registers about 40%RH.


The other important thing is to leave the machine running 24/7 for the first week and regularly empty the holding tank. Many clients have reported that they had removed 6 to 8ltrs of water in the firs 24hrs. Then, as the week progressed, the amount reduced to about 1 ltr per 24hrs.


[There is more about what that means in the science part below].

The more in detail science involved answer

Condensation is basically water vapour that has slowed down to become liquid. In the air H2O can be termed 'steam', 'mist', 'fog' 'rain'. Yes, rain is just water vapour that has lost some of its energy and become water.


Air at a specific temperature and pressure can hold a specific amount of water vapour up to becoming 'saturated'. Water is the liquid state of the molecules of H2O. They can be also be as a vapour. Water hasn't got to be boiling to become a vapour. The oceans are not boiling, yet continually vaporise sea water.


In H2O molecules condensation forms when the electrons orbitting the nucleus of the atoms slow down. Temperature is a comparison of energy levels in the electrons. High energy levels produce more impacts = kinetic energy = thermal energy (heat) than lower energy levels. No energy levels = zero Kelvins (0K) = no thermal energy.


When vapour slows down it condenses to become a liquid - water. If it can soak into a material making it damp. If it cannot soak in it forms globules of water on the surface.


Cool surfaces and areas will cause the vapour to condense. That is why behind furniture, under furniture and in furniture such as drawers and cupboards moisture forms = dampness. The air that carries the energy cannot easy flow around narrow gaps, under furniture or into drawers and cupboards. They become cold zones.


The diffusing effect of H2O gas state still enters drawers, cupboards etc. It is driven by the thermal energy of the air in the room being at a higher pressure than the static air in the drawers and cupboards / wardrobes. That is why clothes become damp and can smell mouldy. Leather especially can support mould growth.


Humidity (H2O as vapour in the air) can only float in between the atoms of the air (mainly Nitrogen and Oxygen). As the air atoms and H2O molecules collide thermal energy is produced.

Conversion from:

  • Electromagnetic energy powering the electrons
  • Kinetic energy bouncing them off each other
  • Thermal energy is the product of Kinetic impacts.

Humid air is dry air with water molecules (vapour) in the spaces. H2O can be solid as ice, liquid as water or gas as vapour. The molecules are exactly the same with a ratio of 2 hydrogen atoms to 1 oxygen atom (H2O). They remain as singular molecules whilst in gas / vapour format.


Water can vaporise at temperatures well below boiling point (100°C). The evidence is water evaporates from the oceans and seas etc.


Leave a glass of water and eventually the water will evaporate. (If you want to find out more it will be in Construction Science Explained – www.buildabooks.co.uk out soon).


The molecule H2O remains the same whether it is water, ice or vapour. As vapour they can float in the spaces  between  the other gases in the air. The process is termed humidity.


A common myth-statement: ‘Steam is water vapour’ . Not true. Steam is 'condensation' suspended in the air. Vapour is in the gas state and is not visible.


Molecules are more than one atom. They can be the same type or different types termed compounds. Atoms have weight termed 'Atomic weight' that is a comparison with a carbon atom. They have a mass that can be attracted by a greater mass. The greater the mass the stronger the attraction. (Similar to weight).


Weight as a vertical force perpendicular to the Earth's surface is commonly referred to as 'gravity'. However, substances with large numbers of atoms attract substances with lessor numbers of atoms. The force is perpendicular to the centre of the larger substance. It can be in any direction.

The Science Museum in London had a demonstration of the force in action between two steel balls suspended on steel wires down a stair well.


The two steel balls attracted each other and the force could be measured by comparing the distance between the cable at the ceiling and between the suspended balls]. The force was at right angles to Earth's gravity.

/The planet Earth is made up of an enormous number of atoms. The combined attraction (pulling force) we call ‘gravity’. It’s a bit like a very large magnet will attract a smaller item such as a nail. The nail, if magnetised will attract a smaller pin, and so it goes on down the sizes.


Gravity however is a strong attractive force that can go through most things and still attract. It can go through the tallest buildings and still have an effect on an aircraft flying at 35,000 feet in the air. So logically it also has an attractive force on every atom be it in a solid (the aircraft), a liquid (rain comes downwards) and the gases.


If it didn’t have any effect on the gases they would all have gone off into outer space. Irrefutable evidence: the higher the altitude the less molecules. We state that the air is ‘thinner’. What it actually means there are less molecules, less nitrogen, less oxygen etc. and a lot more space.


So what has all this proved?

Air can be dry – no water vapour in it. Very rare though, most air has some water vapour in it even in the hottest driest deserts.


Atoms have a mass. (Mainly the Protons and Neutrons. Electrons are so small they are considered as having no mass). The more mass in the same volume means there are more particles for gravity to act on. We term that ‘weight’. If you could take say a gold bar that has a weight on Earth of 6kg then send it to the Moon it would then only weigh about 1kg. The gold bar hasn’t changed, it still has exactly the same number of atoms in it. The only difference is the attraction force ‘gravity’. The Moon is that much smaller, so less atoms to to do the attraction.

If air is heated, the atoms and molecules will become more energised. They will move more quickly and collide with each other. The collisions tend to end up as the molecules bouncing off and going in a different direction to their next collision. All the time that is happening gravity is trying to pull them down to the lowest point and slows them down. The result is less collisions = less heat generated = the air cools down.


If a cubic metre of air is considered, there will be a given number of air molecules in it at a specific temperature. If the molecules are given more energy (electromagnetic energy) they become more energised and travel faster and further. There will be fewer molecules in the cubic metre as the temperature rises. Fewer molecules means the pull of gravity has less effect. The cooler air will push the warmer less dense air upwards. Warm air rises and that is the reason why.


Evidence: the attractive force of gravity can be compared to magnetic attractive force. The closer together metal particles are the easier the magnet can attract them (ignore friction).


Warm air is less dense than cooler air as there are fewer molecules for gravity to act on. Warm air therefore is displaced by cooler air that has more molecules in the given volume. The end result is warmer air will be pushed upwards by the cooler air trying to get as low as possible.


That was considering dry air only; Nitrogen, oxygen and trace gases argon and carbon dioxide.


Now add water vapour.

Water vapour (H2O) has mass so will be attracted by gravity. The molecules fill the spaces between the other atoms by diffusion. Eventually the molecules fill the spaces and the air becomes saturated at 100%RH.


Nitrogen and Oxygen are both diatomic atoms. That means they naturally float around bonded in pairs. (N2 & O2). The atomic mass therefore is doubled making Nitrogen about 28 amu and Oxygen about 32 amu. When compared with the H2O molecule they are both 'heavier'.


In theory, that suggests that humid air rises as stated on various Websites. However, there are other factors to consider: Temperature and pressure. The atoms and molecules are not stationary but floating around in space. They are bouncing off each other at phenominal speeds. When the H2O are highly charged they will bounce around and be pushed upwards by the cooler air by displacement.


As the energy reduces by transfer to cooler surfaces, plus gravitational pull, the molecules will lose momentum and the atomic attraction will pull them closer together. The consequence is the vapour becomes liquid = condensation.


Although the molecules have not changed their atomic structure they have slowed down, therefore less collisons and less thermal output. H2O molecules naturally try to bond with other H2O molecules in preference to other atoms. In liquid form they will also try to bond with other atoms and molecules that are polar such as silica.

The gravitational pull on Nitrogen is greater than hydrogen. Hydrogen has 1 Proton and 1 electron (no Neutrons). Whereas, Nitrogen has 7 Protons, 7 Neutrons and 7 Electrons.


The number of Protons and Neutrons govern the atomic attraction indicated as Atomic Mass.


An atom of Nitrogen has an atomic mass of about 14 amu. In contrast, Oxygen is about 16 amu and Hydrogen about 1 amu.


All three atoms are diatomic, meaning they are naturally bonded in pairs.

However, as a molecule of H2O the atoms are singular.

  • Hydrogen is about 1 amu x 2 atoms = 2amu
  • Oxygen is about 16 amu x 1 = 16amu


H2O is therefore about 18amu.


In contrast:

  • Nitrogen x 2 = 28amu
  • Oxygen x 2 = 32amu

It is said that humid air ‘rises’ but it is actually it is being  pushed upwards by the cooler more dense air. (Nothing can go upwards without energy being used, not even atoms).


So dry air is more dense than humid air at the same temperature. The warmer the air the fewer number of molecules therefore they get pushed upwards (rise). *[Avogadro's theory relates to the number of particles in a given volume (Mole). Particles are the smallest matter, they make up the atoms. The theory suggests that a mole of any substance at the same temperature and pressure will contain the same number of particles].


That means there must be less Nitrogen and Oxygen plus trace gases in humid air. The H2O molecules not only have filled the spaces between the other atoms, they have displaced them in the process.


The air in the shower will be saturated with water vapour. At its maximum temperature the humid air will be pushed upwards by the cooler air. If an extractor in the ceiling or high up the wall is close to the shower outlet it will pull the humid air out of the room.


The hot water will be flowing over the person and surfaces of the materials around the shower (walls, shower curtains, panels etc.). Those surfaces will be cooler than the hot water but as there is so much water any condensation will blend in with the water.


The surfaces will warm up via heat transfer from the hot water. They will also cool the water down in the process. (example; water at 40°C and surface at 20°C will become about 30°C)


The air around the shower area will be cooler still and condensing the vapour into steam. The visible condensation in the air. Steam will further condense on cooler non-porous surfaces to form globules of water. The polar molecules of the surface material attract the water until there are too many water molecules to support and the globule is pulled downward. Commonly known as ‘streaming’.

Naturally the humid air in the bathroom will be warmer than the ambient air temperature and therefore be pushed upwards as it is less dense.


The cooler air has more density and displaces the less dense air.

Having a clear gap about 10mm below the door and a working extractor fan, the humid air will be pulled out of the room.


It is beneficial to have the fan operating BEFORE the shower is used. There will be an air flow formed that will reduce the humid air from filling the room.

The bathroom / en-suite door if closed is keeping much of the humid air in that room. The extractor is pulling the humid air out at a design speed (we will come back to that in a moment). Air, humid or dry cannot be pulled out of a room unless other air is going in to replace it. (otherwise you would end up with a vacuum).


'Mind the gap!' The gap under the door

If the gap under the door is at least 10mm (3/8”) it will let enough air in to the bathroom / en-suite to replace the humid air being pulled out.


For those who are interested: If the door is 762mm wide (2’-6”) then the gap area will be 7,620mm². A pipe 100mm diameter will have a cross sectional area of 7,854mm² which is slightly more meaning there will be a greater negative pressure but not enough to worry about.


The gap is normally at the lowest level in the room where the air is at its coolest and most dense. It cannot escape under the door as there is a negative pressure (suction) as the extractor is pulling the air out of the room.


Why do I need to know all that?

Simply having an efficient extractor running before the shower goes on (or running a hot bath) will start the negative pressure so any humid air will be going out of the extractor outlet as opposed to building up in the room.


Opening a window – Hmmm. 

Based on the science it isn’t going to do much. The main objective is to pull the humid part of the air out of the building and opening a window will not do that.


Having an extractor going with the window open isn’t much good either. It is possible that the RH during the hotter days or wetter days is greater than 60%RH. Opening a window will hardly help to reduce much water vapour by diffusion.


The window is unlikely to be at floor level so all that cooler humid air will still be there. Open the door and it can then roll out into the rest of your home.


[On the day of writing it is a very hot and dry day in June 2025. The outside relative humidity level is 56%RH with a temperature of 27°C. There is also a strong breeze. The RH in the house is also 56%RH as the H2O molecules diffuse into the house air].



Bottom line: An efficient extractor operating before the humidity rises in the room. Door shut and the drier warm air from the rest of your home is used to replace the very humid air being extracted.


Extractor continues to operate for about 30mins after the shower has stopped. (Door still closed or just very slightly open). That will remove the very humid air that is formed by water evaporation. By wiping up as much as possible any water or condensation left on the hard surfaces around the shower it will reduce further humidity.


An experiment: Use a highly absorbant cloth to wipe up as much condensation and water from all the hard surfaces including the bath or shower tray. Wring the cloth out into a container to catch all the water. That would have all had to evaporate into the air. Leaving the extractor on for 30 mins will help to remove the rest of the water you haven’t mopped up. (Shower curtains tend to hold a lot of water on the surfaces).


1cc of water is equal to about 1 gram at 20°C. To enable easier comparison between moisture content and relative humidity a psychrometric chart has been developed. A graph-like chart enables comparison between grams of moisture per kg of dry air. For example: at 20°C 40%RH will contain nearly 6 grams of water per kg. Increase the moisture by about 3g to about 9g of moisture and the RH increases to 60%RH.


Compare the room temperature at a more realistic 25°C:

40%RH will be about 8g of moisture. 60%RH will be 12g and at 80%RH about 16g.

[Air mass: about 1.2kg per cubic metre]


The comparisons indicate as the air temperature increases more humidity can be held in dry air. In contrast, as the temperature falls, less vapour can be held = condensation. It is therefore important that as much of the hottest water vapour is removed by the extractor before it cools down.

 

Extractor efficiency

The manufacturers provide figures showing how many cubic metres of air an extractor fan is capable of pulling down a tube per second, per minute, or per hour. Some extractor fans have an adjustable amperage meaning the amount of energy used to turn the blades of the fan. More revolutions normally equals more of an air flow.


The humidity will fill the whole volume of the room and all volumes of containers in it such as drawers, cupboards, wardrobes and that includes the waste tubes in the shower tray, bath, basin and inside the pan and water closet of the WC.

Conclusions:

  • Opening a window doesn't actually reduce much condensation. It all depends upon what the relative humidity outside and temperature actually is.


  • Relative humidity should be about 40 to 45%RH ideally.  The occasional peak to 80%RH may occur directly after bathing, but should fall quickly when an extractor is pulling the humid air out of the home. [today - 15th August 2025; London air has a Relative Humidity recorded as 79.9%RH]


  • Humid air is less dense than dry air at the same temperature and pressure. However, energy levels within the molecules also has a factor. Humidity in air diffuses, therefore it also fills spaces right down to floor level. It is often mis-diagnosed as 'rising damp'.


  • To reduce condensation the water vapour must be removed from the home.