Home Condensation Explained Condensation 12 - “Where does all the water vapour go?”
Condensation 12 - “Where does all the water vapour go?” PDF Print E-mail

As we go about normal activities in our properties we unavoidably introduce more and more water vapour into the atmosphere and by so doing we increase what is referred to as 'vapour pressure' within our homes. So where does all the water vapour go?

To help illustrate part of the process the table below lists common building materials and their vapour resistivity. The higher the value the more impervious the material is to the passage of water vapour.



Vapour Resistivity (sMN)/gm

Open window














Polythene (1000 gauge)


Aluminium Foil


Glass sheet


More values can be found in BS 5250 and CIBSE Guide Section A10


In a property with open or partially closed doors it is clear to see that water vapour is able to distribute itself quickly throughout a property irrespective of where the water vapour is actually generated. The figures above illustrate how water vapour is able to pass with relative ease through walls and ceilings to areas of lower vapour pressure which is usually the outside in cooler climates.

Water vapour is not able to move significantly through materials such as polythene, glass, or aluminium foil and this inability to allow the passage of water vapour is important and useful. Some of these materials are termed vapour barriers or vapour checks and when used correctly they overcome certain problems that the uncontrolled passage of water vapour through a structure could cause.

The high pressure of water vapour gas in our homes will always move to areas of lower vapour pressure. Having first distributed itself from where the water vapour was generated around the inside of a property it will then move outside by escaping through the structure. To do this it will pass through openings such as air vents, gaps and porous materials such as plaster, bricks or masonry. However on its way to areas of low vapour pressure it will encounter other materials that are resistant to the passage of water vapour such as paint film, various wall papers, wall linings etc. all of which delay its progress whilst at the same time more and more water vapour is being generated by the life style of the property's occupants. It is for this reason why the extraction of water vapour at source (extractor fans) and ventilation are important to control of water vapour levels in our properties.

Contact Peter: Email - info@dampdecay.co.uk - Telephone: 015242 71794