Your Questions Answered "Where does all the water vapour go?"
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"Where does all the water vapour go?"
As we go about our normal day to day activities in our properties we unavoidably introduce water vapour into the atmosphere and by so doing we increase what is termed the vapour pressure within our homes. So if it is continuously being introduced where does it all go?At most times of the year the vapour pressure inside our homes is higher than the vapour pressure outside. The higher vapour pressure inside will always seek to equalise its pressure with the low pressure area outside by escaping out through the structure to the lower pressure area. It does this by passing through openings such as air vents and gaps as well as through porous materials such as plaster, bricks and masonry.
However water vapour is not able to move significantly through materials such as polythene, glass, or aluminium foil and its movement is restricted by materials such as vinyl wall coverings. The inability of some materials to allow the passage of water vapour is important. Such materials are termed vapour barriers or vapour checks and when used correctly they overcome problems that the uncontrolled passage of water vapour through a structure could cause.
In the table below we have listed common building materials and their relative vapour resistance. The higher the value the more impervious the material is to the passage of water vapour.
| Material | Relative Vapour Resistance |
| Mineral Wool Brickwork Plaster Timber Plasterboard Gloss Paint Film Polythene Sheet Aluminium Foil |
Negligible
1 2 2 2 20,000 45,000 1,600,000 |
From these figures it is clear to see why water vapour is able to distribute itself quickly and uniformally throughout a property irrespective of where in the property the water vapour is actually generated. So where does all the water vapour go? The figures illustrate how water vapour is able to pass through walls with relative ease to areas of lower vapour pressure which is usually the outside in cooler climates.
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