Water Vapour Barrier
May be defined as "a layer intended to prevent water vapour diffusion". The vapour barrier may be a relatively thin layer of impermeable material which is applied to the outer surface or warm side of the insulation, e.g. liquid-applied barrier coating, PIB sheeting, reinforced aluminium foil or alternative barrier.
Alternatively the vapour barrier may be "built in" to the material, as in closed cell structures.
However, it is important to realise that a closed cell structure does not, in itself, guarantee a vapour barrier which is sufficient to meet all performance requirements. We must also consider the nature of the insulation material and ensure that the integral vapour barrier is associated with a very high resistance to water vapour transmission. Armaflex is a closed cell material so that the vapour barrier is "built-in" to the material. It also possesses a very high resistance to water vapour so that in almost all installations there is no requirement for an additional vapour barrier.
With surface vapour barriers it is very important to ensure that they are properly installed to give complete protection, even a small tear or puncture will be sufficient to render the total vapour barrier ineffective.
For the insulation of low temperature systems the use of a proper vapour barrier is a technical requirement to provide long term efficiency of the system. It may also be necessary to have some additional protection for the vapour barrier by means of a weather-barrier. Armafinish FR Paint will satisfy this requirement when applied as recommended.
Vapour barriers are usually expressed as a minimum permeance (defined below). Thus BS 5422 : 1990 para 14.2 recommends that “the permeance of the material (as a vapour barrier)” shall not exceed the following values:
 | The partial pressures for water vapour will be:
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| Cold water supplies | 0.05 g / (s.MN) |
| Chilled water supplies | 0.015 g / (s.MN) |
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The use of less than 9 mm Class O Armaflex would satisfy these requirements. |
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