One of the worst things you can do to your home when applying any exterior coating or cladding is use any product that has a poor vapour permeability rating ie is not very breathable.

Water Vapour Permeability Explained

In laymans terms a water vapour permeability test measures the rate at which a solid material lets air pass through from one side to the other. In terms of the walls of a house this is crucial as the warmer internal  air contains  more moisture vapour than the colder outside air and this imbalance forces the more moist internal air to pass through the walls to the outside cooler air.

This natural balancing of  air pressure is constant and is one of the Laws of Thermal Dynamics. People tend to explain it by way of pointing out that warm air/heat will always be attracted or move towards cooler air such as when we have a shower and then open the bathroom window and watch the moisture-laden  air rush out the window to meet the colder external air.

Trapped Moisture = Damp

If any unsuitable material blocks or restricts this natural flow it will lead to a build up of vapour, a cooling of the air and the vapour will then condense into liquid ie water or as we usually refer to this phenomenon- DAMP .

The most obvious signs of this are surface condensation leading to asthma- causing black mould spores or white furry deposits. The cure for this is good ventilation, a consistent heating level and most importantly a good level of wall insulation.

However, the most structurally harmful result of trapped moisture is hidden from view as it occurs within the fabric of the wall.

Interstitial Damp

Aside from poor insulation, the main cause of  interstitial damp or interstitial sweating, is a build-up of moisture vapour as a result of the outer surface of the wall having a vapour restrictive coating on it.

A brick wall for instance is assessed as having a breathability factor ( water vapour transmission rate for the techies ! ) of around3 to 5 using a British Standard test in which pressured air takes 5 millionths of a second to pass through a solid surface in simplified terms.

Using the same scientific testing methods a material needs to have a  ” breathability factor ” of below 25 to ensure no build up of trapped moisture occurs within the body of the wall.

If moisture vapour cannot pass through at a sufficiently fast rate it will build up, condense into liquid and begin to slowly rot any timber joists set into the wall for example.

Are Wethertex Coatings within the British Standard ?

Yes. All our coatings are well below the 25 maximum recommended limit and in fact our accredited wall coverings and exterior paints  range from 3.7 to 12.8 and rank as possibly the most breathable- friendly wall coverings in the world.

Surely all Masonry Paints & Coatings are Breathable ?

No. Unfortunately not all never paint again type products are within the British Standard and in fact one of the most popular and heavily promoted spray applied coatings has a vapour permeability rating in excess of 31 !! Imagine the danger this presents especially if you have a wall that is currently damp and needs to be able to dry out .

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Breathable Coatings & Damp Prevention

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