What are Breathable Paints
A breathable paint is one that allows water vapour to travel or permeate through it. Paints with high rates of permeability allow water to evaporate away rapidly while those with a low permeability are likely to trap moisture. These values are measure as an Sd value. An Sd value will measure how much of a barrier the material is to water vapour so therefore a lower figure is more breathable.Typical vapour permeable or breathable paints have an Sd value of around 0.02m to 0.5m.
This would mean that an Sd value of 0.3 is the equivalent to 30cm of air barrier. Non-vapour permeable paint with an Sd value of e.g. 3 is the equivalent of 300cm of an air barrier, making it unable to breathe.
Paints with high Sd values have a detrimental effect on the surface they are applied to as they do not allow it to breathe. Most non breathable paints will not usually disclose their Sd values as they can be much higher than 3. Paints with an Sd value of <0.1 offer adequate breathability to cope with most applications.
Applying Breathable Paints
There is no real difference in using breathable paints to conventional emulsions although we would always advise following the manufacturers’ guidance as each product will differ from the next. Preparation is key and by making sure that the walls are dry, sound and any existing petrochemical paints are removed where possible will help to ensure that your breathable paint acts in the desired way and allows the free movement of any water vapour through your building like a natural dehumidifying system.
Why use Breathable Paints
Damp is commonly found in older buildings. It was believed that the only way to eradicate this issue was to waterproof the entire building or by injecting damp proof courses with the aim to stop water penetrating the building. Whilst this offered a temporary solution it was and is still not the answer, with the majority of these applications ultimately failing.
One of the main reasons for this failure is that a building can undergo significant movement, both structurally and thermally (heat). Once a crack appears water can penetrate the crack and be held within the wall behind the non-breathable or waterproof coating. (Cement renders are also largely impermeable).
A build-up of moisture can lead to damp within the walls which may cause ‘blistering’ and ‘bubbling’ of the paint where the water is trying to escape. In more serious cases the render may be ‘blown’ or forced off by the pressure of the trapped water.
Water that is trapped withina wall can lead to serious deterioration of the building fabric. Any non-breathable paint applied to the building will act like a film around its surface. If you imagine wrapping your walls and ceilings in cling film, this will stop the moisture from going in and out but will trap it and the water will build up within the surface. This is where the term “film-forming” paint comes from as it seals the building and stops the building from breathing and allowing moisture out of the walls.