The Breathable Paint Guide

What is breathable paint?

Breathable paint allows water moisture or vapour to pass through microscopic pores in the surface of the paint. This is important because the underlying structure needs to be able to naturally regulate its moisture levels, as moisture will cause problems if it becomes trapped within the structure. However the paint is still weatherproof, because water as droplets are too large to pass in through the microscopic pores.

Breathability is beneficial to both the paint and the fabric of the underlying structure, as it means that water moisture is not trapped under the paint and against the structure. Trapped moisture can result in paint blistering and cracking, or, more significantly, problems with decay, damp, and mould.

Paint is classed as breathable when it has been tested to a standard requirement; meaning a certain volume of water in the form of a gas (moisture or vapour) can pass through it.

Why should I use breathable paint?

If your building is old and conservation is crucial - or if other breathable materials have been used in its construction, such as lime mortar, lime render, or lime plaster - then the use of breathable paint is essential. Old buildings, particularly, are highly susceptible to environmental damage from pollution, water or temperature.

Good quality breathable paints, such as Keim Royalan or Earthborn EcoPro products, will work to protect the underlying structure from any potential damage from the environment. This helps to reduce the chances of decay, damp or mould becoming issues later on.

When should I use breathable paint?

Below are some typical examples of when breathable paints should always be considered for use:

Older buildings without wall cavities/sufficient air gaps, or in damp environments.
On external and internal delicate masonry where conservation is a primary need.
Internal and external surfaces where lime mortar, plaster, or render has been used.
On a surface that has been treated for damp or mould. For example, a wall or ceiling.
If breathable paint or lime wash has been previously used.

In most buildings, protection from any water getting in or moisture being trapped in is of significant importance. But in older buildings specifically, this concern is due to the large amounts of water old masonry can easily absorb. And so, using breathable materials that work to avoid serious issues, such as damp, are necessary to the health of the building.

How can I tell if a paint is breathable?

It is useful to know how to ensure a paint is breathable, as "breathability" can be often used as a marketing term and be misleading. Fortunately, it is easy to check how breathable a paint is.

Initially, when choosing a breathable paint, it is important to take notice of one or two values that should be on either the paint container, or in the accompanying product data sheet. The values to look for is: the Steam Diffusion (Sd) value.

Sd values are important to check to ensure that the paint you are choosing conforms to breathability standards, as follows:

Sd Value:

is the measure of how breathable a paint is;
should be a value of between 0.01 and 0.1m;
the lower the number means more vapour can be diffused, making it more breathable;
a higher number indicates greater resistance to vapour diffusion, making it less breathable.

What types of breathable paints are there?

Exterior breathable paints (masonry, render)

Silicate mineral masonry paints

Silicate mineral paints are suitable for both internal and external use on highly absorbent mineral surfaces - such as porous stone, masonry, lime render and plaster. If applied correctly, masonry paint will protect delicate surfaces where damage from harsh external environmental conditions is a risk. For example, in the UK where very wet or cold weather can be common.

Silicate mineral masonry paint works by forming a bond with the underlying material's surface, unlike ordinary paint which dries as a film. Notably, the bonding process needs a mineral surface to work effectively. This is because the paint bonds chemically - a process called silicification - with minerals in the surface it is covering. These specialist masonry paints can also be used on surfaces with a glazed finish, such as tiles, glass, or ceramics.

A highly weatherproof solution, silicate mineral masonry paints are best suited to protecting old and delicate substrates or lime-based construction materials, where maintaining breathability is key. Silicate mineral paints are UV resistant and will not fade when exposed to strong direct sunlight.

Hydraulic lime wash - also commonly known as Whitewash.

A traditional wash and not technically a paint, lime wash can be used in the same way as a breathable paint. The main benefits of hydraulic lime wash over other breathable coatings, are its hydraulic characteristics when used in wet or damp environments.

Hydraulic lime wash, however, can also be used on interior surfaces. But its ability to dry in the presence of water, makes it much better suited to exterior uses that are predominantly exposed to wet or damp conditions.

Lime wash is also considerably cheaper than other breathable paints, as there are no solvents or chemicals added to it during manufacture. However, because it is 100% natural and breathable, lime wash is not as durable as modern breathable paints. And so, annual or more frequent applications may be needed to maintain its weather resistance and colour.

Interior breathable paints (plaster, masonry)

Claypaint

As the name suggests, claypaint is made from various types of naturally occurring clays, such as kaolin. This produces a natural, eco-friendly and highly breathable interior paint that has very low levels of odours and fumes (VOCs). A thick and creamy emulsion, claypaint has an ultra-matt finish, is hard wearing, and requires fewer coats than most other emulsions.

Clay-based paints are best suited to covering lime plaster, or where damp has been an issue. Claypaints are also available in a range of contemporary colours.

Non-hydraulic lime wash - also commonly known as Whitewash.

A traditional wash and not technically a paint, lime wash is used in the same way as a breathable paint. Non-hydraulic lime wash's main benefits are that it is 100% natural and breathable, and is sympathetic to delicate surfaces. Although it is slow to dry, due to it being non-hydraulic, traditional lime wash is best suited for internal use on delicate masonry or lime plaster.

Lime wash is also considerably cheaper than other breathable paints, as there are no solvents or chemicals added to it during manufacture. However, because it is 100% natural and breathable, lime wash is not as durable as modern breathable paints. And so more regular applications may be needed to maintain its colour - or weather resistance if used outside.

Why not just use waterproof paint?

Unlike breathable paint, modern plastic-based waterproof paint provides an almost impermeable film on the surface that it is covering. And because this film mostly blocks any form of water from passing in or out through it, moisture is trapped under the paint.

If a structure is unable to 'breathe', then trapped moisture can lead to expensive issues to repair such as mould or damp - which can eventually damage the structure if left untreated. An early indicator of these issues can be the cracking or blistering of the waterproof paint itself.

How to prepare a surface before using breathable paint.

Unlike plastic-based paints that form a surface-film, breathable paints are surface-bonding. So the paint needs to be in direct contact with the underlying surface, to be able to create a good bond. And a good bond means better breathability.

There are a number of basic steps to be carried out to ensure the surface is ready, so the paint or lime wash can bond well with it:

The surface should be dry, free from debris, dust, organic growths, and oils.
Remove any unsound or loose material and repair any damage, including cracks or holes.
Any old non-mineral and water-soluble coats must also be completely removed. This can be done with a clean stiff brush, light sanding, or for some stubborn areas a pressure washer can be used.
Prior to priming or painting, rinse the surface with clean water and allow to dry thoroughly.
All non-suitable or adjacent surfaces that are not to be painted, should be masked off or well covered.
Any primer coats should be left to completely dry, as per the manufacturer's instructions, before applying the final coats.

Tips on using breathable masonry paints.

Below are a useful set of points to remember before using a breathable paint:

Check the weather forecast to ensure that rain is not imminent and temperatures are expected to remain within the paint's minimum/maximum temperature application range.
High humidity can also affect how well the paint will adhere.
Ensure that the surface, and any primer coat, is fully dry and that the surface temperature is above the individual paint's minimum temperature for application.
Textured or highly absorbent surfaces will use much more paint than smooth surfaces.
Paint can be applied using a brush, roller or spray gun.
Always allow each coat to dry thoroughly for the recommended time before applying further coats. Just because a coat looks or feels dry, does not mean it is.
Remember to always follow the instructions provided for each individual product.