Understanding Breathability: Why the Term Is So Controversial in Building Conservation

In historic building retrofits and increasingly in new builds, the word “breathable” is used everywhere. It evokes images of walls that “breathe,” freely allowing moisture to escape and preventing damp problems. Yet technically, the term is often misleading and over-simplified. True moisture management in buildings is more nuanced and understanding how water actually moves through traditional materials is essential for successful, long-lasting conservation.

The Movement of Moisture in Walls

Imagine a towel left out in the rain. It doesn’t just repel water or let it sit on the surface, it absorbs it, draws it through its fibres by capillary action, spreads it out, and then releases it back into the air as it dries. Traditional solid walls work in a very similar way. They absorb moisture (from rain, ground water, or humid air), transport it through the material, and allow it to evaporate over time. This dynamic process helps keep walls drier overall and is fundamental to the performance of historic masonry.

What is often called “rising damp” is frequently misdiagnosed. Many apparent cases are actually penetrating damp, condensation, or the effects of poor drainage and incompatible modern materials. The underlying mechanism, however, is still capillary action combined with evaporation, the very process that breathable systems are designed to support.

Vapour Permeable, Hygroscopic, and Capillary-Active Materials

Three properties matter when it comes to moisture movement. Products labelled “breathable” often focus on just one:

• Vapour Permeable: These materials allow water vapour (moisture in gas form) to pass through them by diffusion. This is helpful for drying, but vapour is rarely the main form of moisture in thick historic walls, so vapour permeability alone is rarely sufficient.
• Hygroscopic: Materials that actively absorb and release moisture from the surrounding air in response to changes in relative humidity. Natural materials with this property act as humidity buffers, they help stabilise indoor conditions, reduce the risk of surface condensation, and protect both the building fabric and occupant health.
• Capillary-Active: Materials with a pore structure that can absorb, transport, and redistribute liquid water through tiny capillaries (wicking). Traditional brick, stone, and lime mortars excel here: they draw in liquid moisture and allow it to move and evaporate safely rather than accumulate.

Many modern “breathable” paints, renders, or coatings only address vapour permeability. In historic walls, where liquid water from driving rain or minor ground moisture is common, this can be inadequate, or even counterproductive.

What True Breathability Really Means

Genuine breathability is the ability of a wall system to handle moisture in all its forms, liquid water as well as vapour, without trapping it or causing damage.

Capillary-active materials, such as non-hydraulic lime mortars, plasters, and renders, allow walls to wick and redistribute liquid moisture naturally from inside to outside. Hygroscopic materials buffer humidity swings and help prevent condensation when temperatures drop below the dew point. When vapour permeability, hygroscopicity, and capillarity work together, the wall behaves like that metaphorical towel: it absorbs, transports, redistributes, and releases moisture efficiently, maintaining a healthier equilibrium.

The Risks of Getting It Wrong

Using modern, non-hygroscopic, or low-permeability materials (such as many acrylic paints, cement-based renders, or certain synthetic coatings) can block natural drying pathways. This often leads to trapped moisture, condensation, mould growth, and accelerated decay of the original fabric. In worst cases, it turns a “breathing” historic wall into a sealed, damp environment.

Making Better Choices for Heritage Buildings

When repairing, retrofitting, or insulating old buildings, prioritise materials that support the full moisture cycle. Look beyond the “breathable” label and ask specific questions:

• Is the material capillary-active (able to handle and redistribute liquid water)?
• Is it hygroscopic (does it buffer humidity effectively)?
• Is it sufficiently vapour permeable while remaining compatible with the existing fabric?

Traditional lime-based systems (especially non-hydraulic or feebly hydraulic limes), along with certain natural insulations like wood fibre, often perform best because they combine all three properties. Always consider the building as a whole: good maintenance (gutters, pointing, drainage) frequently matters more than any single material choice.

By respecting how historic walls manage moisture, we can extend their life, improve indoor comfort, and avoid costly future problems.