When choosing the right render for traditional or older buildings, lime render is often the best option. One of the main reasons for this is its breathability, which makes it ideal for maintaining the health of walls, especially in period properties. But what exactly makes lime render so breathable? In this blog, we’ll explore the natural composition of lime render, its capillarity, hygroscopicity, and porosity to better understand how it allows walls to breathe.

Natural composition of lime render

Lime render is made primarily from lime (calcium hydroxide), sand, and water. Unlike modern cement-based renders, lime render sets through carbonation, which reacts with carbon dioxide in the air, turning it back into calcium carbonate. This natural composition gives lime its breathability, as it lacks cement’s dense, impermeable qualities.

Lime is a naturally occurring material used in construction for centuries. It is inherently less rigid than cement, allowing for slight movements in a building’s structure without causing cracks or damage. The softness of lime also allows moisture to pass through the material, preventing the build-up of damp within the walls.

Capillarity

Capillarity is the process by which liquids, such as water, move through porous materials, often against the force of gravity. In lime render, the capillary action allows moisture to migrate through the render and evaporate on the surface. Unlike impermeable materials, lime does not trap moisture within the wall but facilitates its movement, which helps maintain a dry and healthy structure.

When rainwater or moisture from the ground enters the wall, the lime render enables it to move through the structure and escape. This prevents issues such as damp, condensation, and mould growth, common problems in buildings with non-breathable renders.

Hygroscopicity

Lime render also exhibits hygroscopicity, which means it can absorb and release moisture from the air. This is particularly important in fluctuating climates, where humidity levels can rise and fall. Lime’s hygroscopic nature ensures that the walls can absorb excess moisture during high humidity and release it when conditions are dry.

This balance helps to regulate indoor humidity levels, contributing to a healthier indoor environment. It also plays a key role in the longevity of the building by preventing moisture accumulation within the walls, which can lead to structural damage over time.

Porosity

Porosity refers to the small holes or voids in the material, which, in the case of lime render, create a network of pathways for water vapour to pass through. These microscopic pores in lime render allow it to act as a ‘breathable skin’ for buildings, as water vapour can move freely in and out of the wall system.

The high porosity level in lime render means that moisture is not trapped within the wall but is allowed to evaporate. This is especially important for older buildings, whose original materials were designed to work with breathable renders rather than modern, impermeable ones.

The importance of breathability in lime render

Breathability is crucial in buildings, especially in older or heritage properties originally built with lime-based materials. The natural composition, capillarity, hygroscopicity, and porosity of lime render work together to allow moisture to escape from the building, helping to preserve the structural integrity of the walls.

In contrast, modern cement-based renders can trap moisture, leading to issues like damp and frost damage over time. Lime render prevents this by promoting the natural movement of moisture, making it an ideal choice for buildings where maintaining a healthy and breathable environment is essential.

Lime and cork thermal render

As our Heritage Range expands, we can improve the insulating capabilities of lime render. We have recently been testing a lime and cork thermal render to ensure that the inherent breathability is maintained. Cork is an excellent insulator due to its structure; each cubic centimetre of cork contains 40 million cells, around 89% of that volume being air. As a result, cork has a thermal conductivity of 0.0055 W/m2K. The air pockets naturally occurring in cork trap heat and prevent the transfer of warm air from the interior of a building to the colder exterior. The insulating capabilities also work inversely, not allowing cold air to pass through.