Does Lime Mortar Absorb Water?

lime mortars used on a hamstone wall

Lime mortars and most other lime-building materials are extremely good. They not only help to protect the building and masonry of the building, but they can also help manage water within a building. A common question that often arises when lime mortars are used is; Do lime mortars absorb water?

Lime mortars have high porosity and permeability. These characteristics allow lime mortars to deal with moisture movements within a building without trapping water. Lime mortars will absorb water and release it when conditions allow without taking damage. Commonly, this is referred to as ‘breathability’. Using lime mortars will greatly assist in maintaining a dry building free from salts that can cause damage to masonry. In addition, the breathability of lime mortars helps avoid condensation issues in areas such as bathrooms and kitchens.

What Are The Benefits Of Using Breathable Mortars?

There are many benefits to using building materials that can absorb and release water without causing damage like lime mortars. It may seem counterintuitive that a material that can absorb water is better for building. But this is the case for many old and new buildings alike. Let’s take a deeper look at the benefits of lime mortars’ ability to absorb water.

They Don’t Trap Water In The Buildings Structure

In modern construction, mortars and plaster are usually waterproof. Usually, this isn’t an issue as modern houses are built in a manner that mostly prevents any water ingress. Because of this, however, any small leaks or cracks can cause major issues. When all the building materials are waterproof, any water that manages to worm its way in can’t always escape. This can cause trapped water issues.

Trapped water can break down building materials over time. Even hard masonry can be damaged. In colder climates, trapped water can be an even bigger issue. Repeated freeze and thaw cycles can cause cracks to get larger over time. Eventually, this will completely break down stone, cement, render and other very hard materials. So how do lime mortars prevent these issues?

Because lime mortars can absorb water, they can act like a wick. Trapped water can be absorbed from inside the wall by lime mortars. Once it’s absorbed, the water will slowly move to the surface of the lime mortar, where it can freely evaporate without causing any damage. This is how lime mortars can eliminate damp, condensation and trapped water issues.

Related article: Should You Use Lime Mortars For Pointing?

Lime Mortars Can Absorb Excess Condensation, Regulating Humidity In Buildings

Because lime mortars (and plasters) can absorb water, they can help to regulate humidity levels. Lime mortars can absorb the excess water when a room is particularly humid, for example, in a bathroom after a shower or in a kitchen while cooking. When the internal spaces in a building are very dry, lime mortars can evaporate any water they have absorbed back into the room.

This process of taking on and then evaporating away excess water will greatly help the regulation of the humidity inside a building. A stable humidity with no excess condensation will lead to less mould growth and fewer damp issues. This is why lime mortars and plasters contribute to the comfort of internal environments. Plus, your home will look better without condensation anyway!

Lime Mortars Can Absorb Salt And Water, Protecting The Masonry From Salt Damage

Salts can be very damaging to masonry. Over time, a buildup of salt can cause the surface of masonry to crack and break down. This is caused by the salts crystallising in the pores of the masonry. Salt attack over time can cause a layer of crystallised salt to form on the surface of the masonry. So not only is it breaking down the masonry, but salt issues are also unsightly.

salt on surface of bricks
Salt on the surface of bricks.

Lime mortars can prevent damage to the masonry by absorbing the water along with the salt. As the water moves to the surface of the lime mortar and evaporates, the salt crystallises on the lime instead. This process is extremely beneficial because the lime gets damaged rather than damaging the masonry, which would be a major job to replace. Repointing with fresh lime is far easier than rebuilding a wall from scratch.

In addition, lime mortars are unique in how they can ‘self heal’. Water dissolves salt and ‘free lime’ within the mortar. When this gets deposited after the water has evaporated, small cracks caused by salt crystallisation can be filled by the free lime, effectively self-healing. This means lime can cope with salt attacks far longer than masonry and would far outperform cement which would trap the water and salt, causing a faster breakdown of the masonry.

What Are The Disadvantages Of Using Breathable Mortars?

Lime mortars’ ability to absorb water is a massive advantage, but these mortars have disadvantages. For a start, lime mortars are far more labour-intensive to use correctly. This can lead to the cost of using lime mortars far exceeding the cost of using cement alternatives. Although, most people don’t factor in the cost of the damage that cement can cause, especially to older buildings. In my opinion, if you have an older building (pre-1920s), you should always use some form of lime mortar for any repairs.

In addition to the added cost of work, lime mortars that aren’t properly mixed or use inferior aggregates and the wrong proportions can completely fail. It’s always worth making sure the person selecting, mixing and using lime mortars knows what they are doing. Today, there are many specialist lime builders merchants in the UK. Companies like Limebase, Cornish Lime and Limestuff are your best bet. They can offer advice and stock pre-mixed lime mortars which take the guesswork out of making lime mortars.

Related article: Can Lime Mortar Be Mixed By Hand?