Why do I need to use lime?
1. Lime based mortar is more flexible and less brittle.
If there is building movement, lime mortar will eventually re-seal the cracks in the mortar. Having a softer mortar means soft bricks and stones won't be forced to break. The mortar is the "sacrificial component" of any masonry wall.
2. Lime based mortar is breathable. This means that moisture that gets in the wall (from damp conditions inside or outside) will escape rapidly. Lime mortar will draw water out of the masonry and allow it to evaporate, reducing dampness and the risk of spalling.
3. Lime based mortar is long-lived. Lime mortar that is made from 98% pure calcium will last 100 years minimum.
4. Lime based mortar is softer. This means it can be removed easily, without damaging the bricks or stones. Masonry bonded with lime can therefore be reused.
5. Lime based mortar is more environmentally-friendly.
6. Lime-based mortar is made at lower temperatures than cement, and therefore produces 20% less carbon dioxide. It also reabsorbs carbon dioxide as it sets. Lime can also be recycled at the end of its life.
7. Lime based mortar is more aesthetically-pleasing.
8. Lime based mortar has a soft, natural, traditional appearance.
9. Lime based mortars set more slowly. This means they can be re-worked for up to 24 hours.
Lime's ability to deal with moisture is one of key issues:
Most materials are vapour permeable. Not many are capillary active. What this means is that moisture in the form of water can move through the material. Modern buildings rely on damp prove courses and cavity walls to essentially make a raincoat for the house. Traditional buildings have solid wall constructions that absorb water when it's raining and release it again when it's dry. The thickness of the walls prevents moisture from reaching the internal plastered surface before it dries out again. When cement based materials are used on traditional buildings moisture gets in (in part, as a vapour from inside) but if it condenses within the wall then it cannot get back out. This is the case with cement pointing. Cement render is even more insidious because if cracks open in the render, wind driven rain can get behind it and accumulate.
A benefit of lime plaster is that it functions in much the same way as the external wall surface. It has the ability to take on moisture when conditions are particularly humid internally, and release it again when conditions are drier. This goes a long way to manage internal moisture levels. In an ideal world, and when environmental conditions allow for it, moisture within the building will travel through the wall and evaporate to outside air.
These factors are the basis behind lime being termed 'breathable'. It must be noted however that in the context of historic building conservation, breathability refers specifically to the movement of both vapour and water. Materials that allow only the passage of vapour would not be considered 'breathable' when used in traditional building environments.
