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Articles and Further Information

The importance of lime

Lime may be thought of as old fashioned, but is actually a very important product both in older buildings and new sustainable builds. Lime has been used since before the Roman Times, and up until the mid 19th century, virtually all old buildings used lime in mortars, renders, plasters and  decoration.   However, cement and gypsum superseded lime as they are faster to work with and they set much quicker.  A demand for housing at the time meant a decline in the use of lime.

In the last twenty years the importance of lime has been re-established. Older buildings are constructed from softer and more porous materials than modern buildings.  In the recent past many of these buildings have been repaired with cement mortars and decorated with modern plastic paints.  The result is damp problems, peeling paint, failing plaster and cracking render - problems which have occurred in these old buildings because they have been treated with the wrong materials.  The building has been suffocated with the cement and plastic paint and not allowed to breathe as it was originally intended.  These problems can usually be rectified by removing any external hard render or pointing replacing with lime mortars and limewash.  The same should be done internally.  Delaying these repairs can cause severe damage to the main fabric of the building in the long term.

Now, in light of our global problems and significant carbon emissions from our construction industry,  building with sustainable materials such as straw, cob and rammed earth has become a viable and popular solution.  A unique aspect of buildings constructed from these materials is that they are breathable. But, if they are externally rendered in cement, or internally decorated with modern plastic paints, these buildings will no longer breathe.    Unlike modern cement, lime mortar is porous and allows moisture to freely evaporate, and it is this quality that is referred to as ‘walls being allowed to breathe’ which helps to keep a building dry inside. Limewash has long been known to have mildly antiseptic properties, and lime’s ability to breathe can prevent mould and condensation as well as an overly dry atmosphere inside buildings, which creates a very healthy living space.

Older buildings are built on shallow foundations and rely on an ability to ‘flex’ with the seasons.  Sustainable construction also benefits from these properties, and can be built on very low impact foundations.  But, if cement renders and mortars are used, they set very hard and tend to be brittle, and if there is any movement within the building the cement render will crack.  These cracks will allow moisture to get in which is damaging to any building, but particularly so to straw, timber and earth walls.  Lime mortar is softer and weaker than stone, brick or cob therefore it is able to withstand a certain amount of movement that comes with thermal and seasonal changes in ground conditions.

The environmental credits of lime are huge.  During the production of lime, limestone is burned which gives off carbon dioxide.  Later in the cycle, when the lime is being used on a building and is setting, it re-absorbs carbon dioxide back into itself.   This means that lime has a low carbon footprint.   Cement also gives off carbon dioxide during production, but it never reabsorbs it.  The carbon footprint for cement is 100, or in other words, for each tonne of cement produced, a tonne of carbon dioxide is also produced. It is estimated that cement production represents around 8.3% of man-made carbon dioxide.  A change from cement to hydraulic lime in the building industry could make a major contribution to the reduction of carbon levels.  Materials built with lime can also be recycled, old lime mortar rubble can be used in a new mix, and  non-hydraulic lime will eventually revert to the same material as when it was quarried. 

Many people are now opting for sustainably built homes on self build projects.   Sustainable construction methods such as straw bale builds are relatively simple for the self-builder or first-time builder to achieve, and lime is also a good choice for  these buildings.  As long as the simple rules of working with lime are followed, it is easy to work with.  It takes longer to set than modern materials and cements which is why it is much more forgiving, and can be adjusted and re-worked, and mistakes corrected.  Apart from the conservation and ecological reasons for using lime, it is a much more enjoyable product to work with, most builders that we introduce to lime for the first time become passionate converts.

By Rob Buckley
Dorset Centre for Rural Skills