Lime: it’s better for buildings – and for the environment

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Lime: it’s better for buildings – and for the environment

It is now fairly well known that cement is not good for old buildings and that lime mortar should be used. But why? What are the advantages and what are the disadvantages? In order to begin to answer those questions it is necessary to understand the nature of traditional building, the process by which buildings used to be built, and how it differs from modern construction, the process by which we build today.

Here, James Simpson OBE Hon DSc FRIAS, chairman of the Building Limes Forum, discusses the differences between traditional and modern building techniques.

Traditional building has its origins in the pre-industrial age, when buildings were made from natural, local materials. In the UK, earlier buildings of timber were superseded by walls of stone or brick, bedded in lime mortar, plastered internally and sometimes also finished externally with a floated or thrown plaster coat and limewash.

Those buildings were assembled by hand with skills developed over the generations and nurtured through the trade apprenticeship system. They were practically detailed to exclude and shed water. Viewed from today’s perspective the process was almost entirely benign: everything was ‘renewable’ and carbon emissions in construction, if not in use, were negligible. The system, developed over centuries, worked.

Paradoxically to the modern mind, that is because, in traditional building, almost all the materials and the construction as a whole were, in relative terms, soft, weak and permeable. Rainwater was shed or otherwise carried away from roofs, but the walls, protected by overhangs, cornices or drip-courses, were permeable. Moisture absorbed in wet conditions penetrated the outer surfaces of the walls, but evaporated out again when conditions were dry – and dry walls are warmer than wet walls.

Many problems in traditional buildings follow from the application of strong and impervious finishes like cement, which invariably make the walls wetter. That is because moisture from minor leaks, tiny surface cracks and ground water is prevented from evaporating through the external wall surfaces. Attempts to exclude moisture with damp courses and membranes only exacerbate problems by causing a build-up of wetness on one side or the other.

Timber decay is commonly associated with wet masonry and cement mortar also damages the outer surfaces of walls: if it is harder and less permeable than the stones or bricks to which it is applied, it traps moisture and concentrates vapour movement, causing sacrificial decay.

Modern construction is the opposite of traditional building in almost every respect. It employs materials which are manufactured, processed and transported, and which are hard, strong and impermeable. This system is extravagant of energy and resources. In particular, the cement industry creates up to 8% of worldwide man-made CO2 emissions.

Several conclusions may be drawn:

The unnecessary replacement of existing buildings is inherently damaging and should cease;
Modern construction must be further developed, to reduce its environmental impact – particularly that of Portland cement;
Traditional buildings should be maintained, repaired, restored and repurposed sustainably, by using compatible materials and methods.

Symposium reflects new thinking on lime use

Masons and conservators across the UK and beyond are rediscovering traditional mortars through research, use and experience. They are moving away from the default specification of natural hydraulic limes for conservation works and are determined to practise like-for-like repair.

Click here for more information

Lime is the essential ingredient of traditional mortars, plasters, renders and washes. There are across the world, and have been over the centuries, many different sorts of lime and ways of preparing and using it. At its simplest, however, lime is made by heating pure limestone in a kiln, driving off CO2, which is then re-absorbed as the mortar cures. If the energy used for heating is renewable, as it can be, this ‘lime cycle’ makes so-called ‘air lime’ carbon neutral. There has been a revival in recent years of the traditional method of making mortar by adding water to the quick lime and aggregate to produce ‘hot mixed lime mortar’.

Some limes, due to natural or added impurities, have chemical setting properties analogous to those of cement, but weaker. Such ‘hydraulic’ mortars are useful – necessary even – in harsh conditions, but are less permeable and less environmentally benign.

In conclusion, it is important to understand how traditional buildings work; traditional building and modern construction are entirely different systems, which should only be mixed with proper thought and care.

The modern construction process, as it is currently understood and practised – particularly when it involves the use of concrete and cement – is a relatively cheap way to build, but it has significant consequences for the global environment.

In the context of traditional buildings, the use of lime mortars, plasters, renders and washes, and the skills required to use them, will be to the medium and long term benefit all.

• The Building Limes Forum is a registered charity. It exists to encourage expertise and understanding in the appropriate use of building limes and education in the standards of production, preparation, application and aftercare. Throughout the year the organisation invites its members and others to learn, discover and share knowledge about the use of building limes through talks, practical demonstrations and tours of historic sites. To find out more visit www.buildinglimesforum.org.uk.

YOU CAN FIND A LIME SUPPLIER OR CONTRACTOR IN YOUR AREA IN OUR ONLINE DIRECTORY HERE

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