Lime rendering on French property

MANY houses throughout France are built in limestone, and others are coated in it, in the form of a lime rendering, writes Neil Parkinson.

Lime, in French la chaux, also termed quicklime, burnt lime or calcium oxide is an essential product in the successful restoration of old houses and the intelligent construction of new ones, as a substitute to cement.

The French generally associate lime with rustic country houses, but its use in architecture across the Mediterranean basin goes way back to the 5th Century B.C. and use by the Roman civilisation is evident in the fabulous Pantheon in Rome.

The main advantage in using lime is that it allows a building to breathe. In comparison cement traps humidity. However others factors play in its favour, as we will see herein.

Contemporary restorations of stone buildings often employ some lime rendering, sometimes inside in order to counter the excessive ‘mineral’ look of too much visible stone, to create smooth or semi-smooth surfaces, on the exterior of a house as a substitute for cement to repoint visible stonework, and sometimes in and out with pigments to create decorative coloured walls.

Equally where the restoration of a stone wall has resorted for economy to the use of modern bricks to fill in excessive gaps in a stone wall, these can be hidden by a lime rendering to maintain uniformity.

The chemical process used to create the end product for lime rendering consists in industrial steps that break down the limestone extracted from the earth by the application of heat and the subsequent addition of water.

Its use in the house requires the addition of sand and water, and results in the evaporation of the water and the absorption of carbon dioxide. This magical process is indeed a cycle, that creates a malleable limestone from a raw material of quarry limestone.

Besides being visually appealing, lime rendering has other benefits over traditional cements:
– excellent thermal and acoustic properties
– very durable
– supple and unctuous – to obtain a rugged finish use sands of 0.2 or 0.3 mm. Smoother finishes are produced with sand of 0.1 to 0.2 mm
– it is economical to use
– addition of pigments result in a regular tint when blended in a cement mixer. Note that pigments must be added in the right proportions in order to not produce too powdery a mix
– excellent adherence to the wall
– anti bacterial qualities
– slows the build up of dust
– excellent light reflection from white lime
– no shrinkage while drying, meaning no cracks
– it lets walls breathe, thus preventing the build up of mould

Alleviation of humidity using lime rendering
All walls contain a certain amount of humidity – it would be ridiculous to try and change this. Rainwater can penetrate exterior walls to a depth of a few centimetres, but the wind and sun enable quick evaporation.

Humidity in the ground beneath the house has a tendency to rise, but will not rise in excess of 1 metre above ground level if evaporation is possible. The photograph indicates the consequences of not creating a sound environment for humidity to evaporate.

Neither wall has an external rendering but the application of cement or plaster board to the interior, in practise a very common occurrence, is not a good solution, as can be seen on the left side.

External cement rendering, another common occurrence, is good in preventing water from penetrating the wall, but has a tendency to crack with the natural expansion and contraction of the stones, and allowing water in.

Another common practise is to install interior plasterboard with an air gap separating it from the stone wall. This is also a damp trap and needs ventilation. A solution is in the use of hemp fibre insulation over which is laid a layer of quick lime.

La chaux is an essential product in both house restoration and construction in France, and not only is it friendly to the earth, it is friendly to man.

Neil Parkinson is the founder of James Properties France based in Montpellier. The company addresses requests for existing houses, apartments and building plots in the Gard and Hérault, and for off-plan property across France.