Why Lime?
Most buildings constructed before 1900 were built using lime mortar as the binding material between the stones or bricks.
Lime mortar allows accumulated moisture to escape through the joints, which is important to prevent the build up of moisture within the structure.
Repairs to these old buildings should be done using traditional lime mortars rather than more modern materials which may be quicker to apply but can cause significant damage.
Our illustration opposite shows how poor moisture management from cement mortars and gypsum plasters can lead to accelerated rates of decay in the brick or stone. In many cases the core of the wall becomes damp. This is not always visible on the inside of the building until non ‘breathable’ plasters de-bond from the wall.
The presence of damp makes a building less thermally efficient and also mobilises soluble salt leading to the masonry units becoming defective.
Humidity is always present. It is how we deal with it which is
important.
Sources of water in a typical residential building include:
- rising damp : tens of tonnes per year
- People : 3.5 tonnes per year
- Wind-driven rain: tens of tonnes per year
In the South East of England between Brighton and Eastbourne we are in the severe exposure zone for wind driven rain receiving an average of 57-100 litres of water per m2 per rain spell. How this water is dealt with can greatly impact the comfort and condition of an old building.
Lime mortar allows us to control where the drying front is.
The principal drying front is wind going across the surface, which creates convective drying.
Moisture moves to the drying front, in a process of non-saturated capillary flow. The principal decay agent is salt, which is harmless in solution but causes damage at the drying front, when crystallisation occurs. This breaks down the mineral binder. Moving water by capillary flow takes the salt with it.
Cement mortar, or unsuitable natural hydraulic limes, act as a plug holding the salts in. The drying front is diverted from the joints (once highly effective) to the stone / brick surfaces.
See cement plug image and before and after plumpton to illustrate cement causing poor drying surface vs restored highly efficient drying surface.
Airspace represents a potential problem, because it is an additional barrier to capillary flow. Where joints have deteriorated, water would need to evaporate and then recondense in order to cross the space (holes in the lime mortar joint) and escape. This is slow and inefficient for drying, when voids are present. Poor quality pointing often does not address the issue of voids within the masonry : it only fills surface voids, leaving a large number of open spaces in the joint behind the surface. Voids should be filled as far as possible during a thorough repointing process.
