Most traditional buildings are built with stone, soft bricks, timber and earth using earth or lime-based mortars and renders. These materials allow moisture to be absorbed and then to readily evaporate away, we often say it allows the building to 'breathe'. In such buildings the levels of dampness in the building are 'controlled' by this ready evaporation of moisture.
Externally the porous materials are dried out by the wind and sun. Internally, air movement - through the roof covering, windows and openings - all help the evaporation of moisture from the internal porous surfaces. Where moisture can evaporate freely and the traditional 'breathing' performance is not impaired, the walls of traditional buildings will remain relatively dry.
Of course even the evaporation from porous materials may prove insufficient if building fabric is subject to excessive wetting. Traditional buildings were designed with good details to keep water out of the building, for example deep eaves, wide gutters and plinths at ground level. If those features are made inoperative, e.g. by blocked gutters or raised external ground levels, then the building is at risk.
|A breathing building|
The Industrial Revolution and the increased movement of materials brought rapid changes in the methods of construction and the building materials available. By the late 19th century there had been a significant move away from buildings constructed using traditional materials and methods to those buildings with cavity walls and damp-proof courses built using cement mortars and renders.
Modern building materials (hard dense bricks, cement based mortars and renders, modern masonry paints and external sealants) are generally impermeable and rely on providing physical impervious barriers (cavity walls and cement renders) to the elements to keep out driving rain and physical damp-proof courses to prevent rising dampness. These modern materials and methods work well together in modern buildings designed to exclude the elements.
|Close up showing the porous nature of limewash|
|Another inappropriate repair: the cement render on this building is too rigid and has cracked. Water will now be penetrating through this crack, be dampening the fabric but be unable to evaporate.|
If impervious modern materials are used on a traditional building there is a risk that the balance between water entering the fabric and water evaporating from it will be disturbed. When the balance is disturbed the traditional performance will be adversely affected and problems - such as of dampness and decay - will occur.
The increased use of impervious materials, in well-meaning but damaging programmes of repair and maintenance, dramatically transforms the way historic buildings perform. The application of cement renders, masonry paints or sealants to the wall of an historic building will significantly reduce the amount of evaporation that can take place. As the moisture content of the wall increases, the likelihood of decay increases. Timbers and weaker materials, such as soft external masonry where hard cement pointing has been used, are particularly at risk.
Also modern cement based renders, mortars and plasters are hard, inflexible and impervious. If they are bonded to softer, more flexible traditional materials these modern materials are susceptible to cracking. Even the finest hairline crack is prone to being exploited by moisture and any moisture drawn in may become entrapped behind the hard impermeable finishes.
Where modern impervious materials are causing problems of dampness and decay they should ideally be removed, but only where this will not cause more damage to the building then leaving them in place. Unfortunately, the removal of many impervious materials will cause damage to the building and needs to be done with great care to prevent any unnecessary damage to historic fabric.
Before removing any materials from an historic building seek the advice of the Local Authority Conservation Officer or a qualified specialist in the field of building conservation to avoid the risk of a breach of planning law (Legislation) or unwittingly causing damage to historic fabric.
|The consequences of mixing technologies. Here a cement repointing has led to damage to the soft bricks in the arch.|