Modern buildings are built to be waterproof – they incorporate impervious materials (hard dense bricks, cement based mortars and renders, modern masonry paints and external sealants), which rely on providing physical barriers to keep out driving rain and damp-proof courses to prevent rising dampness. Used correctly in the construction of new buildings, such materials and methods work well and will exclude the elements, as long as they are properly maintained.
Old buildings are usually built of stone, brick, timber or earth (cob), held together with earth or lime mortars and covered with lime render, lime plaster and limewash. These materials are permeable and allow moisture to penetrate the fabric, both externally (rain) and internally (moisture from condensation, steam and drying washing). Moisture then evaporates away when conditions are favourable – these walls and their materials are “breathable”.
Dampness in old buildings is controlled by this regular evaporation of moisture. Externally, porous materials are dried out by wind and sun. Internally, air movement through windows, roof coverings and open fireplaces promotes evaporation of moisture from the internal surfaces. Where moisture can evaporate freely the fabric remains relatively dry.
If impervious modern materials are used on an old building, the balance between water entering the fabric and evaporating from it will be disturbed. The breathing performance will be adversely affected and problems, including dampness and decay, will occur. Vulnerable materials, such as timber and soft bricks or stone, are particularly at risk. Modern “improvements” such as replacing earth, stone or timber floors with concrete and the installation of double glazing also add significantly to the problem.
To make matters worse, modern cement-based renders, mortars and plaster are hard and inflexible. If they are used over softer, more flexible traditional materials they will tend to crack. Even the finest hairline crack will allow water to enter and any moisture drawn in will become trapped behind the impermeable finishes. Where modern impermeable materials are causing problems of dampness and decay, they should be removed. However, this can itself cause considerable damage if it is not done with care. All work to old buildings should be carefully specified and carried out by skilled craftsmen in order to minimise risk.
Although the above advice may seem restrictive, there is an increasingly wide availability of traditional materials, including lime renders and plasters, breathable paints and natural insulation materials.
There are many excellent internet sources of further information on the correct use of traditional materials in old buildings. Here are just a few:
Many books are also available – we highly recommend:
“Old House Handbook - A Practical Guide to Care and Repair” by Roger Hunt and Marianne Suhr (In association with the Society for the Protection of Ancient Buildings)
Haynes “Period Property Manual – Care and Repair of Old Houses” by Ian Rock