The truth about damp in old buildings
Bob Bennett MBE, The Lime Centre
There is little doubt that nearly all the old houses that have survived until now must have been built reasonably well and it is unlikely that they had any problems with damp in the beginning. The problems that do develop often fall into one or more of the following categories.
In spite of the advice given by some damp proofing companies, rising damp is fairly unusual in older buildings. Where a dwelling was built on low lying land or an area of poor drainage, damp may well be a problem but I find that most of the buildings that have survived until today have been carefully sited on well drained ground. Damp at the foot of internal walls may at first seem to be associated with rising damp but there are often other reasons for the problem which may be any of the following.
If the soil level is above the internal floor level, it will almost certainly have invalidated any damp proof course. A house built into a sloping site; a new raised flower bed with a dwarf retaining wall; newly laid paving around the house or patio area that drains towards the house instead of away. All of these features may drain water to the foot of the wall and above the Damp Proof Course. I recently examined a fine old manor house where the drive had been ‘upgraded’ by nearly half a metre, to the point that it had obscured the under floor ventilation air bricks and was 180mm. above the d.p.c. By installing a simple French drain we were able to rectify the problem.
Cold Bridge Syndrome
Is probably the most common source of damp, as nearly every historic building has solid masonry walls, the introduction of cavity wall construction being a comparatively recent innovation. During prolonged periods of low temperature, these walls slowly chill from the outside until the internal surface is cool enough to allow condensation. Providing that the wall remains vapour permeable, moisture generated within will migrate through the walls without causing a problem. However, the misuse of cement for pointing and other repairs, including cement renders and gypsum plastering, has ‘sealed’ the structure and prevented the transfer of vapour. Furthermore, when the house was first built, the production of vapour was mainly as a result of cooking and washing.
The advent of showers, the installation of bathrooms and drying of clothes within the house, have all contributed to a large increase the moisture that cannot now escape. The resulting condensation, which often occurs at the lower levels on the interior surface of the external walls, is easily confused with rising damp. Most modern paint systems are not vapour permeable. The manufacturers of so ‘called masonry paints’ often boast of the ability of these materials to resist water. Unfortunately most of the products are also vapour impervious and, when applied either externally or internally, (few emulsion paints breathe), as a finish to solid masonry walls, the result will be ‘cold bridge syndrome’ and all the associated misery.
Downpipes and gutters
How often do we see clogged rainwater goods discharging large quantities of water down the face of buildings? During prolonged spells of bad weather the soaking can go on for days. The result can be twofold, damp penetration (although this is seldom the case), or the drop in temperature due to the chill factor is significant and can, once again, result in cold bridge syndrome.
One factor which many of us overlook when improving our houses, is the use of the chimneys. The simplest of artisan housing always had a chimney that terminated in the kitchen and often with a fire in the sitting room. The fire in the kitchen was alight every day of the year as that was where cooking was carried out. Stand outside a house with a fire alight in the hearth and you will see smoke coming from the chimney. The rate at which the smoke billows out is exactly the rate at which air is entering the building, providing excellent ventilation. It was not by accident that armchairs have the front of the seat down to floor level and the high back and sides wrapped around the occupant so that in this way he can face the fire for warmth and avoid the draughts.
There is a tendency to forget that the rapidly changing air within the old houses also meant that any damp air was evacuated rapidly. These houses benefited from both heating and ventilation. Unfortunately, most of the open fireplaces are no longer functional and with the advent of central heating our houses are much warmer but the ventilation is almost nil. It is hardly surprising that some dwellings now suffer from damp. Fortunately, recent improvements in the provision of ventilation can help to rectify the problem.
Lime has been the principal binder in mortar, plaster, render and wash for the last ten thousand years. One of the earliest surviving examples of lime concrete is the floor to a fisherman’s cottage on the banks of the Danube, which was laid 5600 B.C. Although the cottage structure has long since failed, the lime floor survives. While this example of an old lime structure may at first appear unique, it is worth remembering that all the cathedrals, abbeys, fortified castles, stately homes, churches and almost half the housing stock, all of which are in regular use to-day, were built of lime. Lime is one of the most vapour permeable building materials and was almost certainly used throughout your house when it was built. It must therefore be the ideal material with which to carry out maintenance.