Old cob and stone buildings can be warm, dry, and comfortable to live in when they are treated correctly. There are a number of strategies that may be employed to help control levels of damp within them.
Using Air Flow to Control Damp
The more air there is flowing through a building, the less risk there is of damp building up internally.
Damp is generated inside a building in two main ways. The first is through normal living activities carried out by the occupants such as washing and cooking, the second is through the evaporation of moisture from the walls and floor into the internal living space.
Because the temperature of the air inside a property is generally higher than the temperature outside, excess moisture sitting in the walls or floor will be drawn to the internal faces and from there will evaporate and increase the internal humidity of the property.
Old stone and cob buildings are particularly prone to this as they do not have effective damp proof courses and often have wet walls due to the application of inappropriate finishes such as cement and masonry paint rather than lime and mineral silica paints.
Maintaining a constant low flow of air through a building allows damp air to exit to the outside world. This may be done through trickle vents, installing a positive pressure ventilation system, or just by leaving the windows open a little.
Air flow is probably the cheapest and easiest way to manage damp. It is a useful tool, but to dry a building out properly other areas must also be addressed.
Perimeter Drainage to Control Damp
The function of a damp proof course is to prevent water wicking up through the footing of a building into the walls.
Old stone and cob buildings rarely have this feature; instead they were designed with the emphasis on good perimeter drainage and the use of breathable finishes such as lime and clay. Cob and stone walls are designed to be able to deal with a certain amount of moisture wicking up into them, though this is reliant on them being finished with lime rather than cement.
Even when these properties are correctly finished with lime, care must be taken to limit the amount of water that comes into contact with the base of the wall.
The most common vector for large amounts of water coming into contact with the footings is through downpipes that are not routed into drains or downhill away from the building. Attention must also be paid to the direction of any runoff from the surrounding ground; for example where the level of the ground runs toward the property.
Maintaining Gutters and Downpipes to Control Damp
Gutters are the first line of defence for your building when it comes to offering protection from the weather. They are commonly neglected, and when faulty will lead over time to the need for major re-rendering works and even structural repairs.
Gutters and downpipes collect and concentrate rainwater, and when they spill they can project a massive amount of water directly on to the wall. This often starts or exacerbates the vicious circle of cracking cement render.
To prevent damp in cob and stone buildings the maintenance of rainwater goods is absolutely essential.
Gutters should be regularly cleared of debris and downpipes should be firmly connected and must discharge water away from the property. It is advisable to observe them during a downpour to assist in spotting any faults, though tell-tale signs such as dark staining and algal growth are also useful in diagnosing issues.
Using the Heating to Control Damp
Cob and stone buildings are likely to feel much drier when they are heated consistently and at a low level. Because the walls have such a high thermal mass, they are prone to developing condensation if they are exposed to short bursts of heating.
When the heating comes on in a room, the air warms up first. The walls warm much more slowly. As the air is warmed, its capacity to carry humidity increases and it picks up any residual moisture from the walls and floor.
If the heating is then turned off, the air cools long before the walls. As it cools its ability to hold humidity drops and condensation will form in the coldest areas of the room, promoting mould growth.
This issue can be managed by setting the heating to run at a consistent background temperature. Providing the walls are reasonably dry, they will then be able to heat up and a high temperature differential with the air will be avoided, reducing the risk of condensation.
Maintaining Chimneys to Control Damp
Along with faults in the rainwater goods, leaky chimneys are the main cause of water damage to old cob and stone buildings.
A significant proportion of chimneys on old properties have inadequate flashing, and many are nearing the end of their lifespan. A leaky chimney can hasten the need for re-pointing and re-rendering works, and should be addressed as a priority.
Once a chimney starts to leak, water trickles down the flues, which are almost always integral to the walls in old Cornish cottages.
Stone walls in Cornwall are most commonly mortared with clay, and as clay has no chemical set it washes away when exposed to water, meaning that the stone lining of the flue may be damaged and become loose. Additionally, the water resulting from a leaky chimney is full of impurities, and these impurities become deposited on the stonework and cause damage, particularly to softer stone.
It is often necessary to re-build leaky chimneys using a strong hydraulic lime mortar, but if they are dealt with promptly a robust lime render may suffice. Because of their exposure to the weather, a very strong hydraulic lime such as NHL 5 is recommended to be used.
Breathable Finishes to Control Damp
Lime is not a panacea for the control of damp in old stone and cob cottages, but is a very important tool in getting them to function as they were designed to do. It permits walls to shed moisture by means of evaporation, deals with small movements in the building effectively, and in some circumstances acts as a sacrificial element.
Old (pre 1900) buildings that have been finished with cement render or cement pointing are much more likely to build up excess moisture within their walls than those that have been properly finished with lime. The most significant reason for this is that unlike lime, cement does not permit the easy passage of moisture and moisture vapour - it acts as a barrier.
With conventional modern buildings this is considered a desirable trait as they are designed to keep all moisture out of the building envelope. With old buildings it is a recipe for disaster, as they are designed to reach an equilibrium of moisture within their walls.
Old buildings inevitably pick up moisture, and if they are denied the means to shed it through their pointing or render they are likely to experience a build up which can lead to unpleasant living conditions and damage to the fabric of the building.
Lime should be used in conjunction with the other strategies mentioned here. At Muddy Mortars we are experienced lime masons and can advise you as to the importance you should place on each element, and can help you prioritise where to spend money on your old building. Please call or email us if you think we can help.