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Water penetration through walls is generally avoided through precautions in design and construction. Overhanging eaves considerably reduce rain penetration into wall surfaces, except during windy weather, and even where flush eaves are employed, gutters and downpipes are provided to ensure that walls are not exposed to roof water discharge. However, these precautions should not be necessary as walls should be capable of resisting rain penetration in any case. In solid walls very thick construction is used so that absorbed penetrating rainwater can be accommodated without becoming apparent at the internal surface, the accumulated water later dispersing by evaporation. In cavity walls complete penetration and saturation of the external skin is expected, but precautions are taken to prevent absorption of this water into the internal skin. In both cases defects can occur and internal dampness problems are often encountered.
Causes of dampness in buildings
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In solid walls penetrating dampness may result from insufficient absorptive capacity, penetration occurring because the wall is too thin, either throughout the building or at local thin points, such as window reveals. Alternatively, penetration results from excessive permeability. With carbonaceous stones, that is limestones or sandstones with a carbonate cementing matrix, atmospheric acids will cause slow erosion and a progressive increase in permeability. In many cases dampness follows the pattern of the masonry, developing in either the mortar or the stone, whichever is more permeable. This effect can be seen most clearly if the internal surface is finished in limewash, the custom in many old churches in some parts of the British Isles. Construction in impermeable granite presents a particularly interesting example. The impermeability of the stone would seem to be a good protection against penetrating dampness, but the same amount of rainwater will be incident on an impermeable granite wall as on any other wall; all the water flowing down the wall will be absorbed into the porous mortar which, with its limited absorptive capacity, will quickly become saturated throughout the thickness of the wall, although the mortar might have perfectly adequate capacity if used in combination with a stone of average porosity which could absorb some of the water. In fact, many granite walls are not constructed in this way, but comprise two separate skins with a rubble core. Penetrating water can drain through the core, emerging at the interior at some distance from the original source, making it difficult to positively identify a defect such as an area of faulty pointing.

Penetrating dampness resulting from inadequate absorptive capacity tends to result in uniform internal dampness, except where the upper parts of walls are drier as they are protected from rainfall by overhanging eaves. However, rainwater absorbed into a wall will tend, at least in durable macroporous materials, to drain towards the base. If a damp-proof course is provided it is likely that this water will accumulate on top, giving an appearance very similar to rising dampness, although often the wall is dry underneath the damp-proof course. Whilst this accumulating dampness may appear similar to rising dampness, treatment is obviously entirely different and it is thus essential to ensure that dampness of this type is correctly diagnosed. Similarly, if dampness is concentrated at the top of a wall it must be suspected that there is a roof or adjacent gutter defect, and heavy flow from a defective gutter or downpipe may cause a vertical band of dampness.

Parapet and valley gutters, as well as flat roofs, frequently drain into hoppers which lead to downpipes. In some large buildings the hoppers may be fitted with spouts at a higher level through which water can discharge if the hopper should become blocked. Dampness in many buildings can be attributed to the failure to keep hoppers free from accumulating leaves, moss, lichen and pieces of stone, so that the entire roof discharge eventually occurs through the overflow spout, or simply overflows down the wall if spouts are absent.

Some years ago extensive renovation works at a Scottish castle were followed by the development of dampness at the junctions between walls and ceilings, immediately below every parapet, a problem that had never occurred previously. Investigation showed that the parapet copings had a slight fall towards the roof, but their edges did not project beyond the parapet walls so that water was discharging down the roof side of the parapet, being absorbed and percolating downwards to the accommodation beneath. The problem had not occurred previously because the roof asphalt had been continued up the inner faces of the parapets and across the copings, a detail that had been introduced by the well-known architect Lorimer when the castle was repaired and extensively reconstructed in about 1908 following a fire. This dampproof membrane feature was considered unacceptable by the architect preparing the scheme for the recent renovations and it was removed, but unfortunately the architect failed to appreciate the essential damp-proofing function of the asphalt covering. The dampness was cured by reconstructing the parapets with damp-proof courses coupled with flashings at the top of the asphalt upturns.

In theory the use of cavity walls should completely prevent penetrating dampness, but unfortunately practice is not as perfect as theory! Dampness at the top of walls can arise through roof, parapet and gutter defects, as for solid walls, but even direct penetration can occur, spreading across the cavity through mortar droppings or slovens which accumulated on wall ties during construction, causing small patches of dampness internally. Sometimes this problem develops only when cavity fill insulation is introduced, the original ventilation of the cavity being sufficient to prevent significant water penetration across the ties but the cavity fill destroying this ventilation and causing the dampness to become apparent as patches on the interior surfaces of the walls. Cavities should preferably extend to well below internal damp-proof course level or, alternatively, if the damp-proof courses are continuous, they should be stepped so that the level in the external skin is lower than in the internal skin. Occasionally the situation is reversed, either through carelessness or ignorance, and penetrating dampness accumulating on the damp proof course at the base of the external skin may flow into the internal skin; in one example this defect occurred in all the houses throughout a large building development.

In another house sulphates in the bricks reacted with the cement in the mortar of the external skin, initially causing expansion and distortion. This prompted some reconstruction of the brickwork in some areas, but where the original brickwork had been retained the mortar eventually became very friable; sulphate attack in brickwork will be discussed in a separate article. One day there was a severe thunderstorm, the thunder shocks causing the friable mortar to run from the external skin into the cavities, accumulating on the trays over openings and providing bridges through which the heavy rainfall penetrated from the external into the internal skin, damaging the interior decorations!

It has become normal practice in recent years to restrict cavity ventilation to improve thermal insulation. In fact, cavity ventilation has a very important function in evacuating humid air; if vents are omitted dampness penetrating through the external skin will increase the humidity of the cavity air. If the wall cavity is continuous with the roof space, and the roof space is similarly unventilated, condensation will eventually occur if the roof covering is impermeable, water accumulating in the supporting boarding of a wood roof deck, perhaps resulting in the development of fungal decay within the boarding and the supporting joists, and in severe cases causing dampness staining through condensation dripping onto the ceilings beneath. Cavity fill insulation reduces heat loss by restricting the convection circulation that transfers heat across the cavity from the inner to the outer skin, but fill materials must be chosen with care. Non-wettable pelletised materials, such as expanded polystyrene, can combine excellent thermal insulation with freedom from disadvantages, but some mineral fibre materials can conduct moisture across cavities; the mineral fibre should be treated with a water repellent but there have been many instances of cavity fill without water repellency, either because of a fault in manufacture or because the water repellent has been destroyed by biodeterioration. Even foams formed in-situ can have considerable disadvantages; wettable injected foams can conduct moisture across the cavity, and if they are incorrectly formulated they can actually collapse to release moisture and cause dampness. There are also two forms of dampness development which are common to all forms of cavity fill insulation. If mortar slovens were permitted to accumulate on cavity ties during construction, they will tend to conduct moisture across the cavity from the outer skin to the inner skin, but in a normal ventilated cavity the evaporation from these sloven bridges is usually sufficient to prevent any dampness becoming apparent within the accommodation. However, all forms of cavity fill restrict ventilation and increase the likelihood of dampness developing in isolated patches through sloven bridges. In addition, the vibration caused by drilling into the outer skin in preparation for cavity filling often loosens considerable debris from the inner face of the outer leaf which then accumulates at the bottom of the cavity, perhaps bridging trays over openings and damp-proof courses, causing dampness to develop in the accommodation; this problem is particularly severe in seaside properties in which sea spray absorbed into the outer leaf can cause crystallisation damage to the inner surface which can then be loosened by the drilling vibration.

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