Case study 1 – Unnecessary installation of damp proofing
Quite often we inspect properties that have treated for rising damp through the installation of the chemical damp proof course and damp proofing system. A case in point is set out below.
We were asked to inspect a property in Hertfordshire which was suffering from a damp problem. The property a traditionally built 1930s chalet bungalow was suffering from severe damp problem with peeling wallpaper to a front south facing bedroom elevation. The plaster beneath the paper was also covered in mould.
The owners of the property had purchased the property in 2020 and they were aware the property had suffered from a moisture problem in the past as we found evidence of previous damp proofing works to the property on account of the filled drill-holes to the front elevations at damp proof course level. This was not picked up in their pre-purchase RICS Level 2 Homebuyers Report.
The original damp proof course to the building is of traditional slate construction and the external ground levels were a sufficient distance below the damp proof course. When the electrical resistance moisture meter probes were inserted into the wallpaper the readings were off the scale (99.9% WME). However, when the plaster behind the wallpaper was probed it was found to be completely dry (7% WME). If the moisture problem was caused by rising or penetrating damp then the plaster would have also been damp.
The relative humidity (RH) readings at the time of the inspection were in the region of 65 – 70%, which is too high. A RH humidity of 35 – 60 percent is favourable. A relative humidity of 70 to 80 percent is sufficient for mould growth. It should also be noted that the RH adjacent to the surface of a wall can be up to 10% higher than at the centre of room. So a RH reading of 65 – 70% could actually be 75 – 85% at the wall surface. The RH should not drop below 30% as this can cause dry itchy eyes, dry skin and infections.
Clearly the cause of the damp problem within this property was condensation and nothing to do with rising or penetrating damp. Like most bungalows the bathroom was situated centrally within the property and adjacent to the front bedroom, where the damp problem was most severe.
As part of the remedial works we recommended that the extractor fan within the bathroom was upgraded to a unit with humidistat control and a similar extractor fan installed in the kitchen. We also installed some data loggers and monitored the humidity levels over a period of few weeks before and after the extractors were installed. The RH levels within the property before the extractor units were installed ranged between 55 – 80% and once the units had been installed these reduced to 50 – 60%. These readings are slightly high but no unexpected given the monitoring was carried out in the winter months when condensation is most likely to form. We also recommended the wallpaper within the front bedroom was stripped and the walls redecorated using breathable clay based paints, which can be wiped down.
The previous damp proofing works to the property were completely unnecessary. The property suffered from acute condensation and the damp was nothing to do with the damp proof course failing. The cost of installing the new extractor fans £200 per unit. The cost of the previous damp proofing works would have probably ran into several thousand pounds and it did not solve the problem so it was a complete waste of money.
Case study 2 – Damp Patches to Chimney breast Incorrectly Diagnosed
The misdiagnosis of this type of moisture problem is very common with surveyors. In this particular case a damp patch appeared over the client’s chimney breast soon after they moved into the property. The previous owners of the property had installed an ensuite shower room within the bedroom where the damp patch to the chimney breast appeared. The damp patch was identified in the pre-purchase survey commissioned by the client and the cause was identified as a defective flashings to the chimney stack above the chimney breast. However, the chimney stack and flashings were inspected by a roofing contractor and no defective flashings or potential source of the damp penetration was found. As part of our inspection, we took a small sample of the damp plaster and carried out a salt test This confirmed the presence of nitrates within the sample. The electrical resistance moisture confirmed the plaster adjacent to the damp patch was dry and the thermal image also confirmed this to be the case.
The damp patch had been caused by a hygroscopic salts. Hygroscopic means having the ability to attract and absorb moisture from the surrounding environment. Table salt, which is pure sodium chloride, can cause moisture to condense through a process known as deliquescence when the relative humidity surpasses 75%RH. Salts that induce condensation at or below 75%RH are generally referred to as hygroscopic. Most hygroscopic salts can be found near chimneys as the burning of coal and wood produces nitrates, which can migrate through brickwork and into plaster through leaks or condensation. In this particular situation the problem will have been made worse by the new ensuite bathroom situated next to the bedroom chimney breast as steam from the bathroom will have been absorbed by the chimney breast salts.
On this occasion, we simply recommended the damp patch was covered with a gloss paint to provide a waterproof film between the salts and atmosphere and the ensuite extractor unit upgraded to a humidistat unit. The cost of the remedial works was in the region of £250.