Damp diagnosis case studies
Gone with the wind
Stages 1–4
Property detailsA14-storey block of 2- and 3-bedroom apartments built in the 1960s to provide high-density local authority accommodation close to Blackheath in southeast London Site: in a residential estate of similarly constructed tower blocks; height above sea level and location indicate a moderate exposure to wind and rain Orientation: at each dwelling, external walls are north, west and easterly facing respectively Construction: reinforced structural concrete beams and columns with inset external cavity walls, outer leaf of face brickwork and inner leaf blockwork and integral purpose-formed cavity trays Windows: replacement double-glazed PVC-U windows incorporating closable static trickle vents at the heads of the windows Services: full gas-fired central heating and hot water system provided from a combination boiler and steel radiators Survey date: November 2000 – weather conditions: cool and dry |
Aims of the survey
To identify the cause of dampness to the bedrooms in 4 properties within the same block of flats and to determine whether the external walls to the bedrooms should be dry-lined.
Research before the investigation
The surveyor obtained details of the location and orientation of the property, together with details of the surrounding area and adjoining property from the internet. The landlord supplied the repair history at the subject property. He was also given a history of complaints of damp from previous tenants and a brief history of similar problems reported in other properties within the estate. Dampness within the bedrooms of the subject flats apparently got considerably worse during the cooler months of the year.
Investigation
1. Walk-over – Stage 1
Each dwelling was of similar layout, comprising a kitchen, bathroom/WC, living room, 2 bedrooms and an L-shaped entrance hall (see figure 1). Access to each property was gained either by lift or communal stairs and covered walkway.
Figure 1: Typical layout of flats
The surveyor began with a brief tour of the affected areas. In each case it was the external walls within each of the 2 bedrooms that were affected by black mould spotting. There were crescent-shaped mould formations at the corners of external walls at low level – a classic profile of internal condensation. In some locations black mould spotting was visible at high level on wall surfaces and corners where furniture (e.g. wardrobes and sideboards) was placed tightly against the walls.
Figure 2: A typical mould formation found at the abutment of two external walls
2. Stage 2
The surveyor used an electrical resistance moisture meter to check across the wall surfaces to see if high readings meant that further investigation would be needed. He checked each wall in turn, concentrating first on the areas of the wall surface most affected by mould. The results generally indicated moderately high readings of 18–23 R/R to the wall surfaces at low level. The highest readings were found at the abutment of the 2 external walls at low level. He also made a spot check of the internal air temperature and relative humidity using a hygrometer and digital air and surface thermometers. The results did not support active condensation.
Four inspections could not be undertaken simultaneously, and it is worth noting that the surface moisture readings changed dramatically depending on the time of the inspection. For example, early morning inspections could produce a much higher surface damp reading than late in the afternoon.
At this point it was most important to take on the role of ‘impartial investigator’ (see additional comments). The surveyor needed to hear the tenant’s point of view, while bearing in mind the evidence that he had gathered and the information the landlord had provided.
There were a number of highly critical tenants who had already experienced several visits from other inspectors who had all confirmed that condensation due to lifestyle was the sole cause and that tenants should open windows during cooking and bathing. The tenants and residents felt that the landlord was blaming them in an attempt to avoid spending money on the ‘real cause of the damp’.
Interestingly, some of the residents said that they had thought the old single-glazed windows (which had been draughty and occasionally let in rain) were a major factor affecting the damp and mould internally. But when the new double-glazed windows were installed they were disappointed because the problem had persisted, and in some cases had become worse. This had led to confusion and anger, and some even suspected that the landlord had bought cheap replacement windows that were not up to standard.
Some of the residents said they wanted to be moved to larger, more suitable accommodation to reduce overcrowding and escape the damp, mouldy flats. Several tenants told of asthma and skin rashes among their children. Others theorised that the ‘real problem’ was actually water coming in through the walls from the outside and that the landlord was ignoring the problem and sending inspectors who ‘didn’t know what they were doing’. Many had complained to their local councillors, the residents’ association and the Ombudsman; several were considering legal action.
The surveyor ran through a list of general questions relating to living conditions to find out, for example, heating periods and patterns, methods of drying clothes, types of food prepared and cooking methods (e.g. pots and pans without lids), activities relating to visitors (e.g. dinner parties, additional washing of clothes) and if there was any subletting or renting of the properties either in whole or in part. He also noted the amount of personal possessions and furniture that might contribute to blocking free movement of the air.
The surveyor noted gas and electricity meter readings, and established where there were periods of continuous occupancy. Finally he asked about the specific problem.
- How long had the problem of damp and mould persisted?
- Did it follow an incident of water leakage or penetration, or did it follow an improvement, repair or alteration to the building?
- Was there a general problem of dampness in other properties within the estate?
He identified any properties that were not affected by dampness, and also asked if there were any other reported defects of internal leaks of water, rainwater penetration or defects generally likely to contribute to internal dampness. (The most common problem in flatted accommodation is leaks from the dwelling above.)
Occupancy levels varied from overcrowded to adequate:
- Flat 1 – 2 adults 4 children
- Flat 2 – 2 adults 2 children
- Flat 3 – 2 adults 1 teenage son
- Flat 4 – 1 adult 2 children
Sleeping arrangements were the same in each flat: the children occupied 1 bedroom and the adults occupied the other.
Adults in 3 of the households received Housing Benefit and Social Security Benefit. The adults in the remaining household were in full-time employment. This gave an indication of likely occupancy.
The surveyor also learned that the former laundry rooms had fallen into disuse and were being used as caretakers’ storerooms and a community meeting room. Outside washing lines and drying areas were no longer used because of vandalism and theft of clothing. None of the dwellings had external balconies. In all the properties there was a washing machine and tumble dryer. Each property had a door fitted to the kitchen and bathroom, and in fact internal doors were fitted to each room throughout the dwelling.
The bathrooms and kitchens had mechanical extractor fans mounted through the window glass in each room.
The residents were unable to produce bills for the previous winter gas and electricity bills. This was unfortunate because a check of the fuel bills would, when compared to the fuel suppliers’ models for usage, give an indication of whether the residents were underheating the dwellings. Details of the cost of the fuel in relation to the income of tenants would have identified which tenants were in fuel poverty or severe fuel poverty (i.e. 10% of disposable income spent on fuel).
All of the tenants mentioned that, before the new double-glazed windows were installed, they had to continually wipe excess water droplets off the inner surface of the single-glazed timber sash windows. Often puddles formed on the tiled inner window sills during the cooler months of the year mainly between October and April.
They went on to explain that, since the double-glazed windows were installed, there had been little or no water droplets on the glazed surfaces and around the frames, but that moisture was still noticeable on the tiled inner window sills. Strikingly they all remarked that the mould to the corners of the external walls and to the wall surfaces behind the wardrobes and bedding had become worse.
Although from the initial walk-over and meetings with tenants the surveyor was convinced that this had all the hallmarks of a condensation problem, he could not be certain whether the cause was solely the lifestyle of the occupants or whether building defects were contributing to the cause of dampness. The landlord would be responsible for rectifying any defects in such a situation.
3. Further investigation – stage 3 and 4
To arrive at a sound ‘professional opinion’ in this case, the surveyor needed to proceed with a holistic analysis of the situation, and to engage the tenants in the process, in the hope that if they understood (and indeed took part in) the tests, they would fully accept the ultimate conclusions.
First the surveyor needed to establish a benchmark of dryness. He asked the tenants whether there was any area of the external walls unaffected by the damp; the living room was suggested. He asked permission to take a core sample from the wall to test using the calcium carbide method, and ask the tenant to show a ‘dry’ area of wall. He checked the area using an electrical resistance and a capacitance moisture meter, then took a core sample. The total moisture content (TMC) was just 0.2%.
He showed the result of the carbide test to the tenant, photographed it and recorded the result. He explained that this result was a ‘benchmark’ against which they could gauge the dryness of other external walls, and he made sure that the tenant accepted this explanation before moving on to the next test.
Next the surveyor performed calcium carbide tests on core samples of the inner leaf blockwork walls from the areas within each bedroom that had displayed the highest resistance readings. The results revealed that the core of the inner walls had just 0.2% TMC and therefore were ‘dry’ (when compared to the benchmark taken from the living room). To emphasise the point, the surveyor mentioned that there had been heavy rainfall recently and, although it had not rained on the day of the inspection, if there was a rainwater penetration problem then the inner leaf wall would still show signs of raised moisture levels above the benchmark position.
Here the physical examination of the external walls should have ended. But this was a case of years of an unresolved dampness problem and a group of residents who had understandably become sceptical. Therefore the surveyor carried out an optical examination of the cavity void using a borendoscope to check for any obstructions to the cavity void, cavity trays and cavity wall ties that could transmit moisture from the outer leaf to inner
leaf wall during periods of prolonged or wind-driven rain.
The cavity voids were clear, although there were slight blockages of the lead cavity trays by cementitious ‘snots’. (They were not sufficient to cause rainwater to penetrate.)
After observing these tests, the tenants were beginning to wonder what could be the cause of the damp if it was not coming from outside – a key turning point in the investigation. Indeed, after conducting these tests, listening to the tenants' views and making a plan drawing, one tenant said ‘You’ve now been here longer than all the previous inspectors put together’: but there was still a long way to go.
The surveyor told them that all the evidence gathered so far indicated that the problem of damp and mould appeared to relate solely to the results of active condensation caused by internally generated moisture. Their response was mixed. Most tenants said ‘But these external walls are very cold’ and added that they had heard about proposals to dry-line the walls because the cold walls might condense moisture.
In response the surveyor did a quick U-value calculation and explained how the heat travels through walls (see figures 4 and 5). Regulations require that U values are calculated to BS EN 6946, a more refined and detailed method that would make manual calculations difficult and impractical. New software packages have been developed, for example, by the Building Research Establishment, for the purpose of making U-value calculations easier for practitioners.
Figure 4: Calculating the U-value
Figure 5: How heat travels through the fabric
4. Monitoring
The surveyor decided to thoroughly check and measure the internal microclimate by installing timed data loggers to record:
- air temperature;
- relative humidity of the air;
- dew-point temperature;
- surface temperature; and
- vapour pressure.
The tenants were also asked to keep a simple record of the times that they cooked, washed and bathed.
The data loggers were placed in the kitchen, bathroom, hallway, bedroom 1 and bedroom 2. They were activated by computer software and pre-programmed to record the data every 15 minutes over a full 7-day period. At the end of the monitoring period the surveyor analysed the data and compared it to the resident’s log of moisture activity.
5. Analysis of results
The internal surface condensation would occur if there was excess moisture present at the times when the surface temperature of the wall material was below the dew-point temperature of the air. Therefore the surveyor needed to know (a) the dew point and (b) how much moisture was present in the dwelling.
The dew-point temperature depends on the relationship between the relative humidity (RH) and the air temperature in the dwelling. This calculation can be done quite simply using the ‘psychrometric chart’ (see figure 6) (BS 5250:1989 Appendix A). For example, when the internal air temperature is 20°C and the internal relative humidity is 75%, the dew-point temperature is 15.5°C. Putting this data onto the psychrometric chart, the vapour pressure was found to be 2.0kPa.
Figure 6: Psychrometric chart. Figure 46 taken from BS 5250:1989 is reproduced with permission from BSI under licence number 2003NC0027. BSI publications can be obtained from BSI Customer Services, 389 Chiswick High Road, London W4 4AL (Tel +44 (0)20 8996 9001)
The dwelling could be classed as ‘dry’ if there was proper use of the ventilation, and low moisture generation, where the internal vapour pressure was up to 0.3kPa in excess of the external vapour pressure. (Typical daily moisture generation rates for households are published in BS 5250:1989 Appendix B, Table 3.) In contrast, the living conditions would be ‘wet’ if there was high moisture generation, blocked-off vents, insufficient air changes internally and where internal vapour pressure was 0.6kPa or more higher than the external vapour pressure.
BS 5250:1989 (Appendix C, section C2.1) suggests the average external winter conditions are: 5°C and 0.83kPa (external vapour pressure). The data gathered (during November) suggested that the internal vapour pressure was 2.0kPa – a pressure difference of 1.17kPa. Under these pressure conditions, the dew point would move from outside the wall to the internal surface, as the internal temperature fell.
Under the conditions measured (75% RH and 20°C) the moisture content of the air (expressed as g/kg of dry air) would be around 13g/kg. However, when temperature fell to the dew point (15.5°C) the air would become saturated and water vapour would condense on the surface of the cooler wall. (The moisture content of the air would be rapidly reduced by around 4.5g/kg to 8.5g/kg.)
The results from the data logger (figure 7 shows sample of data logger results) showed that in flats 2, 3 and 4 the heating within the dwelling was switched off overnight. The surface temperature of the external walls was below the dew-point temperature for sustained periods during the early hours of the morning, and humidity levels fluctuated to over 80% RH during and following periods of high moisture production emanating from the kitchen and bathrooms.
Figure 7: Results from the data logger in bedroom 1
Flat 3 had the lowest occupancy level of all 4 properties, with only an adult couple and teenage son. However, on 3 days out of the 7-day test period the humidity levels in the hall and bedrooms rose dramatically to over 94% RH. From the tenants’ records this related to the use of the tumble dryer which, when in use, was moved from the kitchen to the hall where the front entrance door was opened in an attempt to remove the moisture from the dwelling because there was no hose connected to the appliance.
In flat 1 there was a much lower incidence of active condensation despite the property being overcrowded with 4 young children sharing the same bedroom. The quantity of mould colonies formed on the wall surfaces was at a much lower density than that found in the other dwellings. The significant difference was that the heating was left on for 24 hours a day during the winter period and never fell below 25°C. The temperature of the wall surfaces rarely fell below the dew point. Good levels of ventilation were provided.
Table 1: Example – from the box of figures take an internal air temperature of 20°C (shown on the left) and an internal relative humidity of 75% (shown along the top horizontal line)
Where the 2 indices intersect is the dew-point temperature. In this case the dew-point temperature is 15.5°C. If these same coordinates are applied to the psychrometric chart you will establish the same results with the additional information relating to vapour pressure, which would calculate to approx. 2.0KPa. The moisture content of the air expressed as g/kg of dry air would be around 13g/kg at 75% RH and 20°C. However, when dew point is reached at 15.5°C and the air becomes saturated, then the moisture content of the air would be rapidly reduced by around 4.5g/kg to 8.5g/kg. This is the process of active condensation as the air laden with water vapour forms a condensate on the cooler wall surface.
Diagnosis
Homes are considered at risk of condensation where internal RH rises above 70%. Water vapour moves by diffusion from a centre of high concentration to a centre of low concentration. In this case, water vapour generated in the kitchen and bathroom moved to the bedrooms where it condensed on the colder external walls.
Figure 1 illustrates the movement of water vapour. Factors affecting the movement included:
- kitchen doors left open during peak moisture production periods; and
- bathroom doors open during bathing (when young children were bathing).
The estate comprised approximately 160 similar flats. Many of the other occupied dwellings had not reported damp or mould problems and when randomly checked were indeed found to be free of damp associated problems. This closed the loop on the 4 properties featured in this study.
The surveyor explained to the tenants how moisture continues to build up in the bedrooms until a drop in temperature (usually overnight) results in condensation forming on walls and window surfaces where the surface temperature is below the dew-point temperature of the air. This process would be repeated each day in a 24-hour cycle.
Prognosis
Although the U-values of the external walls fell below Building Regulations’ standards, that should not be confused with the need to rush out and dry-line. The Building Regulations (Part L for England and Wales [Part J for Scotland] Approved Documents Parts 1 and 2) are primarily aimed at conserving energy. There is a difference between conserving energy and eliminating damp. In some cases we are achieving warmer homes but not necessarily dry, warmer homes. Increasing the thermal insulation of the external parts of the building will do nothing to prevent the over-production of moisture within an overcrowded flat or where occupants use liquid-based fuels and unvented tumble dryers. In this study, insulation would help with heat retention and it would lower fuel bills – useful to help tackle fuel poverty. But the proposal to dry-line the external walls of the bedroom would not in itself resolve the condensation problems.
In reality, condensation cannot be identified as the cause of the damp and mould. Condensation is a mechanism for the movement of moisture, not a ‘source’ in its own right. The source was a combination of inter-related factors – a complex web of use and occupation as well as design of the buildings.
The surveyor's recommendations were to:
- avoid drying clothes on radiators during the cooler months (in fact at any time);
- permanently vent the tumble dryers through the wall to the outside air;
- keep the kitchen and bathroom doors closed at all times during cooking, washing and bathing (self-closing door mechanisms should be installed);
- tackle the overcrowding;
- encourage the occupants to keep the central heating on for longer periods during the winter months, with a minimum background temperature of around 15°C and top up the heat to higher levels adjusted for comfort;
- remove the mould, treat the walls with an anti-fungicidal agent and redecorate;
- check that the radiators are the correct size for each room to ensure that heat output is sufficient;
- run regular winter campaigns as a reminder to residents on how to control moisture within the home;
- check income levels and maximise take-up of under-claimed benefits
- consider replacing fans in the kitchens and bathrooms with fans of a higher rate of extraction and re-site them through walls at high level with self-activating, self-cleaning humidistat-controlled sensors; and
- consider planned maintenance energy efficiency schemes to improve the U-values to current standards.
Lessons learned
- Avoid looking at condensation problems in isolation.
- Wherever there are similar properties to those being investigated, comparisons should be made.
- Avoid simply treating the symptoms.
- In general terms the other inspectors got it right by suggesting ‘lifestyle’ as the cause of condensation-led mould. But an in-depth investigation involving close cooperation with tenants, and data logging, had made it possible to identify specific activities that could be adjusted to improve living conditions.
- Close involvement of the tenants both during the inspection and in deciding upon appropriate remedies also made it easier for the occupants to support the necessary adjustment in lifestyle.
Additional commentsInvestigations into the cause and origin of dampness in occupied property do not come any harder than cases of this nature. In these cases it is difficult for surveyors to gain the trust of the occupants, even though they may be highly competent at conducting such investigations and may be independent consultants. The first question often asked by tenants is ‘Who are you working for?’ or ‘Is the Landlord paying you?’ The skills required to deal with these types of investigations are unlike most other surveys and can take a long time, and a great deal of experience and patience, to develop. Here is a checklist of essential things a surveyor should do, say and demonstrate (the list is by no means exhaustive but serves to illustrate some of the key points). Effective communications checklist
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