Mould despite multiple damp proofing attempts


Damp was identified at the base of the wall during the homebuyers survey of this recently purchased Victorian house in London. A damp proofer was asked to quote for work. Rising damp treatment was recommended costing £4,000 despite there having already been two previous attempts at damp proofing against rising damp.

image of the front of the Joint Position Statement RICS Historic England PCA (property care association) dated September 2022, featuring picture of drain


RICS (Royal Institute of Chartered Surveyors), Historic England (a public trust protecting sites of historic interest) and the Property Care Association (an association of mainly damp proofers) set out a Joint Position Statement (JPS), in September 2022 – see below.

There has long been disagreement about the causes of damp amongst surveyors. Historic England surveyors represent one extreme. They typically do not believe rising damp is a likely cause of internal dampness and rarely recommend altering walls to alleviate damp. By contrast PCA surveyors typically recommend rising damp treatment whenever they see dampness at the base of a wall and often recommend altering a building. RICS surveyors tend to be somewhere near the centre of these extremes. However, unlike Historic England and PCA surveyors, they rarely specialise in damp and often refer issues to PCA surveyors. We and most other independent surveyors are more closely aligned with Historic England’s position except that we are perhaps more pragmatic. For example, if a hole in the wall is the optimum way of solving internal dampness, then we will recommend it, but not otherwise.

JPS questions and answers

In addition to the content typically included in a professional and impartial survey report (e.g. client, surveyor and property details; limitations and exclusions), the report should include the following information:
1 A summary of the surveyor’s observations, including an Assessment of Significance.

dampness at base of walls and mould up chimney breasts

2 The importance of regular building maintenance and how a lack of even basic maintenance can lead to problems arising. This is especially important and relevant to roofs, gutters, hidden gutters, rainwater disposal and changes in external ground levels.

As is common with all buildings, gutters and other rainwater goods should be checked twice annually, ideally during or soon after a rainstorm, for evidence of overflow and rain not being dispersed properly from the property. You should also check that there is no flow of water flow during periods of absence for signs of a leak, for instance, by note the number on the mains water meter outside your house. external render needs to be maintained otherwise there is a risk of penetrating damp

3 An impartial diagnosis of the damp problem, which should be clearly explained and laid out systematically to communicate the relevant risks to the reader.

dampness from excess vapour

4 Where recommendations for the repair and recovery of moisture-affected structures are stated, the solutions should be clearly explained and prioritised as appropriate.

The solutions are explained in the Recommendations section. The primary recommendation is cover render on external walls and maintain internal vapour consistent with outdoor vapour by improving ventilation and dehumidification

5 The client should be provided with alternative options for which each recommendation should include advantages and disadvantages and the relevant cost and time implications.

This question is primarily directed at PCA damp proofers, to encourage them to consider methods that address the source of water not just the symptoms, such as by hacking off plaster and replacing it with slurry. You can sometimes take this damp proofing approach with short-term success. However, water will eventually find its way through. Furthermore, treatment is disruptive resulting in high cost and a poor finish, with no worthwhile guarantee of success. It likely to lower the value of the property. In our opinion you are better off trying to stop the water at its source, as we recommend. ”

6 Where monitoring and staged interventions are appropriate or beneficial, the client should be informed of the reasons for these options, together with the likely cost and time implications of such recovery strategies.

Properties should be monitored for humidity and temperature using data loggers, see datalogger section of this report. The cost of data loggers is currently less than £20 for two, aim to keep the dew point, as shown on the app screen of each logger (bottom centre on the SensorBlue), within 3°C of the external temperature, especially at night (use ventilation and/or dehumidification to reduce the internal dew point – measure external conditions or refer to weather forecasts for night-time temperatures, such as BBC Weather – a short explanation is; that this will make sure the relative humidity of all surfaces will be below the mould point even if poorly insulated). We also suggest noting values monthly using a multi-functional radio wave and conductance damp meter costing about £24 to monitor and map damp patches on internal walls. This way you can monitoring the dry progress, which typically takes 1 month per 25mm thickness of walls and more if coated. This helps determine if the root cause has been fixed or there is an additional or new source of water, and therefore more actions are required. Ideally this should be started before or soon after starting our recommendations and for about a year, repeating monthly to check progress.

7 Where appropriate, clearly state whether the building concerned represents a financial risk to the buyer or lender and why.

8 Any conflicts of interest should be clearly stated.

There is no conflict of interest. We never profit from our recommendations.

As a background, although we charge a fee for undertaking damp surveys, profit is not our primary motivator. We are motivated to:

  • solve damp problems in properties,
  • encourage clients to spend money wisely for the protection of the property and, where possible to lower the environmental impact,
  • stop or at least play our part to reduce the over-diagnosis of rising damp and so called “damp proofing” treatment by replacing plaster with chemicals and slurry, rather than dealing with the root cause.
Chimney breasts opened up
Questions and Answers
Q: Can you please advise the damp meter you had which you advised would be better to use? Could you clarify what would be considered wet/dry from this so that we can monitor the back corner drying out?
A:Dr. Meter damp meters (see recommendations) should not be considered quantitative, but qualitative (the number isn’t so important), and relative to a known dry section of wall, be it an internal solid wall or high up on a wall. They can be used for profiling damp, identifying the source of damp, if there is a leak or penetrating damp and for determining if a damp wall is drying out.
Q: If we were to plaster over that corner and make good, will the wall still gradually dry out? We’re obviously keen to have a ‘finished’ living room! Do we just risk decorative spoiling that may need touched up again?
A:Personally I would either let the wall dry out without plaster (making sure to protect the outside from rain), or dry line the wall (typically with battens or dot and dab), or an adventurous homeowner would use thermal insulating plasterboard (especially in the chimney), but beware of the risk of condensation forming within the wall potentially causing rot. Please note that insulation is not my area of expertise.
Q: We’ve had one of the dehumidifiers delivered and up and running. I think you mentioned, but did you say to run continuously in the living room in order to speed up drying out the corner?
A:Yes, I would, with a fan running.
Q: Its quite noisy when it gets going (tried to send you a video as attached) – almost as if it goes from fan only to a compressor running, does yours do this? No quiet mode for it to operate in the background?
A:I can’t speak for the dehumidifier manufacturers, but agree that noise versus speed of drying is an important consideration. You could run it for periods of the day. Personally I run the noisier dehumidifiers on timers.
Q: Which piece of equipment would be most accurate for the current RH reading? For example, noting you left your black hygrometer – this read 51% whereas the dehumidifier measured 55% and the sensor/app stated 63% when all in close proximity?
A:They use the same technology, which is a material, I think a specialist type of plastic that curls up depending on relative humidity (“RH”) but not temperature. RH is affected by temperature. In the same room the dew point is likely to be the same (a measure of vapour in air), but RH will differ depending on temperature. Most of these meters say they are accurate to I think about 5%RH, in my experience about 1 in 10 of these meters are more that 10%RH inaccurate, this level of inaccuracy can be tolerated when you are considering relative changes, e.g. does sub-floor RH remain high when external RH drops. You can easily check the accuracy and calibrate using salt and a few drops of water in a sealed container like a clean jam jar. Sodium chloride (pure salt – sea salt has fewer added chemicals) is hygroscopic, causing deliquescence at a relative humidity of 75%RH at room temperature.
Q: Chimney Insulation – I’m keen to take up your advice on insulating the single brick at the back of the chimney.
A:The least risky option is to use wood fibre, but it is not the best. Please don’t take my word for this, I cannot be held responsible if this recommendation causes condensation to form between the insulation and wall, but I might be tempted to use PIR (thermal insulating plasterboard such as Kingspan), measuring RH within the wall behind the insulation, checking joists every 6 months for signs of dampness or rot.
Q: Is there a risk of bad mould growth between the insulation and the brick – given the wall is already wet? I know vapour barriers from the inside are a consideration when doing wholesale internal insulation.
A:Theoretically yes, but in practice I doubt it and can’t remember examples of mould growing within a void, unless it is pre-existing – make sure to throughout kill the mould beforehand such as with bleach. Mould needs oxygen, high humidity and a food source. If an area is sealed such as within a void, mould is unlikely to spread out, so even if it were to germinate, it would not spread within the void and out into the air you breathe. This is my gut feeling from casual observation. I have though about it. It’s probably too obscure for research. If there ever have been scientific papers written on it, I am not aware of them.
Q: How would you recommend I fix the insulation to the wall – assuming 50mm PIR? (apologies if this is a stupid question . . )
A:Most people use dot and dab (that is an adhesive, normally recommended by the manufacturer, if not ask your friendly retailer).
Q: 2no. plug sockets down the side of the living room (towards the back corner) have elements of rust in back boxes and a bit of corrosion on the screws (attached). Noting your initial diagnosis of the living room as condensation, is it possible for condensation to cause this?
A:Clearly any source of water can cause rust and any source of water can cause condensation through evaporation, such as within a socket. So if the root cause was penetrating damp near a socket, it would almost certainly also cause rust. If the root cause of dampness in the wall is condensation, then it will often cause dampness around a metal electrical box as metal causes heat loss. Condensation will often cause condensation on exposed screws.
Q: There is also similar rust on the plug socket in the back bedroom, on the wall that had the apparent low level condensation.
A:See my answer above.
Ask A.I. about your report, in the “Type your message” you could ask:

What does the damp report say are the main causes of damp and main actions to stop damp?

¿Qué dice el informe de humedad sobre las principales causas de la humedad y las principales acciones para detenerla?
नमी की रिपोर्ट के अनुसार नमी के मुख्य कारण और नमी को रोकने के लिए मुख्य कार्य क्या हैं?
Que dit le rapport sur l’humidité concernant les principales causes de l’humidité et les principales actions pour l’arrêter?
Was sagt der Feuchtigkeitsbericht über die Hauptursachen von Feuchtigkeit und die Hauptmaßnahmen zur Bekämpfung von Feuchtigkeit?
Что говорится в отчете о влажности о основных причинах влажности и основных действиях по ее устранению?
Cosa dice il rapporto sull’umidità riguardo le principali cause dell’umidità e le principali azioni per fermarla?

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