Background
Kenwood conducted a survey and quoted £15,000 for a damp proof course, during a property transaction. The buyer was sceptical about their business practices and the severity of the issue. The building is grade 2 listed, so invasive treatment is not an option. The only visible sign of damp is in the bedroom (external wall) extending a few feet up the wall, still damp to the touch. Kenwood’s quote indicated a large area needing damp proofing. Potential damp issues in bathrooms due to poor ventilation are not a major concern.
Data loggers – see damp.ai/detect

Conclusion from data loggers
There was a risk of condensation during the period of the survey. It is a short spike and in the summertime. The dew point humidity drops back quickly. Try to follow Dews Law.
Dew’s law states that internal vapour, measured in degrees centigrade as the dew point, should always be kept below the outside temperature, day and night, using ventilation and dehumidification.

1) Look on the data logger app Sensor Blue or, possibly better still,
2) Buy a dew point meter, such as the one shown here (about £16) to constantly check that the dew is not above the outside temperature, and the forecast outside temperature (MetOffice or BBC weather forecast). In the example above, the internal dew point is 17.6°C. While 68%RH may seem like normal relative humidity, a pipe, wall, floor etc. in contact with the outside risks mould and condensation when the outside temperature is below 17.6°C, in this case.
3) You can check past performance, by using my online data logger analysis tool at https://damp.ai/detect/ ,
One of the interesting points is rain is a form of condensation. When it is raining the dew point the outside temperature and outside dew points are exactly the same. This makes standard vapour removal through untargeted ventilation comparatively inefficient as you are replacing internal air, with vapour laden external air. A dehumidifier is often more efficient and cost effective than ventilation. A combination of both should be used to drive down and manage the internal dew point.
It will take about 9 months for the wall to dry out. You can speed this up by constantly keeping a 20+ litre per day dehumidifier running 24/7, including when away on holiday (dripping into the sink), and a fan directed at the damp wall.
Evidence & Illustrations








To keep the property dry | |
|---|---|
| Establish if there’s a leak | |
| There is definitely condensation, which would have taken place when the property was rented and which can look and act like a leak, with central damp zone and brown stain. However, it is possible that there is also a leak from the old lead pipe and or from your neighbour above. Both of those are less likely and you won’t know for sure until after a prolonged period of drying, after wiping with a cloth, then large fan directed at the cold water-pipe, and nearby dehumidifier (20 litre per day capacity). When completely dry, look to see if water returns. The pipe would probably be the responsibility of the freeholder and it’s management company. | £2 |
| Use a dehumidifier | |
| Maintain relative humidity, especially at night when it’s cold outside, with a dehumidifier such as PureMate 20L/Day Portable & Compact Dehumidifier with 6.5 Litre (£179.99 Refrigerant), Duux Bora Smart Dehumidifier (DXDH02UK £300 Refrigerant) or Meaco DD8L Zambezi (£260 Desiccant – best for colder environments) – Which! Best buys. There are plenty of alternatives available. I choose a dehumidifier based on daily and tank capacity, low noise, whether it has a pipe out, and ease of programmable functions over other considerations such as weight and aesthetics. The technology is old and easy to manufacture. So if these brands are not available, you are sure to find another alternative. Evaluate based on daily capacity (20L minimum) and whether it has a continuous pipe out (use this to drip into a sink when away in winter). | £180 |
| Use fan to expedite drying | |
| Direct a fan onto the damp patch to speed up the drying process. | £50 |
| Monitor dampness in wall | |
| Monitor improvements on the damp wall with a pencil line and or damp meter such as dr.wood moisture meter, logging improvements each month. | £20 |
| Reduce vapour from cooking | |
| Cook with tops on pots and pans to reduce the production of vapour from cooking by 80% and use the extractor fan whenever cooking it appears to work well. | |
| Bathroom Humidity Management | |
| After showering, keep the bathroom door closed and the window open or fan running for at least 30 minutes to allow vapour to dissipate. | |
| Target vapour from clothes drying | |
| Use a tumble dryer (like a heat pump), dry in a vented room with the door closed (possibly a bathroom with the extractor running or window open), or use a powerful dehumidifier. Don’t dry clothes in an unventilated room. | |
| Follow Dew’s Law | |
| Follow Dew’s Law to effectively manage internal vapour, it is crucial to maintain the internal vapour levels, as measured by the dew point in degrees Centigrade, below the external temperature. This can be achieved through adequate ventilation and the use of dehumidifiers. Monitor the internal Dew Point with a reliable device, such as the Habotest Hygrometer or Dew Point Meter HT607, or utilise the sensorblue app paired with the ORIA Wireless Thermometer Hygrometer. These tools help in accurately gauging humidity levels, ensuring they remain optimal. For more detailed information on detecting and managing damp issues, please visit https://damp.ai/detect. Although vapour levels were not excessive during the monitored period, it is essential to prepare for the colder months when the risk of condensation is at its greatest. | |
| Insulate pipes | |
| Insulate around all water pipes, especially cold water. | £10 |
| Remedial actions – estimate of costs | £262 |
Actions if all else fails | |
|---|---|
| Hack for solid floors | |
| I don’t think you will need this hack, but you won’t know for about 6 months after stopping the water and drying. Solid floors can absorb moisture, thus amplifying the effects of condensation. If an absorbent plaster wall is in contact with a solid floor, even one that is tiled over timber, there is a risk of symptoms akin to rising damp. To mitigate this, reduce the absorption of plaster at the base of the wall, which may involve temporarily removing skirting boards. Cut back approximately 10-20mm of plaster to expose the brick at the base of the wall. Apply a ~10mm thick bead of thixotropic cream, such as DryZone by Safeguard, along the exposed brick to reduce absorption. You don’t have rising damp, so manufacturers’ recommendations on rising damp should be disregarded in this context. Once treated, replace the skirting boards. I do not advocate drilling holes and injecting any type of chemical treatment for rising damp. | £1,000 |
| Bathroom extractor fan | |
| In my opinion, the optimal fans are ones that are targeted, with the fan located near the shower head (without being so close that water sprays onto it), removing vapour during and for at least 30 minutes after taking a shower or bath (which typically produces slightly less vapour). Ideally the fan should be separately wired up with an overrun. Installing the overrun can account for a majority of the cost. Consider installing a continuous flow extractor fan such as Brookvent Airstream DMEV 100 HT (Continuous Running Extractor Fan with Humidistat Timer). Note a continuous flow fan will expel warm air, thereby increasing heating cost by about £120 p.a. but this may be a small price to pay. You can use an isolator to manually turn off the fan when the property is sufficiently ventilated, but the risk is it is left off. | £600 |
| Optimal actions – estimate of costs | £1,600 |
I conclude that the primary cause of internal dampness is excess vapour from insufficient ventilation causing condensation to form on the cold water pipe, mainly in winter when the flat was tenanted. Condensation, such as on a cold water-pipe, can act like a leak, transferring moisture across an impermeable floor. It is possible that there is also a leak from the old lead pipe and or from your neighbour above. Both of those are less likely and you won’t know for sure until after a prolonged period of drying with a cloth, then large fan directed at the cold water-pipe, and nearby dehumidifier (20 litre per day capacity). When completely dry, look to see if water returns. .
Rising damp is highly unlikely in London, where groundwater—the source of water in rising damp—is pumped out to about 70 metres below ground. If there is a central brown stain, the damp is most likely originating from somewhere near the stain. It is incumbent upon any damp surveyor to trace water to its source and recommend methods of stopping the source of damp. Otherwise, it will continue to cause internal dampness, whether or not walls are treated against rising damp.
. By taking a root cause approach, rather than a focus on blocking the symptoms of damp the savings are likely to be around £15,000. This contrasts with typical damp proofing contractor recommendations such as chemical treatment for rising damp and untargeted ventilation, such as PIV systems.
Flood risk: No risk See Flood Risk (note most flood risk is not from groundwater).
Sub-soil rocks: Rocks with essentially no groundwater See British Geological Survey.
Therefore risk of Rising Damp: remote possibility, see explanation by Dr Robyn Pender of Historic England.
I confirm that I inspected the property on 240808SH1000. I conclude that the primary cause of internal dampness is excess vapour from insufficient ventilation causing condensation to form on the cold water pipe, mainly in winter when the flat was tenanted. condensation, such as on a cold water-pipe, can act like a leak, transfe. All buildings can be exposed to unvented vapour and external dampness to some degree. You will mitigate the risk of damp if you follow all our recommendations. This report is intended to be read in full including supplementary links in this report. Observations and opinions must not be taken in isolation. Given the right weather conditions almost any house can be affected by rain. Like any building, you need to be aware of the risks of damp arising in the future and should plan a programme of monitoring, prevention and maintenance accordingly. Damp Surveys Ltd reports are designed to provide you with an informed, independent expert opinion as to the cause of dampness in the property, together with any recommendations for further investigation or remedial work. We do not warrant any findings in this report unless we enter into a separate warranty agreement with you.
The survey was conducted during daylight hours. Damp will be more noticeable at night and when the weather is colder and more humid. Gutters are more likely to fail when full of leaves and during periods of prolonged rain and adverse wind. We make best endeavours but cannot guarantee being able to identify all forms of damp, rot and insect infestation affecting the property.
We carried out a careful and thorough inspection of as much of the property as was accessible. However, when a full inspection is not possible, we make a professional judgement regarding the likelihood of a defect being present. In certain circumstances, this may lead to a recommendation for further action to open up an area for additional investigation. We were unable to inspect the entire roof, all the guttering, and some of the drains. Similarly, inspection of woodwork or other parts of the structure that were covered, unexposed, poorly lit or inaccessible such as in the loft, cellar, or sub-floor void was not possible, and therefore we are unable to assert that such parts of the property are free from defect. There were no obvious signs of damp resulting from these limitations
Negligence claims must be made within 90 days of the damp survey site visit. Clients must make best endeavours to monitor humidity and temperature on a damp wall and the closest source of humidity, by purchasing and placing two data loggers for example the Smart Hygrometer measuring humidity on the wall and nearest source of vapour, before and after following our recommendations. These data loggers have sufficient memory capacity to store 90 days of data.
This report is for the sole use of the client (named on the invoice, but omitted here for anonymity) for whom the survey was undertaken and can only be relied upon for 90 days from the survey date. Unless expressly stated otherwise in this report, nothing in this report confers or is intended to confer any rights on any third party pursuant to the Contracts (Rights of Third Parties) Act 1999.
Simon Hichens
Site visit 240808SH1000
Simon Hichens BSc Chemistry RICS Qualified Expert Witness (Chartered Surveyors)
PCA Certificated Surveyor of Dampness & Timber in Buildings (Property Care Association)
Damp Surveys Ltd, 119 Longton Ave London SE26 6RF Reg. 10977488 (England & Wales)
Founder of damp.aï & Member of the Society for the Protection of Ancient Buildings (SPAB)
Our damp surveys are designed to provide a holistic diagnosis that identifies and deals with causes rather than a focus on symptoms. Symptoms will disappear over time, once damp is stopped at source. We look for risks of dampness, but unless moisture is present, we can’t guarantee to identify all risks of future moisture. Our reports:
- identify root cause(s) of major unwanted dampness within the property,
- identify major symptoms of unwanted dampness,
- identify major structural damage caused by dampness,
- recommend actions and estimated costs to stop damp at source and mitigate its effects,
- recognise that treating damp is often a staged approach, treating obvious causes first.
Understanding damp
For survey purposes, damp is defined as unwanted water. Water brings life. We need it. Unfortunately, fungus and insects also thrive in water. While a damp wall can cause decorative spoiling, it is not moisture, but the life it brings that causes the greatest concern in the home environment. FACT: vapour causes 85% of residential damp problems.
Independence and Methodology
Our only income is through damp survey fees. We are independent of contractors and never profit from remedial work. We do not receive or pay any fees or other inducements. Our motivation is peace of mind and practical, durable solutions. We use an array of equipment to identify the root cause of damp within walls. Our damp surveys follow guidance from RICS, Historic England and PCA joint position statement and support RICS’s highest standard, level 3 “Home Survey Standard” (HSS).
During the survey we assess likelihood of rising damp*, mainswater leak, penetrating damp, condensation, timber rot and woodworm. We profile the damp patches and consider the likely sources of dampness and test ventilation against building regulations requirements. We consider changes to the property and factors just before the first sighting of damp.
“Management of moisture requires proper control of temperature and ventilation to avoid excess humidity, condensation on surfaces and excess moisture in materials. Ventilation should be distributed effectively throughout spaces, and stagnant air zones should be avoided.”
FACT: Houses contain 30 – 40 litres of water as vapour typically. Each day, each occupant adds on average about 1.5 litres as follows:
- Each shower results in 1/2L condensing (each additional shower in a series won’t add as much as the first, so long as the door is kept closed)
- Vapour from drying clothes releases about ½ litre per day if dried indoors
- Respiration releases about ¾ litre per day (of which ¼ l at night)
- Cooking produces about ¼ litre per day per person
- Unvented vapour can cause condensation, mould and allergenic dust mites. The simple rule for keeping your property dry and mould free is;
- Vent out as much vapour as produced, each and every day!
- Central heating and double-glazing reduce background ventilation. To overcome this you should use mechanical extractors. Where properties are rented, consider using monitoring devices (such as a Govee WIFI hygrometer) and if necessary, install continuous flow extractors, but beware there can be a thermal cost caused by continuous heat lost.
FACTS about Condensation and Mould
- FACT: warm air holds more water vapour than cold air. The warmer it is, the more air’s capacity to hold water vapour. Conversely the colder it is, the less capacity, until air cannot hold any more vapour. We call that the dew point or 100%RH. As soon air reaches capacity, condensation will form on the coldest surfaces.
- FACT: Mould and dust mites
- FACT: Mould only grows when air is humid for long periods. In the occupied home environment, it is 6 hours over 85% RH. For properties that are unoccupied for long periods, it is 75% RH for many days of consistent and high humidity. Avoid allowing the surface relative humidity of cold walls to exceed 85% RH by ventilating, dehumidifying, insulating, and ensuring airflow, particularly in cold corners, base of walls and ceilings, eaves, and cold window and door reveals.
- FACT: Dust mites are microscopic insects that can cause allergies. They grow in similar conditions to mould. So ventilate more to avoid allergies.
- FACT: Health concerns: There are no health and safety issues from damp noted. It is in the occupier’s interests to keep a property properly ventilated or dehumidified and temperatures above the dew and mould points throughout the property, thereby reducing the health risk associated with dust mites, bacteria, protozoans, as well as decorative spoiling caused by mould.
TIPS To Stay Condensation and Mould Free
- TIP: Reduce the production of vapour at source:
- TIP: Keep bathroom extractor fans running, or window open for at least 30 minutes.
- TIP: Keep bathroom doors closed at all times, and bathroom windows open safely.
- TIP: When filling the bath, run the cold water first then add the hot.
- TIP: Dry clothes outside, in a dryer, in a closed room with a dehumidifier or window open, never on radiators or heated rail without a door closed and a means of removing the vapour.
- TIP: Cook with tops on pots and pans, avoiding excessive boiling.
- TIP: Keep the kitchen door closed and extractor fan on where possible.
- TIP:Mechanical vents are best, but if you only have passive vents, such as a whole in the wall or chimney, then vents open with space for air to move, insulate where possible.
- TIP: Keep trickle vents open or alternatively, open windows on safety locks.
- TIP: Stop mould forming:
* Maintain external wall temperatures above 12°C, with low background heating.
* Declutter leaving space for warm air to circulate around cold surfaces.
* Where possible, avoid placing wardrobes and furniture against external walls.
* Avoid overfilling wardrobes and cupboards as it restricts air circulation.
* Use dehumidifiers or small wardrobe dehumidifying bags and replace regularly.
* Wipe mould and condensation off walls and clothes as soon as it appears. - TIP: Monitor relative humidity
* We recommend monitoring temperature and relative humidity with a data logger to see what is happening.
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?
潮湿报告说什么是潮湿的主要原因和防止潮湿的主要措施?
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?
