Background
The property, a top-floor flat within a 1950s/60s block, is reported to exhibit multiple damp-related issues. Concerns include mould (both black and yellow/white), salt-like deposits, and cold external walls, particularly during winter. Previous measures included exterior work on coping stones and brickwork, alongside efforts to mitigate condensation through ventilation and dehumidification. The purpose of this survey is to investigate underlying causes, including possible structural or environmental factors, that may extend beyond condensation-related issues.
A straightforward answer to the question of whether structural factors are involved is: “No.” While heat loss can worsen the impact of excess vapour, condensation on modern double glazing is a clear indicator of inadequate ventilation or insufficient dehumidification in the property. Every property will have areas of reduced insulation, such as around cables, pipes, and other penetrations—particularly the cold water pipe. These areas can create thermal bridges, increasing the likelihood of condensation in those spots if humidity levels are not well managed.
Data loggers – see damp.ai/detect
💚 Green Line (Surface Relative Humidity):
Represents the relative humidity on the surface of walls or other materials. During the monitoring period, it remained consistently above 85% RH, a critical threshold for mould growth. At 100% RH, condensation forms on cold surfaces, increasing the risk of persistent dampness and mould.
💙 Blue Line (Dew Point):
Represents the amount of vapour in the air and is also the temperature at which condensation begins. To prevent condensation and maintain a healthier indoor environment, the dew point should remain at or below the 🩷 Pink Line (Outdoor Temperature) except for very brief periods. Achieving this requires controlling indoor humidity through ventilation and dehumidification—known as Dew’s Law.
🩶 Grey Line (Dew Point Differential):
Represents the difference in vapour levels between the bathroom and the main room. When the grey line drops below zero, it indicates an increase in vapour originating from the bathroom. Sharp spikes in this line suggest the bathroom door is being left open after showers, allowing humid air to migrate into the living room. The humidity spikes in the living room shortly after showers further confirm this behaviour.
❤️ Red Line (Internal Surface Temperature):
Represents the internal surface temperature, which was observed to be unusually low, around 10–12°C. Low surface temperatures increase condensation risk and contribute to greater heat loss. Drying the walls and addressing damp conditions can improve insulation and reduce heat loss over time.
🩵 Light Blue Line (External Dew Point):
Represents the external dew point, which reflects the vapour content of outdoor air. In a perfectly ventilated property, the internal air is continuously replaced with external air, resulting in internal vapour levels aligning with the external dew point. This highlights the importance of effective ventilation to control indoor humidity and minimise damp risks.
Conclusion from data loggers
Observations:
- The surface relative humidity consistently exceeded 85%, creating ideal conditions for mould growth.
- Bathroom vapour appears to migrate to the living room, indicated by spikes in the dew point differential and living room humidity shortly after showers.
- Insufficient management of post-shower humidity exacerbates damp and mould risks.
Recommendations:
- Keep the bathroom door closed after showers to prevent humid air from spreading to other areas.
- Ensure the bathroom extractor fan remains running for at least 30 minutes post-shower, which can be achieved by leaving the light on.
- Monitor the living room and bathroom humidity levels, increase use of the bathroom ventilation and use of the dehumidifier if the dew point remains above the outside temperature and or the property feels and looks damp and mouldy.
- Encourage tenants to maintain lower humidity levels by adopting simple practices such as keeping the bathroom door closed after showers, using extractor fans, and ensuring regular ventilation. Highlight how these steps can effectively reduce damp and mould risks, creating a healthier and more comfortable living environment.
Implementing these measures should help reduce damp and mould risks significantly.
Evidence & Illustrations
To keep the property dry | |
|---|---|
| Ask tenants to remove vapour from bathroom | |
| Ask tenants to keep the bathroom door closed with the fan running until the humidity from the shower has subsided – typically at least 30 mins after the last shower. Consider adding a spring to the bathroom door, or installing a spring hinge as an extra reminder. | |
| Ask tenants to remove vapour from cooking | |
| Cook with tops on pots and pans to reduce the production of vapour from cooking by 80%. Close the door and open the window when it is steamy, such as when cooking pasta. | |
| Ask tenants to remove vapour from clothes drying | |
| I would provide tenants with a means of drying clothes internally, which could be a powerful dehumidifier or tumble dryer (like a heat pump) and ask them to use it. Alternatively ask them to dry in an vented room with the door closed (possibly a bathroom with the extractor running or window open). Ask tenants to never to dry clothes in an unventilated room without a 20 litre + dehumidifier. Consider offering to pay for the cost of running a dehumidifier or heat pump dryer, in the region of £1 per day, plus it is energy efficient as all energy is converted to heat plus a 50% bonus. | |
| 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. | |
| Remove mould | |
| Consider sanding down mould where dry, and removing it with either bleach or anti-mould foam. I personally use HG Mould Remover Foam Spray, but imagine there is no major difference between brands. | £100 |
| Homework, Pets and Plants | |
| Recognise that all the water given to plants is eventually released as vapour. Additionally, pets, especially large dogs, can release as much moisture through respiration as an adult human. Working from home also contributes to indoor moisture levels. If this vapour isn’t effectively removed (e.g., through windows, extractor fans or dehumidifiers), it will be absorbed by walls, increasing heat loss and raising the risk of further condensation and mould growth. Ensure proper ventilation and dehumidification to minimise these effects – it can take 6 to 9 months of excess vapour condensed into a wall, to eventually dry out. | |
| Use fan to expedite drying | |
| Direct a fan onto the damp patch to speed up the drying process. Warm fans will speed the process up further. | |
| Improve airflow | |
| Water vapour moves primarily through diffusion, while heat is mostly transferred through airflow in a domestic setting. To improve air circulation and reduce dampness on walls, reposition furniture and open curtains, allowing warm air from radiators to reach the walls effectively. If shutters are present, consider removing them, or at least ensure they are kept open during periods of high humidity. Any material, such as curtains or wooden blinds, can trap humid air against cold surfaces, increasing the risk of condensation. Unlike curtains, shutters are often partially closed during the day, further restricting airflow and exacerbating the issue. | |
| Heat & insulation | |
To reduce mould risk, prioritise these solutions, ranked by effectiveness, but use them together:
Monitoring is also essential. No single solution is perfect, but they work best in combination. While insulation reduces heat loss, cold water pipes and structural elements create unavoidable thermal bridges. Heat should be consistent throughout the day, including when the property is unoccupied, and its impact improves with proper insulation, ventilation, and dehumidification. | |
| Remedial actions – estimate of costs | £100 |
Actions if all else fails | |
|---|---|
| Target vapour from cooking with an extractor fan | |
| Consider installing a kitchen extractor fan, ducted out. A tenant should use kitchen extractor hood whenever cooking. | £600 |
| Bathroom extractor fan rental | |
| Consider installing a continuous flow bathroom extractor fan such as Vent-Axia Extractor Fan With Humidistat & Timer Continuous Running and Timer, or Brookvent Airstream DMEV 100 HT (Continuous Running Extractor Fan with Humidistat Timer). | £400 |
| Consider targeted insulation | |
| Consider installing insulation around cold zones, including behind cupboards, around lintels, and any surface currently damp or mouldy. Options such as wood fibre insulation or thermal lining paper (e.g., Wallrock) can be effective. While insulation is often recommended as the solution, it is only part of the answer. Insulation can help reduce condensation, but unless you address the root cause—excess moisture in the air, resulting in a dew point that exceeds the external temperature—the problem will simply shift to another surface. | £2,000 |
| Optimal actions – estimate of costs | £3,000 |
I conclude that the primary cause of internal dampness is excess vapour from insufficient ventilation, specifically the bathroom fan is slow, I have cleaned it out, improving the speed to near building regulation speeds, but the external outlet is made of porcelain, with relatively small holes. The fan would benefit from having an overrun of 30 minutes. But this can be achieved by leaving the light on and the door closed for at least 30 minutes after taking a shower or bath. And the kitchen extractor fan is not externally ducted. Vapour from cooking can be controlled by cooking with tops on pots and pans, turning the flame down as well as closing the kitchen door and keeping the window open when humid. The dehumidifier should be used whenever drying clothes internally or vapour levels are too high. You should monitor internal vapour levels, with reference to the dew point on the data logger, keeping the dew point below the outside temperature – use the https://www.bbc.co.uk/weather to find out current and forecast external temperatures.
Mould only grows when surface relative humidity excess 85%RJ for long periods.
I improved the flow rate from the extractor fan. By taking a root cause approach, rather than a focus on blocking the symptoms of damp the savings are likely to be around £5,000. 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 22 November 2024. I conclude that the primary cause of internal dampness is excess vapour from insufficient ventilation, specifically the bathroom fan is slow, i have cleaned it out, improving the speed to near building regulation speeds, but the external outlet is made of porcelain, with . 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 22 November 2024
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.
Relevant examples of similar damp issues or remediation:
Mould From Ventilation Into Loft Blocked
[mwai_chatbot_v2 id=”default”] 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?
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?