Background
This property, originally owned and later rented out, underwent damp-proofing at the time of purchase but now faces recurring issues. 🦠 Mould has reappeared within weeks of treatment, despite efforts in August to bleach and stain-cover with Zinsser and a top coat. Paint blistering and visible signs of damp remain apparent, raising concerns about the effectiveness of prior remediation. 💧 The property is currently rented to family and friends, with the landlord seeking to address the underlying causes to prevent further deterioration.
Data loggers – see damp.ai/detect
This data is taken from a logger in a representative bedroom. The green line represents the surface relative humidity, which measures the relative humidity on the cold surface of a damp wall. During the monitoring period, it remains excessive, frequently exceeding 85% RH (the mouldgrowth threshold) and often reaching 100% RH (the dew point). 100% RH indicates the level at which condensation forms on cold surfaces, creating conditions conducive to mould growth.
💙 Blue Line (Dew Point):
The blue line represents the dew point, indicating the temperature at which water vapourcondenses into liquid. To minimise the risk of condensation and mould, the dew point should be kept below the outside temperature. This can be achieved through targeted ventilation and dehumidification, which help maintain balanced indoor moisture levels.
🩶 Grey Line (Dew Point Differential):
The grey line represents the dew point differential, highlighting differences in vapour levels between areas. When elevated, it indicates that most of the vapour is originating from or passing through the kitchen. This suggests that moisture generated from activities like cooking is contributing significantly to the elevated humidity in the bedroom.
What is surface relative humidity?
It’s the RH at the surface temperature, not the air temperature. 🌡️ For example:
During the survey, the outside temperature was about 5°C, and the inside wall temperature was 7.3°C.
If a data logger recorded 75%RH at 10°C, the surface RH would be 90%RH—perfect for mouldto grow. 🦠 – see MouldPoint.co.uk.
Conclusion from data loggers
Vapour is not being removed sufficiently.
How to minimise mould risk?
The safest way to reduce the risk of mould is to remove vapour with:
- Targeted ventilation 🌀
- Dehumidification 💧
Follow Dew’s Law:
Keep the internal dew point below the outside temperature by:
- Increasing ventilation 🪟
- Cooking with tops on pots and pans 🍲
- Using a 20-litre-per-day dehumidifier 💨
- Keeping the bathroom extractor fan running until the room is dry, with the door closed 🚿🚪
- Drying clothes next to a dehumidifier in a closed room 👚💧
Important to remember:
- Ventilation cannot lower internal vapour below external vapour levels. The dew point is a measure of vapour. If it’s raining, ventilation is less effective because it replaces moist internal air with moist, rain-soaked external air. 🌧️
- Dehumidification works well as a backup, especially in damp conditions.
Evidence & Illustrations
There is a slight brown stain, suggesting rainwater penetrating damp, I understand that this was painted with a stain block. I suspect the penetrating damp dates back to before the gutters were cleared. The guttering around the property is long and therefore more vulnerable to overflow. Keep an eye on it during rainstorms every 6 months and consider having an additional downpipes, perhaps in this corner.
Internally, the plaster has been coated with an impermeable slurry. Rather than addressing the root cause of dampness, this simply hides the symptoms. Unfortunately, this treatment also reduces the wall’s insulating capacity and prevents it from absorbing and regulating moisture. To illustrate, consider the difference between blowing into an absorbent paper bag and an impermeable plastic bag: condensation is far more likely to form on the surface of the plastic bag. Similarly, walls treated with impermeable materials are more prone to condensation and mould.
The combined effect of these treatments is to increase, rather than reduce, the risk of condensation and mould in the property.
While mould typically requires at least 85%RH to grow, salt does not support mould growth because it lowers, or buffers, vapour: drawing in moisture above 75%RH and releasing it when humidity drops below 75%RH. This buffering effect is one reason salt is used as a food preservative, as mould cannot thrive on its surface.
The problem arises when the surface near the salt (e.g., a wall or window) is colder than the salt itself. Relative humidity is influenced by both vapour quantity and temperature. For every 1°C temperature drop, the relative humidity near a surface increases by approximately 5%RH. For example:
If the salt is in a room at 12°C, but a nearby wall is 10°C, the surface relative humidity on the wall will be about 85%RH—the ideal humidity for mould growth.
In such cases, the buffering effect of salt may inadvertently increase the risk of mould by maintaining high humidity on cold, vulnerable surfaces. In this case the salt would have a marginally detrimental effect, but in a closed environment such as a cupboard the effect can be greater.
To keep the property dry | |
|---|---|
| Cooking Tips | |
| Always cook with tops on pots and pans to reduce vapour emission by 80% and or use an externally ducted kitchen extractor fan – see below. Ideally keep the kitchen door closed and window open – or bathroom vent running, when it is humid. | |
| Target vapour from bathroom | |
| TIP: Keep the bathroom door closed with the fan running or window open until the humidity from the shower has subsided – typically at least 30 mins after the last shower. | |
| 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. | |
| Monitor internal vapour | |
| Use devices like the ORIA Wireless Thermometer Hygrometer to monitor relative humidity and temperature near walls or ceilings, look to keep the dew point at or below the external temperature, The dew point is shown on the app (if not use https://mouldpoint.co.uk to calculate the dew point from humidity and temperature readings). Increase ventilation or dehumidification as required. Note excess ventilation results in lost energy. The cold air replacing the warm air will have a dew point that is below the outside temperature. When it is raining, the dew point will be higher, so you may need more ventilation to achieve the same reduction in dew point. In winter, there is no risk in condensation inside, if the internal dew point is below the external temperature. | |
| Use fan to expedite drying | |
| Direct a fan onto the damp patch to speed up the drying process. | £50 |
| 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 a 6.5-litre tank (£179.99, refrigerant) Duux Bora Smart Dehumidifier (DXDH02UK, £300, refrigerant) 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 capacity, tank size, low noise, continuous drainage (pipe out), and ease of programmable functions over other considerations such as weight and aesthetics. The technology is well-established and functions similarly to fridges, with a comparable fire risk profile. If these brands are unavailable, you can confidently select another alternative. When evaluating, prioritise a minimum daily capacity of 20L and continuous drainage capability (use this to drip into a sink when away in winter). Tip: Use a dehumidifier whenever leaving a property vacant. Set it to 70%RH and drip into a sink—especially in winter. This method is more efficient than other humidity management methods, including ventilation. | £180 |
| Monitor dampness in walls | |
| Mark improvements on damp walls with a pencil line or sticky note and use a moisture meter, such as the Dr. Meter Wood Moisture Meter 2 in 1 Pin & Pinless Multifunctional Water Detector £23.99 or similar damp meter. Log and monitor the reduction in dampness to gain early confidence in these recommendations. | £23 |
| Keep objects away from walls | |
| Consider moving objects away from colds wall, especially in corners. | |
| Check rainwater goods | |
| Clear the gutter of leaves and debris to prevent water accumulation and ensure effective drainage. | |
| Mould Removal | |
| When mould is dry, consider sanding it down and removing it with bleach or anti-mould foam. While I personally recommend HG Mould Remover Foam Spray, the effectiveness between brands is likely similar. | £50 |
| Remedial actions – estimate of costs | £303 |
Actions if all else fails | |
|---|---|
| Duct out kitchen extractor fan | |
| Consider instructing general builder to duct out kitchen extractor fan (hole required to duct outside, which may require freeholder permission). | £500 |
| Consider targeted insulation | |
| Consider targeted insulation in specific areas of the home, including walls, lofts, floors, pipework, and reveals around doors and windows. A typical property could benefit from at least £2,000 worth of targeted insulation, significantly reducing energy costs and improving thermal comfort. Focus should be given to vulnerable areas, such as cold corners, cupboards, and flank walls, where condensation and mould are most likely to occur. For spaces like cupboards, where airflow is limited, consider materials such as thermal lining paper (e.g., Wallrock) or wood fibre insulation, often marketed as floor underlay but equally effective for lining walls behind cupboards. These materials improve surface temperatures, reduce the risk of condensation, and are easy to apply in tight or enclosed spaces. While thermal lining paper and wood fibre are vapour-permeable, making them suitable for reducing condensation risks, vapour management remains essential. Insulation should always work alongside ventilation solutions, such as discreet air vents in cupboards or cold flanks, to prevent moisture build-up. Remember, insulation is not a standalone solution but can help reduce cold surfaces where mould tends to develop, complementing a holistic approach to moisture management. | £200 |
| Optimal actions – estimate of costs | £700 |
I conclude that the primary cause of mould is excess vapour from the kitchen, bathroom and drying clothes.
Mould only grows where relative humidity exceeds 85%RH for long periods of time. The primary cause of mould is poor ventilation, insufficient air movement, made worse by cold surfaces.
By taking a root cause approach, rather than a focus on blocking the symptoms of damp the savings are likely to be around £10,000. 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 . I conclude that the primary cause of mould is excess vapour from the kitchen, bathroom and drying clothes.. 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
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.
- Rising damp concern: You were concerned about damp.
- Damp treatment: Despite the low risk of rising damp, there has been an attempt at chemical treatment against rising damp. Rising damp treatment is normally a sign of insufficient ventilation. The walls have been drilled into by about 200mm and injected with damp proofing chemicals. In theory the chemicals are absorbed into the brick pores reducing the bricks ability to absorb moisture. Walls are replaced with damp proofers slurry. This can cause problems with absorption imbalance, resulting in increased condensation on other walls or vapour becoming absorbed and trapped behind the slurry. There was evidence of surface condensation in areas probably not treated with replacement plaster but no evidence of trapped vapour. The solution is through ventilation and humidity control.
“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
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