Background
This recently extended house has three main areas of concern related to damp. In the kitchen, damp patches have reappeared at floor level despite prior damp-proofing by the previous owners and recent remedial work by builders, raising questions about the underlying cause. In the loft conversion, dampness has been observed on a chimney breast, a problem also affecting the neighbour’s chimney. These chimneys are functional, while others in the property have been capped, and guidance is sought on managing this issue. Finally, the semi-raised cellar, housing fuse boxes installed by builders, shows signs of dampness, prompting concerns about the safety of the electrics and whether tanking the space is advisable. The homeowner is seeking clarity on the root causes of these issues and practical solutions to resolve them effectively.
The property is extremely well-insulated. However, great insulation can sometimes mask poor ventilation. Vapour is invisible, yet it is always present and seeks out cold surfaces, such as near a cold cellar.
Data loggers – see damp.ai/detect
The above is a graph from the data logger placed in the cellar comparing it to the logger placed in the kitchen. The green line is called the surface relative humidity. It reaches 100%RH, when condensation takes place between the start, 28th October and 12th November. The data and this graph are unusual and suggest that two heating systems are affecting the vapour and temperature, one heating system appears to be keeping a constant temperature in the sub-floor, perhaps underfloor heating in the kitchen, and other, perhaps central heating, with a cycle of daily spikes starting on 12th November. The grey line at the base of the graph is called the dew point differential. It shows that there is consistently more vapour in the kitchen than in the cellar.
Conclusion from data loggers
A) There is insufficient sub-floor ventilation,
B) Vapour is greater in the kitchen.
Evidence & Illustrations
Primary Cause of Dampness
Having reviewed all the data, I believe the primary cause of dampness comes from excess vapourin the utility room, bathroom, and kitchen areas. Ideally, monitor dew points (as seen on the Sensor Blue app), keeping them below the outside temperature by using ventilation and, if necessary, dehumidification. Over time, the dampness at the base of the wall should disappear, which you can confirm using a damp meter.
A thought occurred when reviewing this report. While I doubt these are signs of hygroscopic salts, it is not uncommon for hygroscopic salts to be released when plaster is removed from chimneys. These salts come from the historic burning of coal. They cause deliquescence, which is a form of condensation, at lower levels of relative humidity than the standard 100%RH. The treatment is very simple, to cover in an oil based primer, such as Zinsser Coverstain, see recommendations in the “if all else fails” section, below.
Other matters
Roof Issues
This is not a roof survey per se, but rather an investigation of the root cause or causes of internal dampness.
A) A crack in the tile, just visible as a line near marker A.
B) Moss growth, which can retain water and should ideally be removed.
C) Flashing sitting on top of the tiles, as is common in London. However, it is unclear whether corresponding waterproofing material exists beneath the tiles to prevent rainwater ingress.
While (C) is less likely to be the root cause compared to (A) and (B), and condensation, I flag it as a potential source of water ingress for further investigation, should (A) and (B), and improved internal ventilation in the loft, fail to resolve the damp issue on the chimney breast.
To keep the property dry | |
|---|---|
| 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 |
| Monitor sub-floor relative humidity | |
| Place one data logger in the sub-floor void and another, externally protected from rain a heat. Compare the relative humidity and dew points recorded below the floor with those outside. Aim to increase sub-floor ventilation (see guidance above) until: Sub-floor humidity consistently tends towards 85%RH or below. The internal dew point is regularly within 1°C or less of the external dew point. Dew points indicate the amount of vapour in the air and are a critical measure for assessing moisture levels. Both readings will be influenced by air from the property. Efforts should be made to minimise vapour movement from the house into the sub-floor void, where it could condense and exacerbate moisture-related issues. | £20 |
| Have the roof checked | |
| Clear the moss from the roof by the chimney, and monitor loft ventilation. Furthermore, I believe I observed a cracked tile (see images above); if confirmed, this should be repaired or replaced. Consider having the waterproofing material under the tiles above the chimney checked. However, this could be invasive and inconclusive, so I recommend addressing the above issues first and monitoring dampness in the chimney breast. | £500 |
| Remedial actions – estimate of costs | £773 |
Actions if all else fails | |
|---|---|
| Consider improving the ventilation | |
| Consider improving the ground floor bathroom and utility room ventilation, ideally in separate ducts. If together then the backflow shutter should be in the sections of ducting before they meet and not on the section after. | £500 |
| Consider improving sub-floor ventilation | |
If the relative humidity in the sub-floor consistently remains above 85%RH for extended periods, and the dew point in the sub-floor does not align closely with or fall below the outside dew point, consider increasing sub-floor ventilation. This can be achieved by:
Additional Notes:
By ensuring sub-floor ventilation is appropriately managed and monitored, potential moisture-related issues can be minimised, supporting a healthier and drier environment beneath your property. | |
| Hygroscopic salts | |
| Hygroscopic salts may resemble grease stains but rest assured, they’re harmless. They don’t cause damage or mould, yet can appear unsightly, especially during summer heatwaves, potentially complicating property sales or rentals. Keep this report handy to reassure prospective buyers of their benign nature. Effective Solutions: The prime method to conceal these salts involves an impermeable blocker with at least 300mm overlap, preventing the spread of any damp appearance. Aim for a uniform treatment across wall sections to ensure consistent humidity interaction. Remember, these salts often lurk just below the surface. Cost-Effective Remediation Methods: Zinsser Cover Stain: The simplest, most affordable approach. Two coats of this solvent-based primer-sealer should do the trick, costing about £15 plus potential labour. It’s user-friendly, easy to remove and reapply, making it ideal for DIY fixes. For larger areas, consider a gloss paint layer between two Zinsser coats for uniformity. This is a homeowner-friendly task but can also be efficiently tackled by a builder as part of a bigger project. Sempatap/Wallrock: Ideal for mildly cold walls, this lining paper creates a barrier, offering slight insulation. It’s a half-day job costing about £100 per linear metre. Thermal Insulating Plasterboard: For very cold walls, removing plaster and installing this board with a vapour barrier is best, mirroring professional damp proofing. It’s a day-long task, costing around £400 per linear metre. Note on Damp Proofing Slurry: Often less effective, as it can activate salts, it’s also invasive and costly. Starting Point: Zinsser Cover Stain is your go-to. It’s budget-friendly and effective for immediate action, with the option to escalate to plasterboard installation if necessary. Humidity Management: Excessive humidity can highlight salts at the impermeable material’s edge. If this happens, a second Zinsser application is recommended. | £50 |
| Optimal actions – estimate of costs | £550 |
I conclude that the primary cause of internal dampness is excess vapour from poorly installed ventilation in the downstairs bathroom and utility room extractor fans.
Having reviewed all the data, I believe the primary cause of dampness comes from excess vapourin the utility room, bathroom, and kitchen areas. Ideally, monitor dew points (as seen on the Sensor Blue app), keeping them below the outside temperature by using ventilation and, if necessary, dehumidification. Over time, the dampness at the base of the wall should disappear, which you can confirm using a damp meter.
I moved the ground floor bathroom backflow shutter to improve the performance of the ventilation. By taking a root cause approach, rather than a focus on blocking the symptoms of damp the savings are likely to be around £20,000. 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 internal dampness is excess vapour from poorly installed ventilation in the downstairs bathroom and utility room extractor fans.. 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.
“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:
See past damp cases
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What does the damp report say are the main causes of damp and main actions to stop damp?
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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？
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Cosa dice il rapporto sull’umidità riguardo le principali cause dell’umidità e le principali azioni per fermarla?