Background
In this large semi-detached Victorian property, both the front ground floor studio flat and the lower ground floor flat have been affected by mould. Various issues have been documented in the lower ground floor flat:
- Rainwater flooding has been reported multiple times, particularly during flash floods. It is suspected that the flooding may have entered through the patio door, although the exact entry point remains uncertain. Detailed information will be gathered during the survey, and several preventive measures are under consideration. Additionally, a minor amount of water ingress has been recorded in the bedroom, located along the same external wall.
- Water ingress has been observed on an internal wall and is believed to be occurring where the original building joins the newer section.
- Damp stains have been identified on the ceiling above the stairs. The cause is suspected to be related to the chimney or its flashing. Although this issue is not regarded as severe, confirmation is required.
- Damp patches have been observed on sections of the external walls.
Data loggers – see damp.ai/detect
Conclusion from data loggers
Studio: The vapour in studio flat is exceptionally high highlighting the insufficiency of ventilation and or dehumidification. To follow Dews Law, always make sure the internal vapour never exceeds the external temperature, by installing and using extractor fans and dehumidifiers and monitoring vapour and temperatures.
Lower ground floor: Use a dehumidifier dripping into a sink, and fan directed and the damp zones, to speed up the drying process.
Evidence & Illustrations
Studio flat
The issues were explained at the time of the survey and highlighted in the video. In summary:
1) The kitchen vent is not used. Even if it were used, it is not running sufficiently quickly and, for open plan rooms, it is far better targeted and therefore more effective to have a hood over the hob. Again it needs to be used whenever cooking. Vapour from cooking can be dramatically reduced by cooking with tops on pots and pans.
2) The bathroom fan is light switch operated, but lights are not always used when bathing, which explains why the vent has stopped working and this wasn’t reported to the landlord. I recommend installing a constant flow bathroom extractor fan, with light switch and humidity boost function. I would ask the tenant to keep the door closed so as to reduce the amount of vapour that flows back into the flat.
3) I would supply a more powerful dehumidifier, extracting 20 Litres per day and ask the tenant to keep it running when drying clothes and internal vapour, as measure by dew point, below the outside temperature. It should be kept permanently running, set to 70%RH and safely dripping into a sink when away.
Lower ground floor flat
The lower ground floor flat is vacant. It was recently flooded by rainwater spilling over the rear French doors.
Wherever possible, ensure that rainwater is directed into the public rainwater drainage system rather than into a soakaway—see my AI supplemented article on soakaways. The Building Regulations in the UK recommend that a soakaway should be situated at least 5 metres away from the building’s foundations to avoid potential structural damage. This distance helps ensure that water seeping into the ground does not undermine the stability of the property. Presumably it should be built below the ground you are trying to protect, i.e. a majority of the soakaway should be built below the lower ground floor of your property and any neighbour within 5M.
Other matters
To keep the property dry | |
|---|---|
| Studio flat | |
| Install and use a hood over the kitchen hob to improve ventilation. Use lids on pots and pans to reduce cooking vapour. Install a constant flow bathroom extractor fan with light switch and humidity boost function. Encourage the tenant to keep the bathroom door closed to prevent moisture from spreading. Supply a more powerful dehumidifier (20 litres per day) and ensure it runs continuously, draining into the sink when away. 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. Remove dry mould using sandpaper and treat affected areas with bleach and/or anti-mould spray. 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. | £1,000 |
| Lower ground floor flat | |
| Maintain relative humidity with fans blowing air around mouldy corners, consider using two or three dehumidifiers with daily capacity (20L minimum) with pipes out dripping into a sink. | £180 |
| Reduce risk of flooding | |
| Direct rainwater from above-ground rainwater goods, such as gutters and downpipes, into the public rainwater drainage system rather than into a soakaway. This will significantly reduce the risk of flooding. If for any reason this cannot be achieved in full, request that the neighbouring property drains their rainwater separately and away from both properties, for example, into the public drain, of if into a soakaway that it should be below the lower ground floor, and at least 5 metres away from any property. See the alternative option of using a soakaway below. | £1,000 |
| Repair render cracks | |
| Check and repair any cracks in the exterior walls to prevent water ingress. Address external render cracks by raking out, that is slightly enlarging and removing loose material and filling with an acrylic based caulk for small gaps and cement based mortar for larger gaps over 3mm. Remove loose render and reapply an impermeable external render. | £1,000 |
| Repair flaking masonry paint | |
| Use a wire brush to remove the paint from the external walls, taking it back to the brick as much as possible, especially to eliminate surface salts. Ideally, leave the bricks unpainted, or remove at least two-thirds of the paint from each brick, as painted bricks typically require regular inspection and repainting every 5 to 10 years. If repainting is necessary, choose a high-quality, breathable masonry paint, such as Zinsser or Dulux impermeable external masonry paint. Apply two coats, carefully following the manufacturer’s instructions. | £500 |
| 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 |
| 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. | |
| 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. | |
| Remedial actions – estimate of costs | £3,780 |
Actions if all else fails | |
|---|---|
| Install or replace the soak away | |
| For residual rainwater, not being directed to the public drain, either replace the current soakaway, or ideally install one more than 5 M from your property and any neighbour, and below the ground level of the lower ground floor, direct drainage from the rear of the property to the soakaway. | £5,000 |
| 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 |
| Render plinth | |
| Install a render plinth about 800 mm high to prevent rain splash back. | £2,000 |
| Aco drain | |
| Consider installing an impermeable surface with drainage (e.g., concrete patio with Aco drain) to reduce water percolation towards the property. | £1,000 |
| Consider targeted insulation | |
| Consider installing thermal insulating plasterboard (e.g., 30 mm Celotex) to reduce heat loss and prevent condensation. | £4,000 |
| 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) – I have installed one in a rental property I own myself, that was bought cheaply because of mould. Ideally the vent should be located away from the door, closer to the shower. I would also install a bathroom spring to encourage tenants to keep the door closed and not far from the toilet. Ideally the ducting should be rigid, for reduced air resistance, and insulated to reduce risk of condensation in pipe. | £600 |
| Consider replacing the kitchen extractor | |
| Consider installing kitchen extractor fan either above the hob, or 150mm sized fan capable of drawing out 60L/S, light switch operated. Alternative install the hob fan and a bathroom size fan that is quiet and triggered by a sensor with 30 minute overrun. | £600 |
| Optimal actions – estimate of costs | £13,250 |
I conclude that the primary cause of internal dampness is excess vapour from insufficient ventilation, as seen in the ground floor studio flat. Additionally, the lower ground floor flat is affected by rainwater flooding, penetrating damp, and condensation, with the possible presence of hygroscopic salts.
Mould grows where relative humidity exceeds 85% RH for extended periods. It can be widespread, as in the case of the studio, due to insufficient ventilation. Although heat loss increases the risk of mould, excess vapour is the root cause. Mould growth may also result from a one-off event, as observed in the lower ground floor flat, though it is possible that this flat also had pre-existing mould from poor ventilation.
. 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: Risk of surface water (i.e. penetrating damp from rainwater, not rising damp from groundwater) See Flood Risk (note most flood risk is not from groundwater).
Sub-soil rocks: Near highly productive aquifer with intergranular flow See British Geological Survey.
Therefore risk of Rising Damp: low risk – because groundwater is pumped out in London, see explanation by Dr Robyn Pender of Historic England.
I confirm that I inspected the property on 28 October 2024. I conclude that the primary cause of internal dampness is excess vapour from insufficient ventilation, as seen in the ground floor studio flat. additionally, the lower ground floor flat is affected by rainwater flooding, penetrating damp, and condensation, with the possib. 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 28 October 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.
- 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.
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?