Background
A Homebuyer Survey for the garden flat in a large semi-detached Victorian house on a hill, in a part of London not prone to groundwater—the source of rising damp—identified significant damp problems, leading to a treatment quote. A damp-proofing contractor, with a vested interest in the outcome, diagnosed rising damp based solely on high meter readings and proposed extensive treatment, including replacing sound plaster up to 1.2 metres. The freeholders, questioning the diagnosis and liability, sought an independent review to determine the cause, highlighting issues such as deteriorating external render, drainage problems, and past structural alterations.
Data loggers – see damp.ai/detect
The data logger graph highlights significant surface relative humidity, the green line, regularly hitting 100%RH when condensation takes place. Indeed condensation was visible on windows during the survey. If it is visible on windows, it will likely to be absorbed into cold walls. The dark blue line, the dew point line, is regularly above the outside temperature, which it should not be.
Conclusion from data loggers
Vapour is not being managed effectively.
Follow Dew’s Law: To eliminate the risk of condensation on cold walls, the internal vapour, expressed as the dew point 🌡️, should always be kept below the external air temperature 🌬️.
Dew’s Law Explained
- Dew Point: This is the temperature at which air becomes saturated, and water vapourcondenses into liquid. It’s a direct measure of how much moisture 🌫️ is in the air.
- External Dew Point: The dew point outside cannot exceed the external temperature—if it did, rain would occur as the air becomes saturated (100% relative humidity). When it rains 🌧️, the dew point equals the outside temperature.
Managing Moisture Efficiently
- Targeted Ventilation: Focus ventilation efforts in specific areas, like bathrooms 🚿, by using extractor fans with doors closed, keeping the fan on until moisture has evaporated away. This targeted approach removes excess vapour without unnecessarily ventilating the entire property, conserving energy and maintaining warmth.
- Dehumidification: Using a 20-litre-per-day dehumidifier is an energy-efficient way to lower internal vapour. Unlike ventilation, which can lead to heat loss, dehumidifiers utilise enthalpy, providing a heat gain. Remarkably, 150% of the energy consumed is converted into heat, functioning similarly to an air-source heat pump 🔋🔥.
By consistently following Dew’s Law for the next 6 to 9 months, the damp at the bases of your walls should gradually disappear, evaporating from the top downwards. See additional recommendations for further steps ❄️✨.
Evidence & Illustrations
For groundwater—the water found beneath the water table—to cause rising damp, the soil would need to be saturated at a consistent level across the entire area, and the dampness in the walls would need to be uniform to the same height. This scenario is highly unlikely on a hill, where groundwater levels are typically low. Moreover, saturated soil could easily be verified by drilling into the ground and directly measuring moisture levels, providing straightforward evidence of rising damp. In this case, dampness was not consistent across the property and was found only on cold walls, strongly suggesting that condensation is the primary cause.
It stands to reason that rising damp is not the only, or even the most likely, source of water found at the base of a wall. Dampness at low levels is far more commonly caused by a leaking pipe, rainwater penetrating through a wall, or, most frequently, condensation. All water sources tend to accumulate at the base of walls, as gravity naturally draws moisture downward.
It is also important to note that it is not in a damp-proofer’s commercial interest to recommend simple solutions, such as opening a window or testing for leaks. Issues like rainwater drainage problems or high subfloor humidity—which are often the true causes of dampness at the base of walls—are typically beyond a damp-proofer’s expertise.
The reason rising damp is specifically defined as originating from groundwater, rather than rainwater in soil, is as follows:
Rainwater falling onto unsaturated soil is absorbed through capillary action, moving both downward and upward. However, when combined with the much stronger force of gravity, the upward movement of water is negligible.
Rainwater should be effectively dispersed through proper drainage and adequate sub-floor ventilation.
The normal levels of relative humidity observed in your sub-floor (as shown in the accompanying video) indicate that neither inadequate drainage nor sub-floor ventilation are contributing to dampness at the base of your walls. This evidence further supports the conclusion that rising damp is not the cause in this instance.
I will discuss the damp proofer’s assertion that: “Render has been completed to ground levels, this will bridge any DPC in place allowing rising damp to occur” later.
The logic behind this theory is flawed. For water to bypass a DPC, the gap between the renderand the DPC would need to be the right size to facilitate significant capillary action. In reality, if the gap is too small, only minimal water can pass thereby causing minimal dampness, and if it is too large, capillary action becomes negligible. Additionally, when comparing moisture ingress from a gap at the DPC level to a similar gap in the render elsewhere, the render gap will draw in far more moisture due to the force of gravity.
Modern understanding recognises that render extending over a DPC is unlikely to be a primary cause of dampness. Instead, the most common causes are rainwater ingress, poor drainage, leaking pipework or condensation from internal vapour. This outdated theory about renderbridging a DPC has been debunked through practical observation and advances in building science.
To keep the property dry | |
|---|---|
| Reduce vapour through targeted ventilation | |
| Keep the bathroom extractor fan running constantly until the dew point in the bathroom matches that of the rest of the property. Use a data logger to monitor this. Typically, this process takes about 30 minutes with the bathroom door closed, but your fan may take longer as it only extracts at a rate of 5 litres per second in combination, compared to the recommended 15 litres per second. Use the kitchen extractor fan when cooking. To minimise moisture, cook with lids on pots and pans, and reduce the heat to a simmer. Avoid drying clothes indoors without using a dehumidifier or a dedicated dryer. Measure internal vapour levels (dew point), and if the internal dew point exceeds the external temperature, take steps to increase ventilation and dehumidification. | |
| Consider replacing extractor outlets | |
| Consider replacing the outlets from both bathroom extractor using the plastic flap types, this should increase the through-put of air by about 400%. | £150 |
| Use a dehumidifier | |
| Maintain relative humidity, especially at night when it’s cold outside, with a dehumidifier such as the PureMate 20L/Day Portable & Compact Dehumidifier with a 6.5-litre tank (£179.99, refrigerant), the Duux Bora Smart Dehumidifier (DXDH02UK, £300, refrigerant), or the 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 well-established and easy to manufacture with a similar function and fire risk profile to fridges. 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). I use a dehumidifier whenever leaving a property vacant, dripping into a sink, set to 70%RH, especially in winter – it is more efficient than other humidity managing methods including ventilation. | £180 |
| 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 |
| 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, to log and monitor reduction to dampness in walls. By doing so, you will gain early confidence in these recommendations. | £23 |
| Monitor sub-floor relative humidity | |
| Place one data logger in the sub-floor void and another outside. Compare the relative humidity and dew points recorded below the floor with those outside. Aim to increase sub-floor ventilation (see guidance above) until the sub-floor humidity consistently tends towards 85% RH or below, and 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 in assessing moisture levels. Both readings will be influenced by air from the property. Efforts should be made to minimise vapour travelling from the house into the sub-floor void, where it could condense and exacerbate moisture-related issues. | £20 |
| Fill render cracks | |
| Use a wire brush to remove loose paint and render. Check and repair any cracks and gaps 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. Paint can hide render cracks, permeable paint can cause moisture to pass through covered gaps. Impermeable masonry paint should be applied to impermeable coatings. Consider rendering the section of unrendered wall behind bathroom, this will add a few hours work. | £500 |
| Remedial actions – estimate of costs | £923 |
Actions if all else fails | |
|---|---|
| Reduce Risk of Rainwater | |
| Ideally, the patio should be at least three bricks below the sub-floor vent to minimise the risk of rainwater splashing into the vent. Additionally, the patio should be lower than the damp-proof course (DPC). However, this may not always be practical. As an alternative, an ACO drain can be installed, connected to the rainwater drainage system. A 75 mm (3-inch) fillet of mortar, sloping away from the wall, is recommended to direct rainwater away from the wall into the drain effectively. | £1,000 |
| 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 |
| Consider EWI | |
| Install EWI, external wall insulation, on external walls after filling cracks and removing the downpipe, then reinstalling a plastic one. This will reduce heat loss, risk of condensation and penetrating damp. It will also reduce CO2 emission and heating bills, while adding value to the property. Planning and freeholder permissions may be required. Insulation is not a substitute for ventilation and vapour management as there will always be somewhere in the property at risk of temperatures close to that of the outside. | £20,000 |
| Optimal actions – estimate of costs | £21,050 |
I conclude that the primary cause of internal dampness is excess vapour due to partly poorly installed ventilation from the two bathroom extractor fans and the kitchen extractor, which I improved during the survey.
Vapour will condense on the coldest part of a wall, which will tends to be the base of the wall. Gravity draws moisture down to the damp proof course. A damp brick is a poor insulator, increasing the risk of heat loss and therefore condensation.
I repaired the kitchen extractor fan, which was blowing humid air into a cold void within the wall.. 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 due to partly poorly installed ventilation from the two bathroom extractor fans and the kitchen extractor, which i improved during the survey.. 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.
- Background: The owners sought a second opinion on rising damp claims made in prior reports. The flat had undergone multiple repairs, including bathroom refits, repointing, and chimney removal. Issues like poor ventilation and past alterations may have affected the property’s drainage and overall damp conditions.
W.H.O. indoor air quality guidelines
“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.
