Despite spending a substantial sum on damp-related issues in this rented house, success has remained elusive. Recommendations from Which! TrustedTrader led to interventions targeting mould and rising damp, evident from colourless patches along the party wall. Concerns began in 2017 with bubbling and peeling paint in the ground floor hallway, addressed by installing a 1-meter damp proofing treatment course and a 3-in-1 sand and cement render with a specialist additive (Triton as known as damp proofer’s “slurry”). Yet, in 2020, the appearance of peeling paint above the damp proofer’s slurry and ongoing condensation and mould growth in the ground floor bathroom highlighted the unresolved issues.
In conclusion, from the data loggers
- It is clear that condensation is the root cause of dampness in this property.
- Tenants should follow recommendations see section below, including using a dehumidifier 24 / 7 until vapour is brought down. It is both economic and environmentally friendly.
Evidence & Illustrations
Consider a raincoat over a sweater: the raincoat, like the damp-proofing slurry (Triton), blocks rain but traps body moisture, making the sweater (similar to the original plaster) damp. This trapped moisture reflects how vapour is caught behind the slurry, akin to sweat under a raincoat.
For cooking, imagine a lid on a boiling pot: steam (moisture) rises, hits the lid (the impermeable slurry), and condenses, dripping back onto the ingredients (the original absorbent plaster). This cycle keeps moisture circulating inside, much like vapour trapped behind the slurry.
In healthcare, it’s akin to a waterproof bandage on skin: it protects a wound from external water but can trap moisture from the skin, creating a damp environment under the bandage. This trapped moisture is similar to the vapour caught behind the damp-proofing slurry.
To mitigate the risk of mould on the bathroom ceiling, it’s recommended to install thermal insulating materials on the colder surfaces. Options include thermal insulating plasterboard or specialised thermal lining papers, such as Wallrock Thermal Liner or Sempatap Thermal Solid Wall Insulation. These materials help maintain a more consistent temperature on the surface, thereby reducing the likelihood of condensation and subsequent mould growth.
To keep the property dry
|Ask tenants to remove vapour from bathroom
|Ask tenants to 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. Consider adding a spring to the bathroom door as it doesn’t always stay closed by itself.
|Ask tenants to remove vapour from clothes drying
|I would provide tenants with a means of drying clothes internally, which could be a powerful dehumidifier or tumble dryer (like a heat pump). Alternatively ask them to dry in an vented room with the door closed (possibly a bathroom with the extractor running or window open). Ask tenants to never to dry clothes in an unventilated room. Consider offering to pay for the cost of running a dehumidifier or heat pump dryer, in the region of £1 per day, plus it is energy efficient as all energy is converted to heat plus a 50% bonus.
|Ask tenants to remove vapour from cooking
|Cook with tops on pots and pans to reduce the production of vapour from cooking by 80%.
|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.
|Remedial actions – estimate of costs
Actions if all else fails
|Consider installing kitchen extractor fan either above the hob, or light switch operated.
|For increased targeting, the fan should be located above the hob and ducted outside with rigid ducting measuring 30 l/s (as a quick test of effectiveness, the fan should be audible by the outlet outside and push a sheet of A4 placed over it almost horizontally). Alternatively the fan can be on a kitchen fan on an adjacent wall. The hole size is larger than a bathroom fan, 150mm, and the speed 4 times faster 60 l/s plus light switch operated fan to the side. Then make it a requirement for the tenants to use the fan whenever cooking.
|Consider supplying 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 6.5 Litre (£179.99 Refrigerant), Duux Bora Smart Dehumidifier (DXDH02UK £300 Refrigerant) or 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 old and easy to manufacture. 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).
|Consider installing a continuous flow bathroom extractor fan
|I don’t know if the current fan is not set up correctly, or just doesn’t perform to specifications. But although it is making a continous sound, there is no flow on air in it’s continuous mode – I have seen this fan in many properties with damp issues. Consider replacing the fan with a continuous flow bathroom extractor fan such as Vent Axia or Brookvent Airstream DMEV 100 HT (Continuous Running Extractor Fan with Humidistat Timer).
|Consider install Wi-Fi data loggers
|With Wi-Fi data loggers you can monitor dew points for time to time, and set clear tenancy agreement for vapour (dew point) management. I think your tenants understand the need to minimise the dew point, to below or near the external temperature.
|To reduce the risk of mould to the bathroom ceiling, consider installing thermal insulating plasterboard or thermal lining paper, such as wallrock thermal liner or Sempatap thermal solid wall insulation to cold surfaces.
|Share this report with your neighbour
|Suggests your neighbour follow similar guidelines.
|External cracks and blistered paint
|There are cracks on the outside wall, consider monitoring movement. These cracks would ordinarily cause a brown stain from ingress. There is no stain, but high damp meter readings. As a matter of good practice, I would seal the cracks with an acrylic caulk and repaint. Likewise the paint is blistering. I would sand or wire brush crystals behind paint and repaint in the same paint as the original coat, or if unknown, a more impermeable paint, such as Zinsser or Dulux impermeable masonry paint.
|Optimal actions – estimate of costs
I conclude that condensation due to past proofing treatment.
Disruption between damp proof slurry and original plaster, trapping vapour
Improved the bathroom extractor fan, advised tenants. By taking a root cause approach, rather than a focus on blocking the symptoms of damp the savings are likely to be around £5,000. This contrasts with typical damp proofing contractor recommendations such as chemical treatment for rising damp and untargeted ventilation, such as PIV systems.
Flood risk: No risk See Flood Risk (note most flood risk is not from groundwater).
Sub-soil rocks: Near moderately productive aquifer with intergranular flow See British Geological Survey.
Therefore risk of Rising Damp: highly unlikely, see explanation by Dr Robyn Pender of Historic England.
I confirm that I inspected the property on 231212SH1000. I conclude that condensation due to past proofing treatment. 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.
Site visit 231212SH1000
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.
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: Landlord concerned about rising damp
- Damp treatment: Past damp proofing treatment evident at front and party wall
- Weather: it appears to have rained recently
- Property: Likely Edwardian house
- Background: Dampness along party wall, following stairs line
- Changes: Bathroom addition, damp proofing treatment
- Occupancy: Mostly occupied post-Covid during daytime
- Mould & notes: Mould and dampness signs elsewhere in property
- Primary damp concern: Dampness about a meter up the wall
- Compounding: Damp proofing treatment
- Airbricks: There is no reason to be concerned about the sub-floor ventilation.
- Sub-floor humidity: Not a concern
- Tenure: House
- Attachment: Terraced
- Build: Edwardian
- Walls: Solid, 9 inch
- Exterior comment: No major concerns outside
- Penetrating damp assessment: Penetrating damp not significant
- Drains: No apparent problems
- Leak signs: None
- Leak assessment: No issues with leaks
- Floor: The floor is of no special concern.
- Loft: Not of concern
- Timber assessment: Timber appears undamaged
- Last dry: Long-standing damp issues, hence past treatment
- Ventilation assessment: Insufficient in bathroom and kitchen
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.