In New Eltham, SE9, a mid-war semi-detached house is facing damp problems in a specific corner, affecting both upper and lower floors. The upper small bedroom exhibits severe dampness, leading to mould, which was discovered after moving furniture. Despite using a dehumidifier and regular cleaning, the issue persists. Similarly, the corresponding corner in the downstairs hallway shows wetness, with brick outlines visible through the plaster. The residents, wary due to past negative experiences with builders and constrained by limited repair funds, are eager to identify the cause before proceeding with professional intervention. This damp issue is particularly concerning as it worsens seasonally and affects the daughter’s bedroom.
In conclusion, from the data loggers
- We can say that a majority of the vapour is coming from the bathroom and that on many days there is more vapour in the bathroom throughout the day, than the bedroom.
- However, it is also true that the highest levels of surface relatively humidity are from midnight to mid morning, when the bedroom is presumably occupied, i.e. from respiration.
- There is only a marginal reduction in vapour and surface relative humidity since the survey, despite my improving the bathroom ventilation considerably.
- This suggests that either the vent is not always coming on, perhaps because there is a light switch problem, or another source such as the kitchen, drying clothes or indeed respiration.
- I would increase use of ventilation and increase the amount of water removed using a higher capacity dehumidifier, increasing the airflow around the corner.
Evidence & Illustrations
Why Good Dehumidification is Essential in Poorly Insulated Properties:
Dehumidification plays a crucial role in combating mould growth, especially in properties with inadequate insulation. The key is to efficiently manage internal dew points, keeping them at or below the external temperature. This strategy is imperative for several reasons:
- High External Humidity: In conditions of rain or overcast weather, the external humidity can reach up to 100%RH. Relying solely on ventilation means that the air entering your property will have a dew point close to the outside temperature. If the internal wall temperature matches the external temperature, condensation is likely to form on the surfaces.
- Mould Growth Conditions: Mould thrives in environments where the relative humidity stays above 85%RH for extended periods. While it’s true that most interior walls are warmer than those outside, a poorly insulated wall might be only 3°C warmer internally. In such cases, the humidity on the internal surface could hover around 85%RH, creating ideal conditions for mould growth.
- The Role of Dehumidification: Effective dehumidification can significantly lower the internal dew point, thereby drastically reducing the risk of mould formation, even when external humidity is high. This is not just a matter of temperature regulation but a proactive approach to safeguarding your home against dampness and mould. Proper dehumidification is therefore essential in maintaining a healthy indoor environment in poorly insulated properties.
In summary, good dehumidification in poorly insulated properties is not just a supplementary measure; it’s a critical component of a comprehensive strategy to maintain a healthy, mould-free environment. The BBC have valuable insights into the increased use of dehumidifiers: https://www.bbc.co.uk/news/business-67623256
To keep the property dry
|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.
|Maintain airflow close to cold walls
|TIP: To prevent condensation and mould build-up, it’s crucial to keep objects away from cold walls. Insulating cold internal surfaces can be a highly effective solution, consider using Wallrock Thermal Liner or wood fibre behind cupboards sited on cold walls.
|Reduce allergenic dust mites
|TIP: To reduce damp-related allergies from dust mites, regularly turn bedclothes and pillows, rotate the mattress, vacuum carpets near cold walls, and wash bedclothes weekly, especially during high humidity and low temperatures.
|Use fan to expedite drying
|Direct a fan onto the damp patch to speed up the drying process.
|Keep some late night heat
|TIP: Maintain a small amount of heat late at night when it’s cold outside. More efficient alternatives include 1) insulating cold walls, 2) strategically placing one or more dehumidifiers, positioned where the night-time dew point is highest – use data loggers to locate the optimal position (typically a damp room near a damp wall, e.g., kitchen, bathroom). Personally, I run a dehumidifier whenever leaving a room unheated.
|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.
|Reduce vapour from cooking
|Cook with tops on pots and pans to reduce the production of vapour from cooking by 80%.
|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 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).
|Monitor vapour with data loggers
|Monitor relative humidity and temperature against the wall or ceiling to observe the effects of improvements, such as ORIA Wireless Thermometer Hygrometer (20m (2 Packs)) – already in place. In addition monitor improvements on the damp wall a pencil line and damp meter such as dr.wood moisture meter, logging improvements each month. Monitor relative humidity and temperature against the wall or ceiling to observe the effects of improvements, such as ORIA Wireless Thermometer Hygrometer (20m (2 Packs)) – already in place. In addition monitor improvements on the damp wall with a pencil line and damp meter such as dr.wood moisture meter, logging improvements each month.
|Remedial actions – estimate of costs
Actions if all else fails
|Consider installing a kitchen extractor
|The main issue to getting ducting out the extractor fan. Ideally it should be using rigid ducting (not flexible as the ridges create eddies slowing air speed), as short as possible with as few bends. It can go up the chimney but would then need to be run for longer to account for the possibility of vapour travelling back down.
|Consider anti-absorption paint
|Consider painting anti-absorption paint on exposed brickwork, especially to the corner. I think this of marginal benefit.
|Optimal actions – estimate of costs
I conclude that condensation and mould due to excess vapour and poor insulation.
Inadequate bathroom and kitchen ventilation, heat loss from exposed brickwork
Improved bathroom extractor fan flow, extended run to 30 minutes. By taking a root cause approach, rather than a focus on blocking the symptoms of damp the savings are likely to be around £2,000. 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 231213SH1200. I conclude that condensation and mould due to excess vapour and poor insulation. 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 231213SH1200
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: No specific concern
- Damp treatment: Treated for rising damp, increased condensation risk
- Weather: Overcast
- Property: 1920s era, probably
- Background: Mould and dampness in bedroom and hall corner
- Changes: open plan kitchen, external masonry changes, lifestyle changes including more frequent showers and clothes washing, including greater occupancy during the Covid pandemic.
- Occupancy: Family
- Mould & notes: Mould in corner, especially behind bed
- Primary damp concern: Dampness and mould in corner
- Compounding: Heat loss worsened by exposed exterior bricks
- Airbricks: There is no reason to be concerned about the sub-floor ventilation.
- Sub-floor humidity: Not a concern
- Tenure: house
- Attachment: Semi-detached
- Build: 1920s
- Walls: Solid, 9-inch thick
- Exterior comment: No penetrating damp concerns outside
- Penetrating damp assessment: The exterior appeared to be OK from a damp perspective. There are no obvious signs of significant penetrating damp.
- Drains: Drains not a concern
- Leak signs: No signs of a leak
- Leak assessment: No obvious leak concerns
- Floor: The suspended timber floor did not feel damaged.
- Loft: Insulation could be better distributed
- Timber assessment: No substantial risk of timber decay
- Last dry: Problems started a few years ago, during Covid
- Ventilation assessment: Bathroom extractor fan slow, improved during survey
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.