Background
In a ground floor Victorian terrace flat, complete with a cellar and side extension, persistent damp issues have been observed in two distinct areas. The first, located in the front room, features mould originating from a corner adjoining the cellar and the building’s front wall beneath a bay window, despite previous mould treatment efforts and remedial actions addressing a misaligned drain and damp-proof injection under the bay window. Despite these measures and the provision of a dehumidifier to tenants, the problem persists, suggesting that the source of moisture may not be fully addressed. Recommendations include the installation of a ventilation system in the cellar to mitigate moisture, though skepticism remains regarding the identification of all moisture sources. The second area, at the base of the wall in the side extension, exhibits dampness and peeling paint despite treatment, indicating ongoing moisture intrusion from adjacent structures. This situation underscores the complexity of diagnosing and remedying damp issues in older properties, where multiple, interrelated factors often contribute to moisture penetration and accumulation.
Data loggers
In conclusion, from the data loggers
- Vapour is being generated from within the flat.
Evidence & Illustrations
I use smoke or perfume as an analogy to help understand why mould tends to grow behind sofas by cold walls
Imagine vapour in a room as perfume spreading out from a bottle—given time, it evenly fills the space. This is how moisture moves through your home, from a humid kitchen to the colder areas, uniformly distributing vapour.
Despite its name, a radiator warms a room through air currents. A sofa placed against a cold wall acts as a barrier, blocking these warm air currents, effectively insulating the wall and trapping cold air. This scenario creates a microclimate behind the sofa that’s cooler, with higher relative humidity and mould, because it allows the uniform distribution of vapour but reduces the distribution of heat.
Just as the perfume analogy illustrates vapour spreading evenly, think of the sofa as a breakwater that stops the flow of warmth, leading to increased relative humidity and the potential for mouldgrowth in these shielded areas.
Mould growth is inhibited by nitrates found in groundwater. The presence of mould at the bottom of a wall rules out rising damp as the root cause and points towards condensation from unvented excess vapour.
As a general rule, if mould and or condensation is seen elsewhere in a property, then insufficient ventilation and dehumidification is the cause. If it is growing in one zone, trapped by an object like a sofa or cupboard and there is a clear leak in that area, then it is possible that it is localised only. In this case it is clear that mould is growing across the property and there is clearly insufficient kitchen and bathroom ventilation.
You have common furniture beetle on exposed timber in your cellar. Common furniture beetle grows independently of damp, but only eats the sapwood (and wood products such as MDF). Sapwood is the fast growing outter-layer of timber, in practice old pine normally has sufficient hardwood for common furniture beetle not to destroy the structural integrity. The best solution is to spray around timber with holes to about 300 mm away with .2% Permethrin (available on Amazon). Permethrin is not considered toxic to humans. It is effective and easy to apply where visible. Active holes can reappear for up to three years. In that unlikely event, repeat procedure to about 300mm around new holes.
1) The property has been damp proofed, despite the low risk of rising damp. Damp proofingtreatment including the use of an internal impermeable slurry increases the risk of mould and condensation, see next report.
2) The sub-floor ventilation to the rear is missing, possibly through bomb damage (as the rear bricks look different to the front ones). This means there is no through ventilation from the front to rear, as the property was originally designer. It may be necessary to put in mechanical ventilationinto the cellar. This is not the cause of dampness inside the property, but could cause rising damplike symptoms, as condensation can form above the damp proof course, risks rot and woodwormto timber.
To keep the property dry | |
|---|---|
| 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). | £180 |
| Use fan to expedite drying | |
| Direct a fan onto the damp patch to speed up the drying process. | £50 |
| 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. | |
| 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. | £20 |
| Reduce vapour from cooking | |
| Cook with tops on pots and pans to reduce the production of vapour from cooking by 80%. | |
| 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. | |
| Monitor improvements | |
| I would buy a Dr. Meter Wood Moisture Meter 2 in 1 Pin & Pinless Multifunctional Water Detector £23.99 to log and monitor reduction to dampness in walls. | £24 |
| Keep objects away from walls | |
| Consider sofa away from the cold corner. | |
| Improve bathroom ventilation | |
| Consider installing a continuous flow bathroom extractor fan such as Vent-Axia Silent 7.5W Extractor Fan With Humidistat & Timer Continuous or Brookvent Airstream DMEV 100 HT (Continuous Running Extractor Fans with Humidistat and Timer) – I have installed one in a rental property I own myself, that was bought cheaply because of mould. Although important to have an isolator, it can be sited in a way that doesn’t tempt tenants to use it. 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. Ideally the ducting should be rigid, for reduced air resistance, and insulated to reduce risk of condensation in pipe. | £300 |
| Remedial actions – estimate of costs | £574 |
Actions if all else fails | |
|---|---|
| Duct out kitchen extractor fan | |
| The main issue to getting ducting out the extractor fan is hiding the ducting. 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. | £400 |
| Consider targeted insulation | |
| I probably would not in this case, as I don’t think the cost justifies the benefit, but you could consider Wallrock thermal lining paper. Insulation is never the solution, but can mitigate the risk. The root cause is the poor ventilation, the solution is improved provision of ventilation, use of ventilation, dehumidification and monitoring. | £500 |
| Consider updating your tenancy agreement | |
| I would replace the current bathroom fan with a continuous flow bathroom extractor fan that is quiet, fast fan, set-up with a 30-minute overrun, connected to a rigid duct (not a flexible duct), with external flaps (check the exit speed is about 15 l/s, see simple to use anemometer below). I would put a spring on the door to ensure that it is kept closed. I would also install a kitchen extractor fan ducted out and provide a 20L per day capacity dehumidifier. In addition I would make monitoring humidity and keeping good ventilation or dehumidification, such that the internal dew point is never above the external temperature for long periods (i.e. more than a day) at a time, as part of the AST (tenancy) agreement, explaining ow the ventilation works, the importance to the property and the tenant of using the ventilation correctly and monitory internal vapour making sure that the internal dew point doesn’t exceed the external temperature. | |
| Repair rear gutters | |
| The green to the rear soil pipe indicates that the rear gutter is leaking. I understand that this is being repaired. | £300 |
| Consider improving sub-floor ventilation | |
| Use a hygrometer probe under or near damp wall. A probe can be pushed through cracks in floorboards or by discretely drilling out a hole. The sub-floor relative humidity will fluctuate. If it remains above 85%RH for an extended period then consider installing a fan into the sub-floor to blow air out. Increasing ventilation is tricky as there is limited accessibility. A company called ELTA made a Sub-Floor Ventilation Fan, which many clients with similar issues have installed with success, but ELTA have been closed down. This site seems to offer the same unit: https://www.builders-superstore.co.uk/product/ventilation-fan-sub-floor-mains-powered/ | £10 |
| Woodworm | |
| The best solution is to spray around timber with holes to about 300 mm away with .2% Permethrin (available on Amazon). Permethrin is not considered toxic to humans. It is effective and easy to apply where visible. Active holes can reappear for up to three years. In that unlikely event, repeat procedure to about 300mm around new holes. I would also apply a timber primer so that you can identify newly bored holes, to know if wood warm activity is ongoing. | £8 |
| Optimal actions – estimate of costs | £1,218 |
I conclude that there was no evidence of rising damp, which comes from groundwater in contact with walls. The risk of groundwater is very low. There were many signs of excess humidity causing condensation and mould.
vapour mainly generated in the bathroom and kitchen is diffusing to the colder walls. The walls behind the sofa are not benefitting from heat as much as they would be in the sofa was taken away from the wall by a foot or so, to allow air movement.
. 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: 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: Rocks with essentially no groundwater 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 240219SH1000. I conclude that there was no evidence of rising damp, which comes from groundwater in contact with walls. the risk of groundwater is very low. there were many signs of excess humidity causing condensation and mould. 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 240219SH1000
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.
- Rising damp concern: You were concern you might have rising 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.
- Weather: dry overcast
- Property: Victorian ground floor flat
- Background: Ground floor Victorian terrace flat with cellar and side extension. Two damp areas (1) Front room in has mould that starts in the corner that borders the cellar one floor down and the front building wall with a bay window. We’ve done mould treatment for 2 years running.
- Changes: conversion into flats, damp proofing treatment including use of slurry, central heating, extensions and lifestyle changes including more frequent showers and clothes washing, including greater occupancy during the Covid pandemic.
- Occupancy: tenant
- Mould Colour: Mould
- Primary damp concern: Mould mainly in living room
- Other damp concerns: Mould by bathroom
- Compounding: heat loss made worse by poor air circulation behind sofa
- Airbricks: There is high humidity in the cellar, which could increase the risk of rising damp like symptom. Ideally there should be increased sub-floor ventilation.
- Sub-floor humidity: Air in the sub-floor void was humid and should be monitored.
- Tenure: ground floor flat
- Attachment: a terraced
- Build: Victorian
- Walls: solid wall made of two brick widths (225mm)
- Exterior comment: There were no major concerns seen externally that would account for internal issues.
- Penetrating damp assessment: The exterior appeared to be OK from a damp perspective. There are no obvious signs of significant penetrating damp.
- Drains: The drains are of no special concern.
- Leak signs: No suspicion of a mains water leak
- Leak assessment: There are no obvious signs of a significant mains or wastewater leak.
- Floor: The suspended timber floor did not feel damaged.
- Loft: There was no loft.
- Timber assessment: Other than a little woodworm, there are no suspicions of significant timber decay.
- Last dry: I understand dampness has been an issue, hence the past damp proofing treatment which has not solved it.
- Ventilation assessment: There is no externally ducted kitchen extractor fan, nor is there a bathroom extractor fan. Clothes are dried internally, often with a dehumidifier.
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.
