Data loggers
In conclusion, from the data loggers
- There are two distinct causes of damp in two distinct locations, one in the flat, one under the flat. There is one common factor, poor damp proofing treatment which as is commonly the case, fundamentally misunderstands the causes of damp, such as in this case.
- During the period under investigation the surface relative humidity on the damp wall (green line) is very high with constant condensation.
- Understanding dew point is key to understanding damp, both above and under the floor. Dew points rise and fall in line with vapour quantity, known as vapour pressure. Ventilation, without dehumidification, can never drop the dew points below those of the outside. This is because air blown out is being replaced with air from the outside. A property that has dew points significantly above external dew points, for long periods of the day, is poorly ventilated – as you have. This goes for both the above floor and sub-floor ventilation.
- The example property’s dew point is, in fact, from my house, where I run a dehumidifier to maintain a healthy environment and reduce the need for and cost of heating. The thermostat can be turned down for the same comfort level when using a dehumidifier, this is because humans feel instinctively colder when humidity is excessive, over about 70%RH.
- At the time of the survey, I recommended using the cord on the bathroom fan during and for at least 30 minutes after a shower and similarly with cooking. There was improvement in dew points for about 18 hours from the time I surveyed the property, 26 October, but the improvement only lasted less than 1 day, suggesting the tenants don’t buy my simple solutions and need additional help.
- I would run a powerful dehumidifier at this property.
See monitor-humidity-with-data-loggers on how to use the data logger app. To work out the external dew point, you can install a logger outside and look at the dew points indicated on the logger app (for example SensorBlue), or as a rough guide if you pick up the Met Weather, look up your area, then take the external temperature and external relative humidity then take off 1° for each 5% below 100%RH to estimate the dew point. So for instance if the external temperature is 14°C and humidity 80%, as a rough calculation, the external dew point is about 14-(100-80)/5 = 10°C.
Evidence & Illustrations
You should install a bathroom extractor fan. Given the challenges I would install a continuous flow bathroom extractor fan. In the kitchen you would be better to install an extractor hood over the hob and ask tenants to use it whenever cooking.
Mould growth occurs where relative humidity exceeds 85% RH for more than six hours, as a rule of thumb. Excessive humidity is usually due to insufficient ventilation, poor air circulation, and cold surfaces. Mould rarely is the cause of respiratory allergies, it is the dust mites that grow in the same humid environments that cause the allergies.
4,000 – 8,000 skin cells per m³ seems unusually high as humans shed a range of 30 to 90 mg of skin flakes per hour.
The root cause of dampness in sub-floor void, causing rising damp like symptoms, is insufficient sub-floor ventilation. A sub-floor should be sufficiently ventilated to cope with rainwater percolating through soil.
To keep the property dry | |
|---|---|
| Reduce risk of allergies | |
| Dust mites are the primary cause of allergies in the residential environment, not mould. They feed on skin cells, which you unusually high levels of. As top consultant, client of ours confirmed, dust mites are the primary cause of allergies and respiratory issues in residential properties, not mould. But dust mites colonise in the same humid environment as mould, so mould is often incorrectly blamed for allergies. See link, as well as reducing dew points as low as possible, to outside through ventilation and dehumidification, vacuum especially carets and around cold walls, turn duvets and pillows daily. Turn mattresses wash linen weekly. | £300 |
| Improve bathroom ventilation | |
| Given the history of issues, I would install a continuous flow bathroom extractor fan such as 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. 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 |
| Ask tenants to remove vapour from cooking | |
| Cook with tops on pots and pans to reduce the production of vapour from cooking by 80%. | |
| Remove mould | |
| Sand 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 |
| Help tenants dry clothes | |
| 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. | £180 |
| Treat Timber Rot | |
| The damaged caused to structural timber is such that I think you need to replace and or strengthen timber. | £5,000 |
| Treat woodworm | |
| In UK there are two types of common woodworm in houses, common furniture beetle and wood weevil. You appear to have both. Wood weevil colonies in rotten timber, mainly cellar rot, it speeds up and exacerbates the damage caused by rot, but is connected to the rot. It will disappear once the timber dries. Common furniture beetle grows independently of damp, but only eats the sapwood (and wood products such as MDF). Sapwood is the fast growing outer skin 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. Permethrin 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. | £600 |
| Improve sub-floor ventilation | |
| Keep monitoring sub-floor relative humidity. Improve the ventilation, given the circumstance I would install a mechanical extractor fan in the sub-floor, see link above. Consider simple fans, like inline bathroom extractors, for basic airflow enhancement. In severe cases, more advanced fans such as the Elta MORI WMF Weatherproof Multi-Functional Sub-Floor Ventilation Fan might be required – see: homevent.co.uk (alternative products might be needed if Elta has ceased trading). | £800 |
| Redirect down-pipe | |
| The front rainwater down pipe appears to be draining through the bay to the front. I would instruct and general builder to install a series of bends to redirect the down-pipe to the side of the building so that water flows down the hill, away from the property. | |
| Remedial actions – estimate of costs | £7,280 |
Actions if all else fails | |
|---|---|
| Further identification of source which could be roadside drain | |
| With the help from upstairs neighbours we tested the mainswater and drains, neither appeared to be leaking. The only other source of water, other than rain percolating through soil, is the roadside drains. They could be cracked, but is it hard to get councils to check and repair without clear evidence. Similar issues in neighbouring sub-floor can help. And constant measuring of water in the sub-floor. There was not a significant increase after the worse storm for 30 years, so I doubt we can prove that it comes from the drains. | |
| consider installing a sump pump | |
| If water remains in the sub-floor despite redirecting the down-pipe and improved ventilation, I would install a sump pump, see link above. | £500 |
| Optimal actions – estimate of costs | £500 |
I conclude that water is entering the sub-floor void. The most likely root cause is a poorly drained rainwater down-pipe and water percolating through soil. The solution will be to increase sub-floor ventilation. Some timber will need replacing or strengthening, and the ventilation in the bathroom and kitchen will need to be replaced with targeted ventilation, with a dehumidifier run to remove vapour such as from clothes drying.
The mould is caused by excess relative humidity, which results from insufficient ventilation meeting cold walls. The root cause is excess humidity from inappropriate ventilation.
Instructed tenants to use the fan manually for at least half an hour with the door closed after using the bathroom to prevent vapour from spreading into the property. Similarly, use the kitchen fan manually when cooking and consider using a dehumidifier. I has not resolved the damp issues, so ventilation will need to be replaced.. 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: Low risk See Flood Risk (note most flood risk is not from groundwater).
Sub-soil rocks: Near low productive aquifer with intergranular flow See British Geological Survey.
Therefore risk of Rising Damp: low as the property is on a hill and in London where groundwater is pumped out. There is no realistic prospect of groundwater coming into contact with walls and therefore no risk of rising damp, see explanation by Dr Robyn Pender of Historic England.
I confirm that I inspected the property on 231026SH1000. I conclude that water is entering the sub-floor void. the most likely root cause is a poorly drained rainwater down-pipe and water percolating through soil. the solution will be to increase sub-floor ventilation. some timber will need replacing or strengthening, and the v. 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 231026SH1000
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 have been multiple attempts 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: Cloudy, 15°C, 90% relative humidity.
- Property: A ground-floor flat in an end-of-terrace Victorian house.
- Background: Various efforts to manage damp over those years – damp course put into some walls in 1998; another one around 2009. In 2014 I began to rent it out and almost immediately had significantly increased mould problems. Installed extractor fans in kitchen and bathroom that are triggered by moisture content of the air. The tenant, currently undergoing chemotherapy, is concerned about mould.
- Changes: The property has been converted into flats. The ground floor flat has undergone extensive damp proofing treatments on several occasions, some of which were inappropriate. So-called heat recovery fans have been installed, which blow air in and out of wet rooms.
- Occupancy: tenant
- Mould & notes: Mould seen around property, there was a recently mould count that appeared inconclusive, suggesting there is as much concentration of mould outside as inside.
- Primary damp concern: Mould is the primary concern, but in in my opinion dust mites and rot are a far more significant issues.
- Other damp concerns: Rot and woodworm.
- Compounding: Past damp proofing treatments have obscured the root causes.
- Airbricks: The air ventilation is insufficient for the dampness in the sub-floor void.
- Sub-floor humidity: Air in the sub-floor void was humid. I would install a sub-floor extraction fan.
- Tenure: lower ground floor flat
- Attachment: a semi-detached
- Build: Victorian
- Walls: 9-inch thickness.
- Exterior comment: No major issues seen on the exterior.
- Penetrating damp assessment: Rain water is percolating into the sub-floor void.
- Drains: No obvious signs of drain defects.
- Leak signs: Probably no leaks.
- Leak assessment: There are no obvious signs of a significant mains or wastewater leak.
- Floor: Suspended timber floor with rotten beams and woodworm below.
- Loft: No loft.
- Timber assessment: There is rot in the sub-floor void.
- Last dry: Understand the property was last lived in some 17 years ago and has always been a struggle to keep dry despite two attempts at damp proofing and a so called heat recovery vents installed in the bathroom and kitchen.
- Ventilation assessment: Heat recovery systems are untargeted, blow air in and out, which is not appropriate for vapour generating rooms. You need targeted ventilation.
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.
