Background
In a Victorian detached house converted into four flats, dampness has been a persistent issue. The ground and first floor flats, owned by the freeholder’s family, as well as the top floor flat, exhibit various signs of damp. The damp on the ground and first floors manifests primarily on the west-facing exterior walls, despite attempts to mitigate it with damp blocking paint, bathroom ventilation improvements, and chemical damp proofing plugs. The top floor flat suffers from damp patches on the north wall and sloping roof, which have recurred despite roof repairs, flashing maintenance, chimney capping, and gutter clearing. Additionally, the basement has a history of flooding, though recent sump pump installation has prevented further incidents. Timber joists in the bathroom and hallway were replaced due to rot. Damp in the bathrooms has improved following fan installation. Drainage issues have been noted, with suspected tenant misuse and nearby construction potentially impacting the water table. All these concerns point to a complex interplay of factors contributing to the damp issues.
Summary of Remedial Actions Taken:
To date, several measures have been undertaken to address the dampness. These include applying damp blocking paint and installing new bathroom ventilation fans to tackle condensation. Chemical damp proofing plugs were inserted into the external wall, but with little effect. In the top floor flat, roof repairs were carried out, including flashing maintenance, chimney capping, and gutter clearing. In the basement, a sump pump was installed to manage flooding, and some rotten timber joists were replaced during renovation. Drains have been cleared following blockages caused by inappropriate disposal of waste by tenants. Despite these efforts, the damp issues have persisted, indicating the need for further investigation and possibly more comprehensive solutions.
Data loggers
In conclusion, from the data loggers
- The left-hand graph is for the empty, 1st floor flat. Here the air is constantly at 80% with temperature around 12°C, the logger was placed centrally.
- The right-hand graph is for the top floor. The logger was sited on one damp wall, with similar temperature to the logger, and therefore similar surface relative humidity. Relative humiditynever exceeds 70%RH, but at the higher temperature of up to 18°C, compared to the 1st floor flat, meaning the dew point is higher, suggesting insufficient ventilation.
Evidence & Illustrations
As can be the case, there maybe more than one source of vapour. This case serves to differential between hygroscopic salts that cause deliquescence, a type of condensation, standard condensation and later mould. The chimney breast is the link between the flats in this property with multiple damp problems.
Top floor flat – leasehold
1. Standard condensation
This is the profile of hygroscopic salts, which caused a form of condensation called deliquescence. Common table salts causes deliquescence at 75%RH, hence pure table salt will clump up when humid. Calcium nitrate is the most common hygroscopic salt in residential buildings, it causes deliquescence at 50%RH.
In period properties, hygroscopic salts mainly originate from the historic burning of coal, or at street level, exposure to equine urine or deicing salts. Most chimneys will have these salts, but unless salts are dissolved in water, they normally remain within the brickwork. I would guess that condensation, perhaps during Covid lockdowns or last year’s energy crisis, caused the salts to dissolve and move to the, visible layer, the finishing coat of plaster.
Hygroscopic salts can absorb and release moisture from the air, maintaining a state of dynamic equilibrium. They are not a source of moisture, per se and can’t cause mould and rot, but often are often mistaken for and treated as coming from rising damp, see the ground floor flat below). The best treatment is to cover the affected area with a non-absorbent layer like an oil-based primer, such as Zinsser B-I-N.
However, in this case there is no particular heat loss. This Infrared profile is common to hygroscopic salts, as the moisture is only surface deep and they are not caused by heat loss, they do not show up on the thermal image.
1st floor flat – vacant rental flat
Note the rust marks left by the nails. Metal causes heat loss increasing the risk of condensation. No doubt there was condensation here when the property was tenanted.
Ground floor flat
Hygroscopic salts inhibit mould growth because these salts absorb vapour when humidity exceeds 50%RH. This removes water from the surface necessary for mould germination, which requires at least 85%RH. The presence of mould at the base of this wall indicates that the mouldisn’t in contact with hygroscopic salts at this location, as it would be if rising damp were the root cause.
Interestingly, the damp proofers have damp proofed well above ground level, probably higher than groundwater can rise, and above an arch, where clearly water could not pass through empty brickwork.
Apart from the disruption and waste of resources, one of the many downsides of rising damptreatment is it is normally accompanied by replacing internal plaster with impermeable slurry. Slurry stops the natural absorption of vapour, thereby reducing the walls ability to buffer moisture. An analogy would be a plastic box compared to a cardboard box, in a cold environment. The plastic will “sweat” more readily than the cardboard. This is described by many as breathability.
I don’t know if this wall originally has hygroscopic salts, like those of the top floor. I guess it did. Hygroscopic salts predominantly come from coal or sometimes equine urine. However, they are also contained in groundwater. They are therefore often cited as evidence of rising damp. This is very rarely ever the case, especially not in London. The important distinction is that groundwater is an ongoing source of moisture. Whereas most hygroscopic salts are not a source of moisture, but are in dynamic equilibrium. In some senses the treatment is the same, that is cover the bricks in an impermeable layer. However, the difference are:
1) The effort and resources required in replacing plaster and applying a coat of slurry is maybe 100 times greater than painting a layer of Zinsser.
2) Slurry is basically a mortar with chemicals used to reduce the amount of water required to work the mortar, thereby reducing pore size. There is still moisture used in the application, therefore hygroscopic salt are at risk of dissolving and moving to the surface of the new layer.
3) Replacement plaster inevitably leaves a ridge. To make it neater and justify the rising dampdiagnosis, the lower half of whole sections of wall are replacement with slurry. Whereas Zinsser only needs applying to the damp patch, with 300mm overlap, and if required can be extended.
4) In both cases, there is a risk of missing another source of moisture. If there is a genuine case of groundwater, leak or even condensation, then there is a risk of rot to sub-floor timber. In the case of Zinsser, dampness will soon show beyond the Zinsser layer, necessitating further investigation.
5) There is an increased risk of condensation in both case, but the risk is greater with slurry, because of the amount of slurry that is applied. Salts often blister along the interface between the permeable and impermeable covering. With Zinsser, a quick rub down and reapplication will suffice, whereas slurry would typically require replacing. Of course, any root cause including condensation would need to be dealt with in both treatments through improved ventilation, airflow, insulation, dehumidification and heat.
In theory pumice, a sacrificial material, such as lime, can draw salts out of a wall. Salt neutralising solutions may work. Insulation involving a gap between the wall and new surface works well, such as thermal lining paper, by Wallrock Thermal liner, as it also reduces heat loss and therefore the risk of condensation, will can occur in the same environments. However, for an easy, inexpensive go to solution, use an oil based primer, such as Zinsser.
To keep the property dry | |
|---|---|
| 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). | £180 |
| 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. | £100 |
| Use fan to expedite drying | |
| Direct a fan onto the damp patch to speed up the drying process. | |
| Keep objects away from walls | |
| particularly for ground floor flat, move items away from walls to allow for air circulation. I would also insulate behind cupboards. Constantly using a dehumidifier will reduce the need. | |
| 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. | |
| Hygroscopic salts | |
| Hygroscopic salts act like grease stains. They are benign, cause no damage or mould, but are unsightly, especially in summer during a heat wave, and cause problems renting or selling a property (keep this report to show prospective buyers). The best solution is to cover the salts, with an impermeable blocking material, with a significant overlap of at least 300mm (to reduce the risk of enlarging the damp patch). Ideally treat each section of wall equally, so that it is homogenous and therefore absorbed or repels humidity in the same way. Hygroscopic salts can be just under the surface. There are three alternative methods; 1) the simplest, cheapest and in my opinion the best is using 2 coats of Zinsser B.I.N. Cover Stain, a solvent-based, primer-sealer. For each patch, it takes about 1 hour and costs around £15 plus any labour if required, the great thing with Zinsser is it is easy strip off and reapply. For significant areas consider painting a coat of gloss paint sandwiched between two layers of Zinsser BIN Cover Stain, and ideally paint all sections of wall affected, as like icebergs, there can be more hidden just below the surface. Most homeowner would attempt this. It will take a general builder a few hours, and so should be considered as part of a larger project. For saturated walls and if it doesn’t work immediately, consider sandwiching a layer of an oil base paint between two layers of Zinsser oil based primer, then you can complete with a standard emulsion. Try an out of sight area first. 2) Sempatap or Wallrock lining paper, see https://www.youtube.com/watch?v=_R1B44fGttE, this creates a slight non porous “cavity” separating the lining paper from the wall. Suitable for slightly cold walls, were insulation is beneficial. It takes about 1/2 day and costs around £100 per linear metre. 3) Remove the plaster and install thermal insulating plasterboard, with a vapour barrier from floor to ceiling (similar to damp proofer’s technique). Suitable for very cold walls where insulation is beneficial. It takes about 1 day and costs around £400 per linear metre. *Damp proofer’s slurry can work, but it often doesn’t (as slurry is applied wet and therefore can dissolve salts bringing them to the new surface). Furthermore it is unnecessarily damaging to walls, normally leaving a ridge, disruptive to occupiers, typically needs replacement every 10 – 20 years and costly, in the order of £1,000 per linear metre. I would always start with the Zinsser B.I.N. Cover Stain, but budget for removing the plaster and installing thermal plasterboard. Note some recommend you cover Zinsser B.I.N. with Zinsser Bullseye 123 for better adhesion to emulsion paint. Excess humidity, can result in the interface between impermeable blocking material and permeable plaster to dampen, which can dissolve hygroscopic salts causing them to become visible near the interface. In such as a case, treat the new surface a second time. Again I would recommend Zinsser BIN Cover Stain. | £50 |
| Remedial actions – estimate of costs | £330 |
Actions if all else fails | |
|---|---|
| Consider targeted insulation | |
| Consider targeted insulation: insulating behind radiators to stop heat loss, insulate behind cupboards, such as with wood fibre, cold chimney breasts and walls under bay windows. Consider using thermal insulating lining paper, such as Wallrock thermal liner. | £2,000 |
| Tenancy agreement for damp | |
| Consider updating the tenancy agreement, see Tenants responsibility to stay mould and damp free, or at least communicate to tenants their responsibilities, and praise them for already communicating. Dealing with damp and mould requires teamwork, communication and appreciation that it is not always easy for tenants, nor do they necessarily understand their challenges and responsibilities. | |
| Consider installing a continuous flow bathroom extractor fan | |
| Continuous extractor fan are useful for rental properties with a history of ventilation issues. | £400 |
| Consider installing kitchen extractor fan either above the hob, or light switch operated. | |
| Each flat should ideally have a kitchen extractor fan. However, they tend to be noisy, not always easy to operate and so people tend not to be good at using them, unless the food is odorous. An alternative is an automatic kitchen fan, these are bigger with 60 l/s flow, but require a 150mm hole. Ideally it is light swich operated with 30 minute overrun. Don’t use a manual fan, tenants won’t switch them on | |
| Optimal actions – estimate of costs | £2,400 |
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 and likely hygroscopic salts on at least one of the chimney breasts on the top floor, external side of building – see report illustrations.
Chimneys are at risk of heat loss causing condensation and mould and of hygroscopic salts, which inhibit mould. Hygroscopic salts are often mistaken for evidence of rising damp.
I significantly improved the ground floor bathroom speed and overrun time 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 £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: 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 risk, see explanation by Dr Robyn Pender of Historic England.
I confirm that I inspected the property on 231122SH0800. 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 and likely hygroscopic salts on at least one of the chimn. 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 231122SH0800
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: A damp proofing contractor claimed your property had 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
- Background: Persistent issued including after damp proofing treatment.
- Changes: conversion into flats, central heating, double glazing with no or insufficient trickle vents and lifestyle changes including more frequent showers and clothes washing, including greater occupancy during the Covid pandemic, possibly reduced heating during energy crisis.
- Occupancy: Ground floor high occupancy, top floor normal, 1st floor vacant, awaiting tenants.
- Mould & notes: mould across ground floor flat.
- Primary damp concern: Dampness up chimney breast
- Other damp concerns: Rot – none seen.
- Compounding: chimney breasts are at risk of condensation and hygroscopic salts
- Airbricks: There is appears to be sufficient sub-floor ventilation, but this should be checked regularly.
- Sub-floor humidity: The sub-floor had normal levels of relative humidity.
- Tenure: building divided into flats
- Attachment: a semi-detached
- 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 floor is of no special concern.
- Loft: There was no loft.
- Timber assessment: Although there has been rot in the past, there are no suspicions of ongoing rot. There is a risk, so there should regularly 6 monthly checked, especially in the cellar and sub-floor.
- Last dry: I assume that the damp issues are relatively recent, possibly since Covid.
- Ventilation assessment: The top floor has insufficient ventilation, in the kitchen and bathroom. The ground floor now has sufficient bathroom ventilation. Both the kitchen and bathroom doors in all the properties should be kept closed until rooms are fully ventilated.
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.
