Damp in the Fireplace – An Unwelcome Guest

Background

A damp survey was requested to investigate potential rising damp and assess colourless damp patches identified above ground level, as noted in a recent home buyer’s survey. 💧 The client, undertaking a larger renovation project, has expressed concerns about dampness and its root causes, which may require thorough investigation, including disruptive testing if necessary. Existing humidity monitors have been installed, and access to the party wall has been arranged to aid in the assessment. The damp issues are believed to be related to rising damp and could significantly impact renovation costs. 📊
A surveyor’s report highlights the importance of addressing these concerns effectively to prevent further damage.

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Data loggers – see damp.ai/detect

💚 Green Line (Surface Relative Humidity):

Represents Surface Relative Humidity, the percentage of moisture relative to the surface temperature. During the monitoring period, the green line indicates that the surface RH of the damp wall frequently reached the dew point—the level at which condensation occurs.

Key Insight: Consistently high surface RH shows that the wall is at risk of condensation, especially in colder zones, like near the sub-floor.
Impact: Condensation forms when the surface RH reaches 100%, increasing the likelihood of dampness and mould growth.

💙 Blue Line (Dew Point):

Represents the Dew Point, a measure of the vapour content in the air. To prevent condensation, it should remain below the 🩷 Pink Line (Outdoor Temperature).

Solution: Maintaining indoor vapour levels below the dew point through targeted ventilation and dehumidification helps reduce condensation risk. This principle is part of Dew’s Law.

Observation: For a significant portion of the monitoring period, the dew point was above the outdoor temperature, indicating elevated vapour levels indoors.

Context: When this happens, colder areas—such as those near the sub-floor—are especially vulnerable to condensation.

This thermal image, showing colder temperatures under the floorboards where I believe the dog basket had been, illustrates the effects of air movement in the sub-floor void, especially near sub-floor ventilation.

Conclusion from data loggers

I believe excess vapour from insufficient bathroom and kitchen ventilation is the primary cause of the dampness in the living room.

Evidence & Illustrations

You expressed concern that your property might be affected by rising damp. Rising damp refers to the upward movement of **groundwater** through building materials via capillary action. While **all water can rise**, only **groundwater is bounded**—meaning it sits above saturated soil, which prevents it from being drawn down by gravity. Other sources of water, such as a mains or drain leak, or condensation, are considered defects and should be dealt with at the source.

The property is situated on rock with essentially no groundwater present. Additionally, in London, groundwater is typically pumped out to depths of 30–60 metres below ground, making rising damp highly unlikely. Furthermore, damp-proofing treatments merely hide damp rather than addressing the root cause.

Rising damp is often diagnosed for several reasons:

Misdiagnosis: When the root cause of damp cannot be easily identified, rising damp is often assumed by default for dampness at the base of a wall.
Profitability: Treating supposed rising damp can be lucrative, leading to overdiagnosis.
Appeal of the term: “Rising damp” is a catchy and seemingly logical explanation, which can mislead homeowners—until the issue is examined more closely, as you are doing now.

It is straightforward to determine whether groundwater is likely to affect a property:

Geological Research: Review the geology under the property and check the height of groundwater at local pumping stations.
Testing: If absolutely necessary, drill below ground to check for saturated soil within 1 metre of the property. If saturated soil is found, repeat the test on all four sides of the building.
- If one hole has water at a higher level than the others, the root cause is likely a leaking drain, which should be repaired promptly to prevent issues such as rot in the sub-floor and subsidence.

By focusing on the root cause of dampness—whether a leak, condensation, or other issues—you can avoid unnecessary treatments and ensure lasting solutions.

There is dampness and calcium sulphate surface salts in patches on both sides of a sofa on the party wall. Calcium sulphate is the primary ingredient in cement and plaster. When dissolved, it migrates to the surface and crystallises. These salts are benign and can be removed with sandpaper before redecorating. Filler or finishing plaster may be required. They are most commonly associated with condensation. Importantly, there is no brown staining, which would be expected if water were passing through the party wall. I also found no dampness on the other side of the party wall.

There appears to be a shared chimney stack immediately above both damp zones – on both side of the sofa. Your right-hand ground floor neighbours have a chimney breast, but your property does not. Chimney breasts can be supported in various ways from below, including with steel joists (RSJs). The wall is hollow near the damp zone. Metal elements, such as a joist, electrical wiring, or other materials entering the sub-floor void, can conduct heat away, increasing the risk of condensation.

Condensation is more commonly found on the external side of a room. However, the external wall in this case has been damp-proofed against rising damp, which hides rather than resolves the root cause of dampness. This treatment can shift the problem to the next most vulnerable surface. Furthermore, there is no risk of rising damp in this part of London—the underlying rocks have essentially no groundwater, the source of water in rising damp.

The sub-floor relative humidity was found to be normal in the sub-floor under the stairs. However, I did not have access to inspect the sub-floor beneath the living room.
If sub-floor relative humidity is elevated, moisture from a source such as a leak can condense on the damp-proof course. While you appear to have adequate sub-floor ventilation, it could be obstructed by spider webs or debris accumulating in the vents.
Before the inspection you found that the rear drain was blocked. It is unblocked now – but it could add to sub-floor vapour. Additionally, the middle analogue dial on the mains water meter was slightly turning indicating a movement of water at the speed of a leak. While this could result from a small, innocuous drip from an outlet—possibly in one of the upstairs flats—it warrants further investigation.
I recommend inspecting all outlets and, if feasible, temporarily turning off the water supply to the top two flats. This will allow you to determine whether the central dial on the water meter continues to turn, indicating a potential leak under your living room floor.
The dampness on the living room wall may be exacerbated by the previously blocked rear drain and potentially a leak in the mains water pipe. Addressing these issues could be essential to keep the sub-floor dry.

Other maters

I recommend thoroughly testing all rainwater goods, roofs, drains, and pipes during a rainstorm. There were clear defects, such as with a pipe to the front and rear roof, although I understand you are replacing these.

While you are replacing the side wall, consider the following:

Blistering salts: When rainwater breaches a wall, salts dissolve and migrate to the surface. These can cause blistering and microcracking, increasing moisture absorption and accelerating wall deterioration.
Damp penetration: In many cases, dampness primarily affects the outer skin of the wall, reducing its insulation and increasing the risk of condensation and mould. In some situations, damp can penetrate fully through the wall, causing more serious issues.
Unpainted and unrendered walls: Unpainted, unrendered brick walls generally perform better at managing moisture. Any covering (paint, render, etc.) can trap moisture and cause issues if improperly applied or maintained.
Regular maintenance: Covered walls require regular inspections. Ideally, permeable coatings should allow moisture to escape. If an impermeable layer exists beneath, the outermost layer must also be impermeable to prevent moisture trapping.
Unknown coatings: If unsure about the existing coating’s permeability:
- Remove it entirely.
- Apply the most impermeable coating available to mitigate issues from moisture ingress and salt migration.

Proper attention to these factors is crucial to maintain the wall’s durability and protect it from further damage.

To keep the property dry
Cooking Tips
Always cook with tops on pots and pans to reduce vapour emission by 80% and or use an externally ducted kitchen extractor fan – see below. Ideally keep the kitchen door closed and window open – or bathroom vent running, when it is humid.
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 internal vapour
Use devices like the ORIA Wireless Thermometer Hygrometer to monitor relative humidity and temperature near walls or ceilings, look to keep the dew point at or below the external temperature, The dew point is shown on the app (if not use https://mouldpoint.co.uk to calculate the dew point from humidity and temperature readings). Increase ventilation or dehumidification as required. Note excess ventilation results in lost energy. The cold air replacing the warm air will have a dew point that is below the outside temperature. When it is raining, the dew point will be higher, so you may need more ventilation to achieve the same reduction in dew point. In winter, there is no risk in condensation inside, if the internal dew point is below the external temperature.
Use fan to expedite drying
Direct a fan onto the damp patch to speed up the drying process.	£50
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 a 6.5-litre tank (£179.99, refrigerant) Duux Bora Smart Dehumidifier (DXDH02UK, £300, refrigerant) 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 capacity, tank size, low noise, continuous drainage (pipe out), and ease of programmable functions over other considerations such as weight and aesthetics. The technology is well-established and functions similarly to fridges, with a comparable fire risk profile. If these brands are unavailable, you can confidently select another alternative. When evaluating, prioritise a minimum daily capacity of 20L and continuous drainage capability (use this to drip into a sink when away in winter). Tip: Use a dehumidifier whenever leaving a property vacant. Set it to 70%RH and drip into a sink—especially in winter. This method is more efficient than other humidity management methods, including ventilation.	£180
Monitor dampness in walls
Mark improvements on damp walls with a pencil line or sticky note and use a moisture meter, such as the Dr. Meter Wood Moisture Meter 2 in 1 Pin & Pinless Multifunctional Water Detector £23.99 or similar damp meter. Log and monitor the reduction in dampness to gain early confidence in these recommendations.	£23
Keep objects away from walls
Consider moving the shelf away from the cold corner.
Check rainwater goods
Check for leaks from rainwater goods, from gutters to drains and clear as required. Consider having a CCTV leak check. The drains maybe made of clay, over 100 years old and subject to heavy properties settling on them, so could be cracked underground.
Investigate mains leak
Testing for a mains water leak is simple. First, check the building for innocent drips from shower heads, taps, and cisterns. Next, turn off any stopcocks in upstairs flat. Then look at the dial on the front water meter. It should stay stable. If there is a leak you will need to fix it, but this could wait until your renovation project is underway.
Remedial actions – estimate of costs	£253

This estimate is based on approximate time, costs and competence to complete repairs. It is not a quote for work. We are independent. A general builder can complete most work. trustedtraders.which.co.uk is good sources of general builders. Once the recommended remedial actions are completed and the walls allowed to dry, dampness will not have a material impact on the value of the property. It is the client’s responsibility to check that a contractor has sufficient insurance and competence to undertake any work that we recommend.

Actions if all else fails
Duct out kitchen extractor fan
Consider instructing general builder to duct out kitchen extractor fan (hole required to duct outside, which may require freeholder permission).	£500
Bathroom extractor fan
Consider installing a continuous flow bathroom extractor fan such as Vent-Axia Extractor Fan With Humidistat & Timer Continuous Running and Timer, or Brookvent Airstream DMEV 100 HT (Continuous Running Extractor Fan with Humidistat Timer).	£600
Optimal actions – estimate of costs	£1,100

I conclude that the primary cause of dampness on the living room wall is excess vapour from insufficinet ventilation, probably made worse by dampness in the sub-floor void from either a blocked rear drain or a possible leak on the mains water pipe. The reason for it being visible either side of the sofa is likely to come from the location of a removed chimney breast.

The property is not at risk of groundwater, the source of water in rising damp.

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: 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 . I conclude that the primary cause of dampness on the living room wall is excess vapour from insufficinet ventilation, probably made worse by dampness in the sub-floor void from either a blocked rear drain or a possible leak on the mains water pipe. the reason for it being. 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

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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.

Note neither condensation nor rising damp are causes. They are symptoms.

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.

“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.

FACT: dehumidifiers are a win-win as all energy is released as heat with a 50% bonus through enthalpy, but they need to be targeted to the source of the vapour or location of dampness and mould, should be insufficient, typically 20 L/ day theoretical capacity with 6 Litre container and decanted daily or drained into a sink or similar.

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.

Help It takes teamwork to solve damp and mould issues in properties. It isn’t easy or obvious. We can help with damp.ai. Time permitting the plan is to run online workshops for solving damp issues, within the community.

Damp in the Fireplace – An Unwelcome Guest

Background

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Video evidence, tips & tricks

Data Evidence Explanations

Data loggers – see damp.ai/detect

💙 Blue Line (Dew Point):

Conclusion from data loggers

Evidence & Illustrations

Other maters

Remedial actions

Recommended remedial actions

To keep the property dry

Optimal remedial actions

Actions if all else fails

Conclusions & reasoning

Conclusion summary for main concern

Reasoning

Root cause fix should save: £10,000

Risk of rising damp: remote possibility

Survey report & limitations

Site visit

Damp & Survey methodology

W.H.O. indoor air quality guidelines

W.H.O. indoor air quality guidelines

FACTS about Condensation and Mould

TIPS To Stay Condensation and Mould Free

Exemplar

AI & related examples

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