103N: Rising damp like symptoms – left side

  • Here are two neighbouring ground floor flats in a Victorian terrace.
  • Both have been damp proofed.
  • Both have dampness rising damp like symptoms at the base of the walls.
  • Both have recently been diagnosed with rising damp.

About £2,000 saving by identifying the root cause of damp, instead of a second round of damp proofing from a London “damp proofing” company, which is typically only a short term solution.

This is the left side of the party wall – click to see the right side property.

Surveyor Tips:

  • Rising damp like symptoms are not a sign of rising damp.
  • There has to be ground-water below to cause rising damp. The following also cause rising damp like symptoms:
    • condensation,
    • penetrating damp at the base of a wall,
    • a leak at the base of a wall,
    • hygroscopic salts possibly coming from a chimney breast,
    • mains-water or central heating leak into the sub-floor void,
    • rainwater penetrating into the sub-floor void,
  • Always test the ventilation, here the fix took about 5 minutes each to fix extractor fans.

Root cause

Vapour causing condensation resulting from two broken extractor fans, fixed during the survey.


The building has been damp proofed against rising damp, the horizontal drill holes across the centre of this image. Note the damage, which could cause ingress.

Hear what Dr Robyn Pender of Historic England says about rising damp.

The primary concern is damp and mould in the front bedroom.

In this image you can see condensation forming at the base of the window, mould and calcium sulphate salts are visible on the surface. Mould is inhibited by nitrates found in groundwater. The presence of mould at the bottom of a wall eliminates rising damp as the root cause and points toward condensation from unvented excess vapour.

There is dampness in the front wall.

I tested the surface on the inside at the base of all external walls every metre, chimney breasts and a sample of internal walls with a Protimeter damp meter in conductance mode. These meters measure electrical conductance of salts in water, a proxy for damp. Readings below 20WME are considered dry. The range is 8 to 99WME. See surveyor.tips/dampmeter

The challenge with the front wall it is heat loss and the curtains blocking flow of warm air from radiators.

The thermal image illustrates the heat loss to the front wall. Mould grows where relative humidity exceeds 85%RH for 6+ hours. Excessive humidity results from insufficient ventilation, poor air circulation and a cold surface. See surveyor.tips/mould. Lack of a radiator to the front wall creates thermal imbalance, i.e. one side of the room is colder.

Wall temperature was 14.8°C, ambient 17.5°C. Each  1°C loss increases surface humidity by ~5%RH.

The secondary cause of high relative humidity is low temperature relative to the source of humidity, see surveyor.tips/humidity. Relative humidity is a measure of how much vapour is in the air compared to air’s capacity to hold vapour. It is a function of vapour pressure (quality of vapour) and temperature. As temperature rises, air can hold more vapour.

Conversely as temperature drops, air holds less vapour until it meets the dew point at 100%RH, when dew or condensation forms.

Part of my instruction was to establish if your flat was the source of water in the neighbours flat.

The above image of the damp meter roughly in line with the neighbour’s damp patch on the party wall around their electrical sockets. The meter reading is low, i.e. the surface is dry. However, I note some disruption to the wall which suggests either past dampness or replacement of normal absorbent plaster with damp proofer’s slurry. Slurry can hide surface moisture. It affects the absorption profile of a wall, but is not itself a source of water.

I measured the inside of the wall with the damp meter in radio-frequency mode.

Water reflects radio waves at a set frequency similar to mobile phone shields. Meters can’t differentiate moisture from other dense matter such as metal and concrete. They help trace damp in a normal, homogeneous wall.

Readings below 300 REL indicate that a wall is dry below the surface, 999 REL is the limit. These meters are for scanning, mapping and profiling, see surveyor.tips/profile. 

The high meter reading suggests that there is dampness deep within the wall.

There’s a lot of dampness on the surface and high up on the rear bathroom wall despise it been damp-proofed.

There are calcium sulphate salts on the surface. Calcium sulphate is a key ingredient in cement and other building materials. If diluted in water salts tend to move to the surface. These can be removed with sandpaper and decorated. 

By contrast the kitchen wall by the rear door, often at risk of condensation is dry.

A dry wall in the kitchen despite all the other issues in the flat, suggests that the damp-proofing treatment has been at least partially successful blocking the signs of dampness. The price for damp-proofing is reduced absorption in areas without damp-proofing.

The root cause of dampness in this property is ventilation.


The Front bathroom extractor fan has shutters that were so blocked up with dust they were stuck shut. The spider’s web illustrates the lack of airflow. There was no meaningful air movement. The overrun was set to  about one minute after lights are switched off.

I removed the shutters cleaned up the fan and extended the overrun to 30 minutes.

The fan now runs at 9.8 l/s (litres per second), which is about average, albeit slightly below the building reg.s 15 l/s. I suspect the ducting is flexible rather than rigid, and that therefore even the best fans will struggle to achieve more than 10 l/s. Consider a continuous flow fan.

There was also negligible airflow from the rear bathroom extractor fan.

The backflow shutters (the two semi-circular pieces of plastic) were permanently held shut by the silver foil of the flexible ducting. Backflow shutters are designed to stop the wind blowing humid air back into property. At best they are marginally beneficial. By removing them I was able to increase the flow rate to 9.6 l/s.

The rear extractor fan set-up was incorrect, with an overrun of about 13mins and humidistat of 85%RH.

Some humidistats (humidity activated fan) are notoriously unreliable when it is cold. You have the twin benefits of a humidistat and light activated fan. It is common for installers to leave factory settings unchanged. I adjusted the fan to come on above 65/70%RH, with an overrun of 30 minutes after lights are switched off. The problem is the fan is relatively loud and near the bedroom. A tenant could be tempted to change the settings. I would consider replacing this with a continuous flow extractor fan like the Elta Mori, see recommendations.

Here are before and after photos demonstrating vent improvements after removing the backflow shutters.

I estimate an increase of roughly 10 times the amount of humid air extracted from both bathrooms.

I tested carbon dioxide levels.

High CO2 is not dangerous but makes a place feel “stuffy”. It is a sign that there is insufficient fresh air coming into the property. 842ppm is not exceptional, but the normal level is 500ppm. High levels of CO2 suggests that windows are not being opened enough for the level of occupancy. 

Even if the bathroom and kitchen extractor fans exceeded building regulations, they could not account for such a high level of CO2. The tenants should be encouraged to monitor CO2 and open windows when the level exceeds 800ppm. 

I left a datalogger close to the front wall, to measure humidity and temperature.


I left dataloggers in the front bedroom and bathroom. I asked the tenant to move them as detailed above.

The above graph is relatively complex, without prior data, so let me highlight some aspects:

  1. The dew point difference, the grey line, which compares two data loggers, helps identify where vapour is being generated or conversely removed.  We see that more vapour is being generated in the bedroom than in the bathroom. 
  2. The down-spikes correspond with use of the shower. Note bedroom humidity (blue line) remains constant. I have no log of humidity before I made the ventilation improvements, but this suggests that my fixes have solved the production of vapour for both bathrooms.
  3. The dotted lines are from the bathrooms. Dew point is a proxy for vapour production. I think we can say the shower in the front bathroom is used more than the rear bathroom. Outside use of the shower, the bathroom’s dew point (green dotted line) is consistently about 12°C most days.
  4. I don’t know if the location of the datalogger in the rear bedroom was close enough to a rear wall to be indicative, so can’t comment on the relative temperatures and relative humilities. However, the dew point (green line) suggests that there is more vapour produced in the front bedroom or better ventilation of the rear bedroom. This may simply be as a result of the rear bathroom acting as a ventilator of the rear bedroom. Whereas the front bathroom draw air from the flat as a whole, not just the front bedroom.
  5. The temperature in the rear bedroom (red line)  varies from about 18°C to 13°C with three daily peaks, which is normal and acceptably consistent. The kitchen is warmer peaking around 22/23°C. By contrast the bedroom peaks at 17°C and drop to 10°C. While placement of the dataloggers is clearly a factor, this at least highlights heat loss to the front wall.
  6. Despite improvements, humidity by the front wall remains close to 85%RH for almost 3 days. This suggest that heat distribution, airflow and insulation need to be considered. A simple solution would be to put a hem into the curtains so that air can flow and move obstacles such as plants.

Conclusions from data loggers

  1. The ventilation improvements have probably worked sufficient to stop condensation on walls. However, consider installing continous flow extractor fans, such as the Elta Mori £110 Amazon, https://surveyor.tips/extractor, see recommendations, which continuously flows with virtually no sound unless boosted by light or increased humidity.
  2. Remove obstacles from the front bedroom wall or consider anti-condensation paint. Alternatively try insulation, such as Sempatap thermal lining paper, see recommendations. 

I would try a continous flow from the rear bathroom, shorten curtains, remove obstacles and paint anti-condensation paint before considering insulation, which can have unintended consequences.

Rising damp

I measured the subfloor humidity in both flats, they both had normal levels of relative humidity ~70%RH.


If there was groundwater then there would be high subfloor relative humidity unless there was considerably more subfloor ventilation, which itself would reduce risk of rising damp. 

The damp bricks on the bathroom exterior wall is a sign that there are hygroscopic salts.


Hygroscopic salts are often cited as proof of rising damp. I have asked the CEO of Property Care Association (“PCA”), damp proofers trade body, if they have evidence to back up such claims. He confirms that there is no scientific evidence but is not prepared (or sufficiently resourced) to undertake a study. I have undertaken a pilot study and am satisfied that movement is predominantly from vapour and rainwater not rising damp (groundwater), see https://youtu.be/EeT05HxvAs0.  There are no signs of internal dampness from these salts.

Groundwater risk assessment

There is no risk of groundwater below the property, see the British Geological Survey map.

  • Rising damp is the “upwards flow of moisture through a permeable wall structure, the moisture being derived from groundwater”.
  • Groundwater is subterranean water at or below the water table. 
  • Groundwater behaves different to rainwater in soil, as it is at or below the water-table therefore there is no gravitational force, it can’t disburse. By contrast rainwater can and should be drained away from a wall.

Flooding from surface water

If there is no surface water, then even if there was high groundwater, it would flow away.

Floodwater can be mistaken for groundwater. But even the risk of floodwater is very low.

Chemical damp proofing for misdiagnosed rising damp

Chemical damp proofing often causes major problems if rising damp is misdiagnosed, as the original absorbent plaster is normally replaced with impermeable (non-absorbent) damp proofer’s slurry, which hides rather than stops the underlying root cause of dampness.  

Example from another property

After a second course of treatment from “The Leading Damp Proofing” specialist.

If, as is commonly the case, condensation is the root cause, excess humidity can become trapped at the interface between the original permeable plaster and impermeable slurry. This damage results from two damp proofing treatments from the same damp proofer you were considering. They said the work was not covered under their guarantee, instead quoted an additional £16,000 to repair.

Example from second property

Like yours, a vast majority of properties that I survey have been misdiagnosed for rising damp and mistreated.

Unfortunately conflict of interests and lack of evidence results in overstated rising damp diagnosis. Contractors can’t be blamed, treatment for rising damp may be their only profit.

Other matters

The bathroom sealant looks in need of refreshing. 

Fans and grout should be cleaned and checked and sealant refreshed roughly every 2 years.

Water is occasionally dripping from the boiler’s overflow.


Overflows are a safety mechanism, so should not continue to flow to the extent of discolouring the brickwork. There is no sign that this is causing internal dampness, but it could in time. You should have the boiler checked to find out why it is overflowing and fixed as required.

There has been at attempt at of damp proofing the cellar walls.

Walls below ground are subject to water from penetrating damp. In my experience it is better not to damp proof against penetrating damp as a) cellar walls were designed to evaporate away water, b) damp proofing only last about 10 years before it needs replacing.

In my opinion cellars are best left as designed, to evaporate out water and for vapour to be removed through effective sub-floor ventilation or airbricks. Airbricks should be regularly inspected and kept free of obstructions, dirt etc.

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