102J: garden maisonette with interstitial condensation and hygroscopic salts

This Victorian semi detached two bedroom lower ground and ground floor maisonette in central London was vacant. The buyer wanted to know the root cause of damp and remedy.

Root causes

The root cause was interstitial condensation (that is condensation forming within a building material). There was also evidence of past condensation.

Surveyor Tips

  1. Exposed areas are subject to increased heat loss, increasing the risk of condensation.
  2. Metal within a building losses heat rapidly, increasing the risk of condensation.
  3. An owner super heating a property well before a survey, reduces the risk of a wall being misdiagnosed as having rising damp.


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The most obvious signs of damp are to the rear addition, used as an entrance to the property.

There is discolouration to the paintwork. 

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Testing the damp looking patch with a damp meter revealed it to be dry.

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 8WME to 99WME. See surveyor.tips/dampmeter. Walls measured were largely dry on the surface except where mentioned in this report.

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I also tested in radio-frequency mode, sensitive to water about 7 cm below the surface.

I tested walls 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. Again the section of wall was dry despite the damp looking patch.

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The area by the door however was damp deep inside the wall.

The whole of this section by the front door, mainly around the ceiling was damp. I noted that the entrance hall is unheated and has fours external surfaces, increasing heat loss.

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There was dampness under the lintel above the front door.

Lintels are generally composed of metal. The staining almost certainly results rust from condensation forming within the lintel.

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I tested and found metal above the door.

Metal causes heat loss increasing the risk of condensation.

Looking outside around the flat roof we see that time and money has been spent trying to stop rain penetrating.

It is my view, that the roof is not the root cause of dampness. And the root cause of damp is internal vapour resulting from insufficient ventilation. However, in the unlikely event that I’m wrong, it should only cost about £500, for the roof to be replaced with EPDM.

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The face of bricks above the entrance, have been covered in a low absorption chemical.

This chemical coating appears to have made no difference to internal dampness.

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Note the flat roof has ponding, increasing the risk of water penetrating.

I had a good look at the bitumen felt and doubt water is passing through it. The internal dampness does not line-up with this water.


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There is no externally ducted kitchen extractor fan, and the bathroom extractor is insufficient.

Ventilation is most effective when air is extracted close to the vapour source; bathroom, kitchen, drying clothes and occupied rooms. The internal ventilation does not meet Building Regulation 2010 Part F requirements. This is best achieved with mechanical extractor fans. 

 See surveyor.tips/vent_regs specifically P39 and P19:

  1. Bathroom 15 l/s with a 30-minute overrun.
  2. Kitchen 30 l/s adjacent to hob; or 60 l/s elsewhere in kitchen.

Consider installing a continous flow bathroom extractor fan.

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Your RICS homebuyer surveyor identified a section of dampness along the flank wall.

There are no signs of visible dampness along this wall, but there are high damp meter readings in radio-frequency mode, suggesting damp deep within the wall.

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Looking outside we can see dark shading to this section of wall.

This dark shading is a sign that there are hygroscopic salt in the wall, see end of this report

Other matters

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Common with period properties there are a few cracks forming in the render.

Rainwater can easily penetrate render cracks. As a matter of good maintenance they should be raked out and filled, with an external water or external caulk and painted, as they occur.

There were no signs of these minor defects are causing internal dampness.

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I tested the whole of the ground floor with a damp meter in conductance mode all the wall surfaces were dry.

I note that your RICS surveyor said there word high damp meter readings on the surface of ground floor walls. The heating was only during my survey, despite it being midday and vacant. This suggests the owners were making an effort to reduce the risk of condensation on external walls. It is therefore unsurprising that there is a difference in my readings compared to that of your RICS surveyor. This just goes to reinforce my conclusions, that the issue is condensation, which can take months to dry out if absorbed deep inside the wall.

Rising damp risk assessment

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Note the drill holes, a tell-tale sign that the property has been chemically damp proofed against rising damp.

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.

Elevation is: 12M above sea level.                 The flood risk is: no risk.

Sub-soil rocks are: Near moderately productive aquifer with intergranular flow.
Signs of groundwater: there are no signs of groundwater.

Therefore the risk of rising damp is a remote possibility, see a good explanation by Dr Robyn Pender of Historic England  https://youtu.be/Jo8oF9ubvtI

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