103P: interstitial condensation RSJ lower ground floor

Lower ground floorVictorian conversion flat, mould, condensation and interstitial condensation.

Surveyor Tips:

  • If there is mould at the base of a mould there is no rising damp, as ground water contains nitrates that inhibits mould.
  • Many surveyors are unfamiliarly with interstitial condensation.
    • It is where condensation forms within a wall, here a cold RSJ is causing condensation to form within the wall.

Root cause

Root cause to be detailed soon.


Damp wall

The concerns in this property are surface salts and mould growing across the exterior walls.

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.

Damp wall

There is a damp patch below a steel beam “RSJ” in the rear bedroom. Dribble marks can be seen on the wall.

Steel can lose heat rapidly, increasing the risk of condensation. Much of the condensation is forming within the wall itself, this is known as interstitial condensation. Dew point is the temperature below which condensation starts. The temperature and relative humidity changes between the inner wall surface and outer wall. On occasions the dew point is not on the inner surface but somewhere in the middle of the wall. This phenomenon is known as interstitial condensation and commonly associated with RSJs, because of heat transfer.

Damp wall

Looking through a thermal imaging camera where blue is about 5˚C colder than yellow, we see the wall is cold. 

Damp wall

I used a metal detector to determent that the joist is composed of metal, as suspected.

Damp wall

The wall was damp at the time of the survey.

I tested the surface on the inside at the base of all external walls 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. Walls measured were largely dry on the surface.

Damp wall

Although there is slight dampness to the front walls, they were dryer then might be expected.

Damp wall

Mould is a primary concern, here we can see the temperature of the mouldy wall is 14.2°C.

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.

Damp wall

The thermostat is about 20 cm away yet measures 17.8°C.

There is an increase of relative humidity by about 5% for every 1°C drop in temperature. The thermostat should be set-up to ensure external walls do not fall below the mould point see – mouldpoint.co.uk for a mould point calculator and daily forecast.

Damp wall

I understand you control the heating manually. 

I encourage use of a modern WIFI thermostat with multiple temperatures set to ensure that a property does not drop below the dew point, or mould point for an extended period.

One alternative is to have the boiler come on for 15 minutes every hour or so.


Damp wall

The kitchen extractor fan is not ducted out, a lot of vapour is generated in baby food preparation.

Damp wall

Make sure to keep all the trickle vents open to help background ventilation.

Damp wall

I opened up your bathroom extractor fan to make sure it was set up correctly, which is was, and clean it.

The vent speed and timer set-up were good. However, the extractor was dirty. I was able to increase the speed by 10% by cleaning it. I encourage owners to open extractors carefully when switched off, the clean the vents such as with tissue paper, and checked that they are running freely and functioning correctly at least once a year.

Damp wall

The rear bathroom is not being used, yet there are calcium sulphate salts on the surface.

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. The issue here is condensation forming on the tiled floor. Plaster from the wall is in contact with the floor increasing the risk of condensation becoming absorbed up the wall.

Damp wall

There is dampness visible along the internal corridor behind the neighbour’s bathroom.

Damp wall

There was a high damp meter reading underneath the electrical door monitor.

Damp wall

I noted that the door monitor has electrical wires below it.

It is likely that the plaster has metal imbedded in the wall under it and was at some stage replaced with bonding, which is a more absorbent form of plaster. In combination these marginally increase the risk of water evaporating out at this point rather than anywhere else along the corridor. However I don’t think that condensation is forming on your side of this party wall.

Damp wall

I looked in the neighbour’s bathroom and noted the fan stops as soon as the lights are switched off.

90% of humidity remains in the bathroom after lights are switched off. Bathrooms especially, internal bathrooms should have either a 30-minute time delay or better still, a continuous flow fan such as Elta Mori £110 Amazon, https://surveyor.tips/extractor.

Damp wall

I could not see behind the toilet. I doubt there is a leak but think that condensation is forming on the cistern.

Mainswater comes in at 8°C in winter, making cisterns vulnerable to condensation.

Damp wall

There is a crack in the grout under your neighbours shower which could be a source of moisture.

Damp wall

There is a crack in the sealant underneath your neighbour’s shower that could also be a source of moisture.


Damp wall

You have been running dataloggers in the bedroom and ensuite bathroom. 

The above graph is relatively complex and difficult to judge how representative the datalogger is of the damp wall, position can make a big difference to °C and %RH.

  1. Overall I see a vast improvement since my survey, the average drops from 74%RH to 61%RH.
  2. This improvement should be sufficient to solve your damp problem.
  3. While there are still some temperature fluctuations, they are less frequent. And probably following from ventilation, rather than the heating being off.
  4. 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 ensuite bathroom, or the bathroom is better ventilated.

Conclusions from data loggers

  1. The ventilation improvements have worked and are probably sufficient to stop condensation and mould growth on walls. However, consider installing kitchen extractor fan and continous flow extractor fan from the ensuite bathrooms, 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 wall to increase air movement  and consider anti-condensation paint. Alternatively try insulation, such as Sempatap thermal lining paper, see recommendations. 

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

Other matters

Damp wall

There are cracks in the external render.

External render cracks and gaps such as around windows should be checked, filled and painted annually. I don’t believe that these cracks are currently causing internal dampness, but it is a matter of time.

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