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104F: Solid floor next to unvented bathroom

Ground floor Victorian flat in need of ventilation.

Surveyor Tips:

  • I always recommend continuous flow bathroom fans for damp properties, Such as the Elta Mori dMEV,
  • Or for those with deeper pockets and suffer from hay fever, try a Positive Input Ventilation PIV unit, but be careful of sort circuiting airflows.
    • PIV, perhaps more than other fans is vulnerable to poor installation.
    • Make sure an installer tests ventilation flow rates with an anemometer in and out of the property and check yourself.

Root cause

Root causes will be updated soon.


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The covered drill holes is a sign that the house has already been chemically damp proofed against rising damp.

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The primary concern was dampness in the rear bedroom.

The rear suspended timber floor has been replaced with a solid floor. Solid floor are prone to condemnation. The plaster is in contact with the solid floor in some places, this amplified the effects of condensation.

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At the time of the survey the wall was largely dry, which it would not be if there was rising damp.

I tested the surface on the inside at the base of all external walls every 1 metre, chimney breasts and a sample of internal walls with a Protimeter damp meter in conductance mode. These meters measures electrical conductance of salts in water, a proxy for dampness. Readings below 20WME are considered dry. The range is 8 to 99WME. See https://surveyor.tips/dampmeter.

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There was a high damp meter reading under the pipe.

Pipes draw out heat, increasing the risk of condensation.

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There issue with the rear bedroom is heat loss.

You have insulated the rear wall reducing the risk of condensation, there. However, the corner is still cold and vulnerable to condensation.

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The bathroom is in the process of being updated.

I recommend you take advantage and install a continuous flow extractor fan.

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The bathroom walls were largely dry at the time of the survey.

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There was a high damp meter reading on the cold kitchen wall.

This is a typical area for finding condensation.


The cellar was dry, with dry walls, which it would not be if there was rising damp.

Monitoring relative humidity

You sent me data back from dataloggers here are a few observations and conclusions.

Relative 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

The relative humidity readings appear to be below the mould risk zone of 85%RH. However, placement of the datalogger has a significant effect. Each temperature drop of 1°C, increases humidity by ~ 5%RH. There is around 5°C difference between the kitchen and external WC. If the internal face of the external wall was 5°C less, the relative humidity would be about 95%RH, i.e. mould risk and close to the point when condensation starts.

I measured a 4.8°C difference in temperature between the datalogger and corner of the wall. The Surface Relative Humidity reflects this difference.

Dew point

Dew point is also a proxy the vapour pressure. The higher the temperature the more evaporation will take place. Comparing the dew point of one area to another makes it possible to see where vapour is being generated of conversely removed (normally through ventilation). The bathroom is consistently producing more vapour than is being vented out.

In conclusion from the dataloggers

We can say with reasonable certainly that;

1, The downward %RH trend tells me that ventilation has been improving, or that less vapour is being generated. However, in late March there were long periods of high humidity.

2, Temperature is stable, with daily changes of about 5°C, which is normal.

3, There are individual spikes in relative humidity, but these are short lasting.

Overall the graph looks good, condensation will not return if follow my recommendations.

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