102O: Suspect penetrating damp; exposed render & insufficient ventilation

A modern ground floor extension, two bed, tenanted. Landlord convinced that penetrating damp was the only cause, but couldn’t work out how.

Root causes

The root cause was insufficient ventilation. There was a possible contributory factor caused by damaged render just above the exposed damp proof course (“DPC”). However, the damp is too localised for this to be the major factor, and the response of mould tells us excess humidity is part of the problem.

Surveyor Tips:

  1. The presence of mould tells us that there is high humidity, poor air-circulation and a heat imbalance.
  2. Mould grows of leather more rapidly than canvass.


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The primary concern is dampness in the corner of the rear bedroom.

You wondered whether there was penetrating damp, rising damp or indeed condensation.

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There was a high damp meter in the corner.

I tested the surface on the inside at the base of all external walls every metre 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. See surveyor.tips/dampmeter. Walls measured were largely dry on the surface except where mentioned in this report.

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That was low temperature in the corner.

The secondary cause of high relative humidity is low temperature relative to the source of humidity, see surveyor.tips/humidity.

Part of the issue is the bed is stopping heat from the radiator warm in the corner up. The solution could be through insolation auntie condensation paint or improved ventilation and a better balance of heat through the course of the day.

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The damp meter reading is at normal levels a short distance away from the corner.

If there was penetrating damp, you would normally expect to see discolouration like a tea bag stain. There was no such stain. The profile is not consistent with rising damp, which dips in the corners, as the extra surface area around a corner increases evaporation. The profile is consistent with condensation where the extra surface area decreases the temperature.

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Looking outside the render is in order. There is a visible damp proof course (“DPC”).

The lack of render to the lower bricks causes insulation imbalance at the base of the wall.

The DPC appears to be uncovered by the render.

The gap between the render and DPC, could cause moisture to penetrate above the DPC causing rainwater to dampen brickwork. I suspect that there is a cavity wall with insulation and a membrane in the centre, this reduces the chance of penetrating damp. However, a damp external brick will can cause heat loss, increasing the risk of internal condensation.

There is mould in various places in the property, this means that there is insufficient ventilation.

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.

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The ensuite bathroom extractor fan runs at about half the required speed, with an overrun of only 5 minutes.

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 (main bathroom is extracting at 22 l/s, with only 5 minutes overrun, I extended it and others in the building to 30 minutes)
  2. Kitchen 30 l/s adjacent to hob; or 60 l/s elsewhere in kitchen (which is is)
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The kitchen extractor fan ducting had fallen off the mounting, resulting in vapour from cooking recirculating.

I fixed the kitchen extractor fan during the survey, 

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Despite providing a clothes dryer, the tenants appear to be drying clothes on a clothes rack.

About ½ L per person per day results from drying close indoors. 

Other matters

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I tested a leak in an upstairs flat. The ceiling is still damp.

Rising damp risk assessment

Elevation is: 14M above sea level.                 The flood risk is: no risk.

Sub-soil rocks are: Rocks with essentially no groundwater.
Signs of groundwater: rising damp is not of concern.

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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