103I: recent purchase deciding to chemically damp proof of not

This Victorian semi-detached house was suffering from mould on the first floor and a damp patch on ceiling below – could it be condensation.

The radiator had been turned off – easy solution. Dehumidifier, insulation, anti-mould paint and ventilation for longer term less expensive solutions.

Surveyor Tips:

  • Check heating system is working and balanced.
  • Trace unexpected leaks up, consider condensation on a cold water pipe.

Root cause

Vapour condensation due to radiator being turned off.


The main concern is mould growing on the first-floor rear bedroom wardrobe, on the flank 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.

You had a particular concern about a vertical green streak.

There was definitely mould in the cupboard, but I think the green streak was paint rather than mould. Using a metal detector I was able to determine that there is metal in the wall. Metal can cause heat loss increasing the relative humidity.

A thermal image, where blue is about 5°C colder than it yellow, illustrates heat loss.

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

Looking immediately outside we see a chimney breast. 

Chimneys are normally made of single skin of bricks with a cold flue in the centre. This can exacerbate heat loss.

One particular concern you had was a water stain seen on the kitchen ceiling.

The tea bag stain like discoloration is a sign that water has passed through building material or over timber. I measured the ceiling and found it to be dry.

There was also water damage to shirts hanging above this water damaged ceiling.

From the stain, I think these stains result from water dripping onto the shirt from above.

Looking upwards, above where the shirts were hanging there are two pipes. 

Mainswater comes into the property at ~8°C in winter, which is often below the dew point, the temperature when condensation starts. The quantity of water produced is a function of vapour pressure (quantity), temperature and surface area. Insulation reduces surface area and therefore the risk and mount of condensation (or cover in cloth such as an old towel).

There is a particular area of mould growth on the cold rear wall.

The rear elevation is north facing and therefore receives little or no daylight in winter.

The temperature of the corner was about 12.3°C. 

The last record on the datalogger is at just after 6PM, when the logger was in the centre of the room. The humidity was 53.6%RH, temperature 19.88°C. This gives a dew point of 10.2°C, see mouldpoint.co.uk.  Using 12.3°C we can calculate the surface relative humidity is 86.94%, applying the August-Roche-Magnus formula: 100*𝑒(𝑐𝑏(𝑇𝐷−𝑇)(𝑐+𝑇)(𝑐+𝑇𝐷))  [c d are constants, TD = dew point and T = temperature). Therefore, despite the centre of the room being warm and dry (54%RH), the wall is so cold and humid, with a surface constantly above the critical relative humidity of 85%RH needed for mould to growth.

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.

I left a datalogger by the wall. It stopped working suggesting electrics have been damaged by condensation.

Here is a datalogger from a house earlier today. They logged humidity and temperature for 4 months.

I won’t go into specifics, but I found a temperature difference between the datalogger and cold wall in this example property of 8°C. The grey lines represent the adjusted relative humidity taking into account the temperature difference.

The takeaway is that the mould is being caused by heat loss, which in part is from poor heat distribution, poor insulation, poor airflow as well as insufficient ventilation from the source of vapour, typically the bathroom.

Next to the first-floor mouldy cupboard, there is a section of wall that looks and feels damp.

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. 

I tested the surface with a Protimeter damp meter in conductance mode. 

These meters measure electrical conductance of salts in water, a proxy for damp. 19.8WME is marginally dry. The salts suggests that the wall goes through daily cycles of condensing and evaporation.

I used a radio-frequency damp meter.

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. This suggests there is dampness deep within the wall. A damp brick has half the insulating capacity of a dry brick, exacerbating condensation – which helps understand why, a) condensation forms in patches, b) and because of the effects of gravity, tends to be at the base of a wall, albeit this wall is on the first floor

I noticed the radiator was not working. It had been turned off at the valve.

Ideally you should have balanced heat across the house, and throughout the day.

If you don’t wish to heat rooms evenly, you can improve local ventilation, such as with a window on safety lock.

The alternative is run a dehumidifier with a tube continuously draining harmlessly away. Or improve the insulation of external surfaces and cold-water pipes, and improve airflow, with holes at the top and bottom of the cupboards.

Other matters

There are external render cracks. There is no sign of internal dampness, but they should be raked out and filled.

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