102Y: condensation leak

The owner wanted to fix damp issues with plenty of time, before considering putting the property on the market for sale.

Root causes

Vapour from condensation, caused by an internal bathroom door being kept open. In addition there was a mains water leak, the likely cause of dampness along a solid floor.

Surveyor Tips

  1. To test for a main water leak, attach a water pressure gauge to an outside water pipe, then turn street side stopcock off.
  2. Ask all occupiers not to use the water / toilet for 20 – 30 minutes. I recommend doing the test early if the pattern of water could be from the main – typically it looks like rising damp on a solid floor, or high humidity in the sub-floor void.
  3. However, as was the case here, if there is often a leak on the outside tap, difficult to fix – then try the trick discussed below, using a glass vessel full of water, with the meniscus pressed up against the kitchen tap, with the stopcock turned off. If there is a leak water will be sucked into the tap, or down from it.
  4. Check if there is any “innocent flow” of water from a dripping tap, shower or toilet cistern. Stop the flow and retry.
  5. Suggest that the owner looks at the main water meter for an extended period when water is not used, noting the reading before and after.


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Mould and droplets of condensation were forming on the front room wall during the survey.

Condensation forms when the humidity drops below the dew point. Mould only 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 wall temperature was 7.6°C, well below the dew point of 9.2°C and mould point of 11.6°C.

The secondary cause of high relative humidity is low temperature relative to the source of humidity, see surveyor.tips/humidity. The front room radiator is away from the wall, various objects are slowing down the movement of air. The insulation of this Victorian wall is relatively poor compared to modern standards.

Refer to MouldPoint.co.uk for a mould and dew point and surface humidity calculators and daily forecasts.

Rising damp risk assessment

Elevation is: 17M above sea level.                 The flood risk is: no risk.

Sub-soil rocks are: Near low productive aquifer with intergranular flow. Superficial deposits: Taplow Gravel Member – Sand and gravel. Bedrock geology: Lambeth Group – Clay, silt and sand.
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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The bathroom vent is running at normal speeds and lasted for about 15 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.
  2. Kitchen 30 l/s adjacent to hob; or 60 l/s elsewhere in kitchen.
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I tried increasing the overrun to 30 minutes.

I was unsuccessful, so put the setting back to the original 15 minutes. Consider upgrading the fan to a continuous flow, such as the Elta Mori.

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To the rear of the property moisture is blistering through the external wall above a solid floor.

The blistering is caused by moisture trying to evaporate from behind a gloss painted wall. 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. 

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There was a high damp meter reading.

I tested the surface of 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 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. I determined that the highest point of a dampness was roughly 1M in from the kitchen, it suggests that this is nearest to the source of water.

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I was unable satisfactorily attach my pressure gauge to your rear tap, so used a simpler leak detection method.

We were able to test for pressure loss by seeing if water from a full glass would be drawn back into a tap. Water was drawn back on every occasion that we tested it, indicating a mains water leak. The damp profile above the solid floor is consistent with a mains leak.

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You can doublecheck for a leak by looking at meter differences over a prolonged period of no water usage.

Note the final digit on the mains water meter is in litres, 1 L over 10 hours would be considered a significant leak. False positives can result from innocent flows such as dripping showers and toilet overflows. I checked and found no obvious reasons for a false positive.

Once confirmed you need to either instruct a specialist leak detector (e.g. leakbusters.net) and plumber to fix the leak. Or live with the leak, perhaps masking the effects, removing the skirting board and applying a thixotropic cream such as DryZone and replacing the skirting boards with higher skirting. There were no signs that the leak is causing any rot or other structural damage. That situation may change over time, so monitor the sub-floor humidity.

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I was unable to look at the flanks wall in detail, but what I could see appeared to be in reasonable order.

Cracks should be raked out. Small cracks should be filled with external acrylic caulk and large cracks filled with a cement-based render similar to the current render – and repainted.

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There is a large crack to the top of the wall above the rear kitchen wall.

I doubt this is the root cause of dampness, as the profile is inconsistent with the damp profile, but the crack should be raked out and filled.

There are high readings across the right-hand flank wall, but these readings are less than on the left-hand wall.

A solid floor, especially one with a membrane, will spread damp out underneath the floor, therefore dampness could easily show up either side of a leak.

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Looking externally along the right-hand flank wall we can see a lot of cracks that need filling.

There is no sign that these cracks are causing penetrating damp, but there is a risk.

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There are also some cracks to the render below the middle bedroom.
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Looking immediately outside the front, I note the left-hand downpipe flows onto the concrete.

Consider adding a horizontal pipe, to ensure water drains away from the property.

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A so-called “French” drain (invented by Mr French) has been cut into the concrete.

Unfortunately, the French drain has no exit, so water will sit in it and seep into the subfloor void. There were no signs of this causing high subfloor humidity or internal dampness.

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