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104P: Damp continues after chemical treatment

This property was recently chemically damp proofed but remains damp, as rising damp was not, and very rarely ever is the root cause.

Surveyor Tips:

  • Look for sources of vapour.
  • Consider property changes since the property was built.

Root cause

Condensation, made worse by changes to the house.


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The primary concern is dampness to the flank wall in the rear extension. Salts have crystalised on the surface.

Calcium sulphate is visible on the surface. It 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 painted.

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The surface was dry. Here I use a Protimeter damp meter in conductance mode where below 20WME is dry.

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Using a radio frequency meter I found mositure deep within the wall, a sign of interstitial condensation.

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. 999 is the highest meter reading.

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The wall was about 3°C colder than internal walls.

Condensation forms where the dew point is reached, which is often within a wall. A damp wall is a poor insulator, increasing heat loss and risk of further condensation.

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The wall has been damp proofed by injecting chemicals. This is used to stop groundwater causing rising damp.

There is no groundwater in contact with the wall.

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I measured sub-floor humidity and found it to be normal. It would be high if there was groundwater or a leak.

I encourage owners to monitor sub-floor humidity as a high reading could signal a leak which could result in rot, especially if the sub-floor airflow is compromised, such as with an rear extension.

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There are calcium sulphate salts at the base front of the bay.

Water is entering the front wall.

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The walls are exposed to rainwater.

Water is being absorbed into the wall through the exposed surface and cracks in paintwork. Ideally render cracks should be filled and flacking paint should be stripped off and the property repainted with a similar paint to the original, probably Sandtex masonry paint.

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There is no grove under the windowsill to stop rainwater flowing back onto the wall.

Ideally cut a groove of about 5mm into the underside of the windowsill, strip and repaint.

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The front wall is cold relative to other part of the house.

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There is dampness deep within the wall from about 1M off the ground downwards.

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The section of wall by the sockets behind the television was very cold.

Sockets are prone to dampness as a section of wall is cut out, a metal box installed and covered in absorbent plaster, which exacerbate the effects of condensation, see surveyor.tips/sockets.

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There is a crack by the front door.

The wall was damp on the surface.

Dampness is highest in the centre above the crack.

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On the other side there is a porch that has been extended into. Paint has come away from a brick.

The porch is unheated and unventilated. On a winters night it will become very cold and humid.

The wall was damp on the porch side, despite no exposure to rain.

The brick is flacking, this is called spalling and results from frost damage to a damp brick.


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Most damp issues are solved through ventilation, here the bathroom is running at about 15% of requirements.

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 P39 and P19 in https://surveyor.tips/vent_regs specifically:

  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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The bathroom overrun was short, I extended it. However, I recommend changing the unit to a continuous flow.

Monitoring relative humidity

I encourage homeowners with property showing signs of damp or mould to monitor the relative humidity. The data can be used to improve ventilation, increase heat balance, insulation and improve airflow through reduced clutter to see the benefits.
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Looking at your data, there is constant heat, but steady increase humidity overnight. This suggests insufficient background ventilation. The simplest solution is to use a continuous flow extractor fan. Consider also using a PIV if you need filtered air, see surveyor.tips/piv.

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