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104K: RICS Chartered Surveyor’s ground floor flat

Two bedroom Victorian garden flat suffering from condensation.

Surveyor Tips:

  • En-suite bathrooms are of notable risk, here made worse by turning the temperature down in the front room.
  • Get the occupiers to install and use a datalogger.

Root cause

Vapour from insufficient ventilation.


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The walls have been damp proofed against rising damp by injecting chemicals. Typically damp proofers replace ground floor internal plaster with an impermeable plaster, known as slurry. This increases condensation risk.

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The front bay wall has calcium sulphate blistering through the paintwork. There are condensation streaks.

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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The surface was dry when using the classical conductance damp meter.

I tested the surface with a Protimeter damp meter in conductance mode. These meters measures electrical conductance of salts in water, a proxy for dampness.

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However, when measured with a radio-frequency meter, damp was detected deep within the wall.

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. These meters are for rapid scanning, mapping and profiling, see https://surveyor.tips/profiling.

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The thermal image hight-lights the area of thermal risk, blue being about 5°C colder than orange.

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The laser thermometer, measured a wall temperature of about 12.8°C compared to ambient of about 20°C.

Every 1°C increases the relative humidity by about 5%RH, until 100%RH is reach when condensation forms.

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The en-suite bathroom extractor fan is running at about 12.3 l/s, but stops after lights are switched off.

Roughly 90% of humidity remains after a bathroom is vacated. You should either keep the lights on for 30. Minutes, change the extractor fan or use the main bathroom.

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The main bathroom extractor fan is running at about 5.6 /s.

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 rusty hock illustrates the issue.

I spent a while considering if the main bathroom is venting out fully. It was inconclusive probably because the vent speed was too low to test against external wind. Consider testing with a smoke pellet, to ensure all air is being drawn out (when the family is not around). Or employ a ventilation expert able to test the inflow and outflow after installation.

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There was a leak from under the bathroom shower panel.

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Not all windows had trickle vents. Not all trickle vents were open, such as this one in the kitchen.

There was dampness to the top of the ducting in the reception, but not discolouration.

Normally if there is penetrating damp, there is a brown, tea bag like stain. The damp looked like condensation.

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Looking at a thermal image, there is a line of heat loss at the top of the wall.

I understand that the neighbour’s extension is lower than your, thereby creating a thermal bridge, or area of heat loss increasing the risk of condensation.

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Looking above the dampness we can see a down pipe.

Downpipes represent cold voids, inside boxing. There is often poor insulation around the downpipe, and a risk of condensation. Note the ridge tile needs cementing back in place and the plant behind the downpipe should be removed. I don’t believe either of these is causing dampness in your property.

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Mould is growing around the share entrance.

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 https://surveyor.tips/mould.

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I measure the sub-floor humidity, which was normal. There would be high humidity if there was rising damp.

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Any cracks around the windows and render should be filled.

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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In this examples there is constant heat, but steady increase in 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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