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104N: Modern ultra-chique penthouse

Amazing views over the Thames distracted by the smell of mould from the second bedroom.

Surveyor Tips:

  • Ask and listen, the occupiers will almost certainly give you the greatest clues.
    • Here the owners stopped using the internal bathroom vent as it was flapping in the wind making noise.

Root cause

Excess vapour from insufficient use of the ventilation.


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The concern was a smell of mould in the second bedroom.

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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Looking through a thermal camera where blue is ~ 5˚C colder than orange, we see heat loss around the sides.

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

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The top of the window is 15.6°C. Heat loss increases relative humidity. Keep trickle vents open to compensate.

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.

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The internal bathroom extractor fan was switched at the isolator.

The extractor fan should be running for at least 30 minutes after taking a shower, otherwise vapour will disperse through diffusion, around the apartment.

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The extractor rate was 19.2 l/s, which is over the Building Reg.s required 15 l/s.

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I increased the overrun time from about 5 minutes to about 30 minutes, by rotating the white dial, see above.

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I removed excess pipework.

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The increase in extraction rate after removing the section of pipe was about 20%.

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I improved the back flow shutters by adding weight, note a spring was missing.

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There was a similar issue with a mould smell coming from the room by the front door.

I believe the issue here is heat loss from the lift shaft immediately outside. The simplest solution is to insulate around pipes and either insulate walls or paint on anti-condensation / anti-mould paint such as DryZone made by Safeguard Europe – widely available.

Monitoring relative humidity

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The purple relative humidity line is reasonably low, mainly below 50%RH. However, the datalogger was far away from the problem surface. There is a temperature difference between the area of mould growth and the datalogger. The dotted line estimates the surface relative humidity is based on a temperature difference caused by heat loss around the metal late at night.

You can either measure the temperature around the top of the window frame with a laser thermometer, very early in the morning (before sunrise) or install the datalogger on the coldest section of window, to get a more accurate set of readings.


The mould is caused by humidity air become trapped by the curtains against the cold window frame, trickle vents are designed to relieve this humidity, but none are open.

The main solutions are;

  1. Run the bathroom extractor fan for 30 minutes.
  2. Open the trickle vent in the second bedroom (if heat loss bothers you, only close the trickle vent when the curtains are open).
  3. Monitor relative humidity, ideally by the coldest section of window which may be at the base of the windows, you should measure before the sunrises.
  4. Consider increasing heat in the second bedroom.
  5. Consider installing a continous flow fan in the bathroom.

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