102C: mould under flat roof, radiator imbalance

This is top floor modern three bed partially terraced flat with significant mould.

Surveyor Tips:

  1. Colourless dampness, especially on ceilings is likely to be caused by condensation.
  2. Tenanted properties, where you have no control over use of ventilation needs:
    • additional ventilation, such as a continuous flow, and
    • monitoring / regular maintenance checks.
  3. Mould results from vapour i.e. poor ventilation AND poor airflow and poor insulation.
  4. Thermal imbalance, such as internal radiators exacerbate damp issues,
  5. Think about where the tenants are going to dry clothes and make provision.
    • In Scotland there is a requirement to provide drying facilities. Most landlords put a drying rack in the bathroom.


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There is water on the ceilings in some of the rooms – here the front left room (when looking from the street).

There was a question over whether the water was rainwater penetrating through the ceiling or whether it was condensation from internal vapour.

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A thermal imaging camera, where blue is about 5˚C colder than yellow, highlights a poorly insulated ceiling.

Condensation occurs when temperature drops below the dew point. The dew point of 8°C was only just below the ceiling temperature during the survey, and above it just before.

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There is mould growing on the ceiling above the unheated staircase.

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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The thermal image picks out a lot of detail such as the slight ripple of heat loss.
Insulating effect of ceiling, as seen from below.

The secondary cause of high relative humidity is low temperature relative to the source of humidity, see surveyor.tips/humidity. 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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I had a good look at the flat roof from above. There are no signs of damage likely to cause penetrating damp.

Bitumen roofs have a lifespan of about 25 years and are especially vulnerable to ponding on the seams as can be seen above. Consider replacing the roof with EPDM adding insulation. There is no urgency.

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The metal detector demonstrates the presence of metal, likely to be steel joists (“RSJ”) supporting the roof.

Metal loses heat rapidly increasing the risk of mould, condensation and interstitial condensation (that is condensation that takes place within the building material).

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The bathroom has an extractor fan, which turns off as soon as the lights are switched off.

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:

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I measured the bathroom extractor speed with an anemometer. This speed is only 2.5 l/s, which is far too slow.

The bathroom extractor fan should have a minimum speed of 15 litres per second (l/s) with an overrun of at least 30-minutes.

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There is no kitchen extractor fan, the double glaze windows don’t have trickle vents.

There should be an externally ducted extractor fan with either 30 l/s extraction rate adjacent or above the hob; or 60 l/s if elsewhere in kitchen.

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The carbon dioxide levels were high, this suggests that windows are not being opened enough.

High carbon dioxide levels are not dangerous but makes a place feel “stuffy”. It is a sign that there is insufficient fresh air entering a property. The meter reading of 984ppm is high, roughly double the normal levels of 500ppm. CO2 meters are a good way on encouraging tenants to take care more interest in ventilation, than they might otherwise do.

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Clothes are being dried internally.

Ideally provided and externally ducted clothes dryer, or clothes rack in the bathroom with a fan running. I tend to recommend landlords install a continuous flow bathroom extractor fan such as the Elta Mori and means of drying clothes, see recommendations.

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There are some internal radiators, this creates heat imbalance that is one area much warmer than another area.

There are some internal radiators, this creates heat imbalance that is one area much warmer than another area. 

I turned the valves down on some internal radiators to reduce the heat imbalance.


Temperature difference is ~1°C between the datalogger and coldest wall. Green dash estimates surface %RH.
Temperature difference is ~1°C between the datalogger and coldest wall. Green dash estimates surface %RH.

The straight green line is the mould point line, 6 or more hours above the line represents a risk of mould. The green dashed line (the surface humidity of the coldest wall) is constantly over the mould point line. See MouldPoint.co.uk for forecast and surface %RH. 

The red dotted line is the external temperature. Note the 5°C decrease in internal temperature, between 7 – 14 Feb. when it snowed and external temperatures dropped by 5°C. This suggests that poor insulation is a key factor causing high humidity and mould. 

The external dew point is below internal dew point as there is always less vapour outside.

Relative humidity seems less effected by showering times, compared to temperature changes. I suspect this is because the tenants started closing the bathroom door and opening the window once they realised that the bathroom ventilator was not working.If you follow the recommendations, you should be able to drop the relative humidity well below the mould point line and thereby stop condensation and mould growth.

Agar gel – Mould growth

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I tested for mould spores. Growth on the medium confirms presence of significant mould growth.

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