102A: mould, high CO2, slow extractor, blocked gutter

This is a two bedroom Victorian ground floor flat with six person family. Damp issues became most apparent soon after double glazing was installed.

Surveyor Tips:

  • Condensation and mould is patchy, although most common at the base of a wall.
  • Condensation tends to rise into corners and cold wall in front of the door..
  • The high CO2 demonstrates how little the property is being ventilated.
  • The key issue is caused by the bathroom, with a slow extractor fan and no overrun.
  • Heat is best controlled with a modern thermostat, with multiple temperature settings.
  • There are additional gutter issues, but they don’t compared to the excess vapour.


Mould condensation
The primary concern is mould and dampness to the front bedroom wall.

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.

The wall was damp as measured with a damp meter.
The base of the wall was damp as measured with a damp meter.

I tested the 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

Damp at the base of the wall does not mean that there is rising damp, the symptom that result from groundwater. In fact rising damp is almost impossible, certainly in London where the groundwater is pumped out.

Dampness high up.
The wall was damp up almost to ceiling height.

We can see heat loss on the wall just in front of the front door, see image above.

Looking through a thermal imaging camera where blue is about 5˚C colder than yellow.

This area of dampness is on the short section of wall by the front door, see blue zone below.

Condensation was forming on the inside window at the time of the survey.

The temperature of the window was below the dew point of 13.4°C at the time of the survey.

Mould was growing up at the front bedroom coving under the ceiling.

Airflow is restricted, partially from curtains and partially because the radiators are some distance from the front bay.

The bathroom extractor fan was weak I did not stay on after lights were switched off.

My recommended solution for the condensation and mould problem is a continuous flow extractor fan. 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.
slow extractor fan
The extractor fan runs at about 4.2 l/s compared to the required 15 l/s.
There is no overrun on the extractor fan speed.

A fan running at half speed for 30 minutes, is better than a fan that meets building reg.s but cuts off after the lights are switched off. Either use a fan with a 30 over run, or continuous flow. It requires installation by an electrician so that it is switched on by the lights, but has a separate electrical connection.

bathroom condensation
There is mould and condensation across the bathroom.

The bathroom is built in a separate building with poor installation, increasing the risk of mould and condensation in this room. Insulation would improve the bathroom, but further increase the risk of transferring excess humidity to other parts of the flat. In my opinion, it is better to access some degree of condensation in the bathroom, and paint with anti-condensation paint to minimise visibility of damp and mould.


I left a hygrometer datalogger in the front bedroom by an external wall. The tenant emailed data after 6 days.

I asked the tenants to live as they normally do. The humidity meter readings in this flat are exceptionally high. It’s interesting that the first 24 hours are humid but not 100%RH. This suggests that the tenants made an effort ventilate the property before I arrived.

Carbon dioxide data logger
Carbon dioxide results from respiration, not from excess humidity. I measured the CO2 in the front bedroom.

High CO2 is not dangerous but makes a place feel “stuffy”. It is a sign that there is insufficient fresh air coming into the property. The meter reading of 1,866ppm is exceptionally high. The normal level is around 500ppm. High levels of CO2 suggests that windows are not being opened enough for the level of occupancy.

Even if the bathroom and kitchen extractor fans exceeded building regulations, they could not account for such a high level of CO2. The tenants should be encouraged to monitor CO2 and open windows when the level exceeds 800ppm.

thermal imbalance
There is no thermostat for the boiler.

The tenant tells me that they turn the heating off at night. I showed the tenants how to set up the heating so that it comes on for 15 minutes every hour at night, to keep some warmth.

steam iron
There is some condensation dampness under the second bedroom window.

I noted that clothes are dried in this room and there is a steam iron, both of which will add to atmospheric vapour.

Damp stain
There is also tea bag stain like discolouration on the side of the wall in the second bedroom.

Penetrating damp from faulty rainwater goods leaves a stain on the wall. 

rain marks
Looking outside we can see dampness falling under the gutter by the left-hand downpipe.

This is a sign that the gutter and downpipe are blocked.

The gutter over the back addition is also blocked and should be cleared.
The gutter over the back addition is also blocked and should be cleared.

The gutter over the back addition is also blocked and should be cleared.

It is possible that the s-bend in the soil pipe is causing water to drip onto the wall.

Gutter blocked
Looking at the front gutter we can see that it is full.

Note the gutter supports mean that the gutters need to be cleared at gutter level such as with a ladder (rather than a long gutter clearer).

There is no gutter on the front bay.
There is no gutter on the front bay.

There is no evidence of internal dampness resulting from the missing front bay gutter, but the lack of gutter is causing deterioration to the external paintwork.

The old coal shoot lid is open.
The old coal shoot lid is open.

It would be wise to seal up these openings.

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