103E: Continuous flow extractor, blocked causing mould

This Victorian ground floor flat is on a hill, yet the owners were advised to have chemical damp proofing against rising damp cost about £10,000 that would have also been very disruptive and not addressed the root cause of damp.

Surveyor Tips:

  • Often the simplest problems and therefore solutions are the most effective.
  • Here the bathroom continuous flow extractor fan was not working effectively.
  • Owners were keeping the bathroom door open.
  • There was no extractor fan in the kitchen.
  • The presents of a line of drill holes at the base of a wall is a tell tale sign that a property has been damp proofed.
  • If it didn’t work first time, why would anyone expect a different result from a second round of chemical damp proofing.

Root cause

Excess vapour from a defective continuous flow bathroom extractor fan.


Lines of injection hole covers shows at least two attempts at damp-proofing treatment against rising damp.

Despite the low risk of rising damp, there has been an attempt at chemical treatment against rising damp. Rising damp treatment is normally a sign of insufficient ventilation. The walls have been drilled into by about 200mm and injected with damp proofing chemicals. In theory the chemicals are absorbed into the brick pores reducing the bricks ability to absorb moisture. Walls are replaced with damp proofers slurry. This can cause problems with absorption imbalance, resulting in increased condensation on other walls or vapour becoming absorbed and trapped behind the slurry. There was evidence of surface condensation in areas probably not treated with replacement plaster but no evidence of trapped vapour. The solution is through ventilation and humidity control.

The primary concern is mould and dampness in the bedroom.

Mould grows where relative humidity exceeds 85%RH for 6+ hours. Excess humidity results from insufficient ventilation, poor airflow and cold surfaces. See surveyor.tips/mould.

The curtain is trapping humid air against the cold wall.

Mould is inhibited by nitrates found in groundwater. The presence of mould at the bottom of a wall eliminates rising damp as the root cause and points toward condensation from unvented excess vapour.

The thermal image where blue is about 5° colder than yellow highlights the heat loss.

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

I measured damp in the wall by the back door. Openings lose heat rapidly at night increasing the risk of damp.

I tested the surface on the inside at the base of all external walls every metre, chimney breasts and a sample of internal walls with a Protimeter damp meter in conductance mode. These meters measure electrical conductance of salts in water, a proxy for damp.

There are salts showing in the party wall.

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. The salts are most commonly associated with condensation.

There is similar damp by the kitchen door.

I also tested walls in radio frequency mode. 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, 999 REL is the limit. These meters are for scanning, mapping and profiling, see surveyor.tips/profile. 

The primary source of damp comes from the bathroom.

I opened up the bathroom  extractor fan, cleaned it and moved the “jumper” see image,  increasing the speed by a factor of about four times.  It now provides sufficient ventilation. 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.
The extractor fan now provides 20.8l/s.

I recommend running the fan continuously, at least during cold spells and until the humidity has dropped in your bedroom. You should also keep the door closed. The thermal cost of a continous fan is about £50 – £100 p.a. see surveyor.tips/cost-of-running-extractor-fans/.

The kitchen extractor is not ducted out. It is worth increasing use of the bathroom fan.

There is damp to the right of the chimney breast.

Hearths are made from bricks in the sub-floor so are at risk of absorbing moisture from the subfloor void.

The dampness in the wall is marginal, I suspect it’s caused by humidity in the subfloor void condensing on the damp proof course, made worse by high internal humidity. Improving internal ventilation should also stop damp appearing on the chimney breast.

The cellar or subfloor void has been tanked with slurry. However, I was able to measure dampness in exposed bricks.

The slurry could cause water to bridge the damp proof course. However, I doubt this is having an impact. Some bricks in the cellar were dry, which they would not be if there was rising damp.

I measured the humidity about 1M in to the subfloor void, it was below 85%RH.

The humidity in the sub can vary with external conditions. Moisture could condense on the DPC. I recommend monitoring the sub-floor humidity with a hygrometer probe (eBay £5). 

In the front reception there is a high damp meter reading in radio-frequency mode.

The dampness was higher under the windowsill then at floor level.

Looking immediately outside we can see the cracks in the render which should be filled.

There were cracks in the wall that should be raked out and filled with an external acrylic caulk. Also, the flanking timber paint could result in Phellinus contiguus or window rot. Window rot is common in old properties, it is a slow growing white rot. None of the timbers are structurally significant. The rot can be cut out, sanded, filled and painted for protection.


There is little to learn from the trace other than humidity is consistently at about 100%RH.

This will give you a good base line to work from, to see improvements. See https://surveyor.tips/datalogger-set-up/ for examples.

I note the last reading is up to the day after I visited. Damp walls will contribute to humidity through evaporation, so it takes time to bring relative humidity down.

£10,000 saving

I’ve been told that since the survey repairs and recommendations the humidity has dropped to normal levels, solving the damp and mould problem saving £10,000 on ineffective chemical damp proofing.

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