Mould & condensation June

Example from damp survey June 2022


Victorian rental garden flat. Walls have been damp proofed against rising damp.

Mould has been increasingly growing across the kitchen rear wall and condensation.

Take away

The ventilation is manual and damaged during a previous tenancy.

Costs, quotes & disruption that could have been saved

An estimated total £5,000.
Actual cost of repairs approximately £600.

Table of Contents

Evidence and explanations

Damp wall of primary concern

The primary concern is dampness and mould in the kitchen.

There are signs of condensation dribble marks on the cooker side of the kitchen, along the party wall.

The wall was dry, as determined by a Protimeter radio frequency damp meter.

The damp meter’s measurement limit is 999REL, below 300REL is consider dry. 999REL indicates water absorbed at depth. High readings illustrate the extent and profile of dampness, and are used to help pinpoint the source of moisture

Mould is growing along the cold external section of wall, that is in the internalised outbuilding.

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
The secondary cause of high relative humidity is low temperature relative to the source of humidity, see

The wall is damp, but not very damp, unlike the dampness seen later in this report.
The rear section of the kitchen would have been an outside WC or shed, with three external walls.

Being an outside shed, it is unlikely that the walls were built with the same level of insulation as the main part of the house. The rear wall and righthand walls are dry lined, that is the internal wall is made of plasterboard separated from the external wall by batons or adhesive, this normally provides some insulation. Thermal insulating plasterboard would be better.

The lefthand wall is not dry lined and about 4.5°C colder than ambient temperature.

Every drop of 1°C increases surface relative humidity by about 5%RH, pushing it into the zone of increased mould risk. The relative humidity on the data logger was 60% for 24.1°C, this gives a calculated the surface relative humidity of 80%RH see

The thermal lens, where blue is about 5 ̊C colder than yellow, highlights heat loss.
Dots at the wall base show the wall has been chemically damp proofed against rising damp.

Damp proofing treatment involves replacing normal absorbent plaster with impermeable slurry, to block or hide damp rather than deal with the root cause. The consequence is there is an increased risk of mould at the base of the wall and condensation forming along the interface between the old absorbent plaster and new impermeable damp proofers slurry.

The small section of wall by the rear door is damp.

The door surround is cold as there is less wall to insulate. Heat loss risks condensation. Water is absorbed by walls. Damp walls are poor insulators. Therefore, condensation is self- perpetuating resulting in damp patches such as those on your wall. Gravity draws moisture down. The ground is cold, increasing the risk of condensation at the base of a wall.

Mould is growing on the opposite side of the doorway.

Mould and condensation form in similar environments, typically mould will grow where humidity stays high and stable for long periods of time. Condensation forming on mould reduces oxygen intake. It is therefore common to have either mould or condensation.


Ventilation will become more important as peopple reduce airflow to minimise energy loss.

There are many views about the best form of ventilation, mainly dominated by manufacturers. A factor consistently missed is that there is a thermal cost to excessive ventilation. In my experience you should:

  1. target the source of vapour with a mechanical vent,
  2. keep the vent running long enough, but not too long,
  3. contain vapour such as with closed doors and extract it efficiently, with minimal risk of leakage,
  4. use long lasting rigid ducts and provide access to inspect or replace,
  5. monitor humidity,
  6. check a vent’s performance annually (clean or replace),
  7. if you have humid or cold zones, such as kitchen or clothes drying, use large dehumidifiers (monitor & decant daily),
  8. PIV (Positive Input Ventilation) are untargeted and therefore considered inefficient by independent experts.
  9. modern houses or large refits often benefit from well designed and installed MVHR (Mechanical Ventilation with Heat Recovery). However, MVHR can waste heat, has risks and is often better as part of a hybrid solution.
The kitchen extractor fan is not externally ducted.

You should provide an externally ducted extractor fan and tell the tenants to always use it whenever cooking.

Culinary styles can dramatically affect the amount of vapour released from cooking.

A pressure cooker superheats water to about 120°C increasing the risk of condensation.

Return after specialist leak detector failed to find leak

The bathroom extractor fan is manual, with a pull cord which was missing, now fixed. It only runs at 8.2 l/s.

I replaced the missing cord so that vapour can be extracted. However, manual fans rely on tenants’ recall and cooperation. Building Regs require fans to extract at 15 l/s. It would be better to a replace the fan with either a light switch operated fan or continous flow fan, see recommendations.

There was a small leak on the stopcock. I don’t recall this leak.
Water was found dripping onto a magazine.

I repaired and stopped this leak by tightening the fitting.

Excess vapour causing: Mould

I checked all nains connections for leaks: taps, shower head, cistern and behind the washing machine.

During my original survey, I checked the assessable mains connections and offered to check the washing machine for leaks. However, your farther was reluctant for me to potentially damage the kitchen when it did not appear to be the central point. My PI insurance is as a surveyor, not repairman. This limits the amount of damage I can risk. On occasions, the only way to positively identify a source of water is by pulling things apart.

I removed the kicker plate, which was swollen and damaged from moisture, and unscrewed a piece of kitchen unit to the right of the washing machine to release it (blue arrow).A vapour source (local from penetrating damp and general from the poorly ventilated bathroom and kitchen). A cold external wall, relative humidity is a function of both vapour pressure and temperature. Reduced airflow such as behind books. Vapour moves through diffusion, like smoke, from high to low concentrations, furniture and other objects reduce the flow of heat from warm air currents.

White rot was growing immediately under the kicker plate, a sign that there is an ongoing source of water.Rot

White rot was growing immediately under the kicker plate, a sign that there is an ongoing source of water.

Cause of the leak

The leak was coming from the hot water pipe, upstream from the dishwasher inlet.

I tried tightening the red T shaped fitting and think I reduced the leak, but couldn’t eliminate it, so suspect that there is a defect with the fitting or pipe, and that it should be tested and replaced by a competent plumber. The mains water should be rechecked for pressure loss over 20 minutes, after replacement to ensure there is no ongoing or other leak.

Looking behind the unit, under the sink, we can see that the wall is exceptionally damp.

You asked:

  1. Why did water spread across to all four corners of the flat and appear to be centred in the corridor.
  2. How do we know if this leak is the cause of the all the dampness.

Answer in order:

  1. Past damp proofing treatment has obscured the effects of the leak, as has the kitchen. Looking behind units we see considerably more water than elsewhere. The damp is being transfer between an impermeable concrete screed and impermeable plastic underlay.
  2. I can’t promise that this leak and condensation are the only sources of moisture, but the profile fits with a leak. It is bad luck to a have leak, by exceptional to have two or more.

Data loggers

This graph comes from your data loggers, the property was vacant part of the time. Numbers detailed below.

Understanding the data logger diagram above

  1. During the period under investigation the surface relative humidity on the damp wall (green dotted line at the top of the graph) is at normal levels suggesting atmospheric is not the primary cause and not a concern, at least at this time of year. The property was vacant.

Conclusion from the data loggers

Vapour does not appear to come from the bathroom (albeit the property was vacant for part of the time). However, the data backs up my opinion that dampness is mainly a leak.

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