103T: damp despite Dutch, English, French damp proofing

1920’s house with open fireplaces resulting in condensation and mould.

Surveyor Tips:

  • When it is cold outside a thermal imaging camera will pick up areas of heat loss.
  • When it is warm you have to explain that chimneys are made from single bricks and that therefore there will be heat loss increasing the risk of condensation when it is cold.

Root cause

Condensation through poor insulation.


Damp wall


The property has been damp proof against rising damp with both the Dutch system and English chemical damp proofing. It is also has an incomplete “French drain”. Unfortunately none of these improve internal dampness.

Damp wall

There is mould and signs of damp inside the front chimney breast.

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.

Damp wall

The damp meter reading show the wall is marginally dry.

I tested the surface of walls 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. Walls measured were largely dry on the surface except where mentioned in this report.

Damp wall

I also use the damp meter in radio-frequency mode, which is able to detect dampness up to 7cms within a wall.

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 inside the chimney was dry at depth.

Damp wall

Looking through a thermal imaging camera where blue is about 5˚C colder than yellow, we see a cold bridge. 

Chimney breasts are typically made from a single skin of bricks, with a flue in the middle, whereas most walls are made from two skins of bricks. Opening out the chimney breast increases the risk of heat loss and therefore high relative humidity.

Damp wall

There is a line of disruption to the plaster about 1.3 m above the ground in the front room.

This is the boundary line between the damp proofers impermeable slurry and the original plaster. The problem with such using slurry is that moisture absorbed into the permeable wall becomes trapped between the permeable original plaster and impermeable slurry, see example of cracking along the top of the damp’s proofers slurry, later in this report.

Damp wall

The high meter reading in radio-frequency mode around the interface highlights trapped damp .

Ventilation and heat balance will solve the problem, but it can take up to 10 months for a wall to dry out.

Damp wall

In the kitchen there are surface salts around electrical sockets.

Calcium sulphate is a key ingredient in cement and other building materials. If diluted in water salts tend to move to the surface. They tend to result from daily alternating condensation and evaporation. These salts can be removed with sandpaper and decorated. 

Damp wall

The thermal image highlights the heat loss around the electrical sockets.

Sockets are prone to dampness as a section of wall is cut out, a metal box installed and covered in absorbent plaster, all of which exacerbate the effects of condensation and absorption, see surveyor.tips/sockets.

Damp wall

Dampness is high on the wall by the kitchen table.

Using a metal detector I determined that there is a large amount of metal it in the wall. I suspect that a steel joist (“RSJ”) is supporting the rear extension causing heat loss increasing the risk of interstitial condensation.

Damp wall

Looking on the outside we see the Dutch rising damp system, which unfortunately increases heat loss.

The incomplete French drain (inspired by Mr French) is counterproductive as it keeps water close to the wall. It would be better without it or at least terminated at the rainwater drain.

Damp wall

Using the meter in radio-frequency mode I found dampness deep inside the wall above the kitchen table.

Damp wall

Looking outside we see around a render crack.

I don’t believe the render crack it’s causing penetrating damp, as if it were there would be brown discoloration like a teabag stain. However if the external skin of a wall becomes damp, it affects the thermal performance of the internal wall increasing the risk of condensation.

Damp wall

Inside the utility room there is damp all the way down the wall including rust on the windowsill.

There is metal plasterers beeding in the windowsill, which will lose heat rapidly causing rust and condensation within the wall.

Damp wall

There are salts on the surface by the solid floor.

Condensation is amplified by cold solid floors. Plaster is much more absorbent than brick and so there should be a gap in the plaster of 10 – 20mm at the base of the wall.

Damp wall

Dampness starts high up on the wall suggesting that the utility room is generating much of the vapour.

Condensing dryers typically only remove about 40% moisture from clothes. Much escapes as vapour around the dryer.

Damp wall

The kitchen hob does not have an externally ducted 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.
Damp wall

The bathroom extractor fan had no air movement.

You’ll see if you look through a majority of my reports (published online) that I am able to fix a majority of malfunctioning extractor fans. I tried to fix yours fan but it was beyond repair.

I recommend replacing it with a continuous flow extractor fan such as the Elta Mori see you recommendations.

Damp wall

There was restricted access behind the first-floor bedroom, cupboard, where I suspect there is mould growth.

Cupboards are at risk of mould growth as humid air is often trapped. Consider forcing down insulation behind the cupboard to both stop air movement and increase insolation. Ends consider placing a hygrometer probe to monitor.

Damp wall

The thermal image highlights the heat loss in corners and under eaves.

The heat loss in corners results from increased surface area. Eaves are poorly insulated compared to normal lofts and or converted lofts.

Damp wall

This image looks at the external heat gain from the centre of your first-floor fireplace.

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