A picture containing tub, bath, dirty Description automatically generated

104G: Recent purchase following rising damp misdiagnosis

Owner negotiated about £10K off the price because a contractor said that there was rising damp. There was no evidence of rising damp. The water table is low in this area. The root cause was condensation from insufficient ventilation.

Surveyor Tips:

  • Following Hart v Large, surveyors are delegating the risk of damp to contractors.
  • Contractors make profit from rising damp diagnosis – So what do you expect?
  • Blame the courts, but what does a judge know of damp – judges rely on experts – which experts?

Root cause

The root cause was condensation from insufficient ventilation.


A picture containing outdoor, brick, building material
Description automatically generated

The property has two types of damp proofing, here the original low permeability bricks with ventilation holes.

A picture containing building, outdoor, stone, cement
Description automatically generated

The disruption to the base of the wall is a sign of chemical damp proofing against rising damp.

A picture containing tub, bath, dirty
Description automatically generated

The primary concern is dampness in the kitchen.

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. They tend to result from daily alternating condensation and evaporation. These salts can be removed with sandpaper and decorated. Note the triangular shape. This is a sign of condensation, not rising damp, which is more horizontal.

A picture containing indoor, projector
Description automatically generated

The key issue is insufficient ventilation, the kitchen is extracting 11.8 l/s compared to the required 60 l/s..

Here the kitchen hob does not have an extractor fan. The fan is an adjacent fan, running at about ¼ of building regulation requirements. 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 P39 and P19 in https://surveyor.tips/vent_regs specifically:

  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.

Background pattern
Description automatically generated with low confidence

The secondary cause of condensation is heat loss.

The area of dampness in the kitchen is on an infill door in an internalised outbuilding. As a consequence the insulation is poor, and out of balance with the remainder of the building. In addition to improving ventilation, heat and monitoring, consider one of three solutions:

1, anti-condensation the paint, which would be my choice,

2, thermal insulating plaster board or external insulation (it would look odd),

3, thermal lining paper Sempatap or Wallrock lining paper, see https://www.youtube.com/watch?v=_R1B44fGttE, this creates a slight non porous “cavity” separating the lining paper from the wall.

A picture containing indoor
Description automatically generated

There is mould in the flat.

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 https://surveyor.tips/mould.

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

A picture containing person, wall
Description automatically generated

Using a Protimeter damp meter in radio frequency mode I was able to detect damp from about 1M down.

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 200 REL indicate that a wall is dry below the surface. These meters are for rapid scanning, mapping and profiling, see https://surveyor.tips/profiling.

A person holding a cell phone
Description automatically generated with medium confidence

The cellar walls are dry, if there was rising damp, then the lowest walls, i.e. the cellar would be damp.

A picture containing indoor, messy
Description automatically generated

While in the cellar I noticed, 1) a good size gap between the ground and suspended timber floor. 2) green timber, suggesting replacement with tanalised timber, probably after replacement following dry rot treatment.

A picture containing indoor, bed, window, wall
Description automatically generated

The master bedroom has slight dampness in the front walls and mould in the cupboard.

Both of these result from excess humidity.

Background pattern
Description automatically generated with medium confidence

Heat loss is visible through into the cupboard.

In addition to ventilation, heat balance and monitoring, consider insulation on the cold side of the cupboard.

A picture containing indoor, person
Description automatically generated

There is heat loss causing mould and condensation behind the bedroom side table.

In addition to improved ventilation, heat balance and monitoring, consider increasing the flow of air by moving the side table away from the wall.

A picture containing tree, outdoor
Description automatically generated

All the trickle vents should be kept open

A picture containing table, indoor, plant, vegetable
Description automatically generated

There are a couple of small damp looking patches on the chimney breast.

These patches could be caused by hygroscopic salts, see https://surveyor.tips/hygroscopic/ .

Hygroscopic salts act like grease stains, they are benign, cause no damage or mould, but are unsightly, especially in summer, and cause problems selling or renting a property. The best solution is to cover the salts, with an oil-based primer, such as Zinsser and a significant overlap of at least 300mm.

Monitoring relative humidity

I encourage homeowners with property showing signs of damp or mould to monitor the relative humidity, so that they can identify the source of vapour and or timing of vapour production. The data can be used to improve ventilation, increase heat balance, insulation and improve airflow through reduced clutter to see the benefits.

Description automatically generated

You sent me data back from dataloggers here are a few observations and conclusions.

Relative 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

The relative humidity readings appear to be below the mould risk zone of 85%RH. However, placement of the datalogger has a significant effect. Each temperature drop of 1°C, increases humidity by ~ 5%RH. There is around 5°C difference between the kitchen and external WC. If the internal face of the external wall was 5°C less, the relative humidity would be about 95%RH, i.e. mould risk and close to the point when condensation starts.

I estimate a 1°C difference in temperature between the datalogger and corner of the wall. The Surface Relative Humidity reflects this difference.

Dew point

Dew point is also a proxy the vapour pressure. The higher the temperature the more evaporation will take place. Comparing the dew point of one area to another makes it possible to see where vapour is being generated of conversely removed (normally through ventilation). The bathroom is consistently producing more vapour than is being vented out.

Leave a Comment

Your email address will not be published.

This site uses Akismet to reduce spam. Learn how your comment data is processed.