Penetrating damp from paint

Example from damp survey June 2022

Background

This Victorian London property was dry when it was purchased about 14 years ago. Dampness started to show over the years. It was assumed that the roof was the cause of penetrating damp. The roof was replaced.

Table of Contents

Evidence and explanations

Damp wall of primary concern

There is staining and dampness on the top floor wall starting from the top.
However, the staining is centred on the lower section of wall in the corner.

The brown discolouration like a teabag stain is a sign of water flowing through building material. In this case it is caused by rainwater and referred to as penetrating damp. Interestingly, there were no reported damp issues when you bought 14 years ago. Stains started to show on this wall, so you had the roof replaced. Other issues have followed the roof replacement.

Most of this wall & chimney breast was damp, 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 surveyor.tips/profile.

Primary cause of damp

Immediately outside there are salt crystals blistering between the bricks and paint.

The blistering is worse immediately below the rendered parapet. Any rain forming on the parapet will accumulate increasing the concentration of water onto the cracks below and risk of absorption.

Though less visible, there were isolated patches of dampness on the chimney breast immediately below.
The chimney breast sits above an open entrance to a coach house. The painted wall has brown stains.
These walls have been treated for rising damp using the Dutch method of aerating inside bricks.

Most damp proofing treatment involves replacing normal absorbent plaster with impermeable slurry, to block or hide damp rather than deal with the root cause. The unabated moisture risks rot and anyway will eventually break through impermeable slurry.

The wall behind the Hydrangea bush is severely blistered.
Evidence suggests a permeable or semi permeable paint applied to an impermeable but cracked coat.

When it comes to coatings, it is important to have consistency, ideally the paint should all be permeable, like the limewashes that used to be applied many years ago. However, unless the old paint is completely removed, all but a few speaks on each brick, then the top layer needs to be at least as impermeable as the layer below it. Otherwise, water can be absorbed through the permeable layer, which could be hiding gaps in the layer below.

Damp stains line up with wall that juts out, it is covered in cement and angled out, but there may be cracks.
Understanding how internal damp lines up with external features is an important part of damp detection.

Features to note: 1) The blue arrows point out increased soot deposits. Long term dampness increases pollution trapped. Soot was common in Victorian times. Bricks have been cleaned relative to the neighbour. 2) This property has a London roof with a central gutter (the roof is shaded blue). 3) The red arrow highlights a wire, in line with a patch of internal dampness. 4) The green arrow highlights a plant on your neighbour’s side, suggesting that they also have a damp issue that could have an impact on your property, although it would be more apparent in your bathroom.

The internal dampness highlighted with the red arrow above is stained brown.

The internal dampness patch is to the right of the central gutter in the London roof.

The flashing immediately above the damp patch looks old in comparison to the new flashing.

It is noticeable that the video the roofers kindly provided you is taken in such a way that this section of flashing is the only part of the roof that can’t been seen. Is it possible that the roofers kept the old flashing on the roof to reduce their costs? In my opinion damaged flashing is the most likely cause of dampness to the front of your building. However, I did not inspect it from above, nor pour water and or dye from above.

The dampness lines up with a wire, which may be redundant, section of render and wall that bulges out.
Bathroom paint may be hiding dampness, so the wall behind may be as damp as the front bedroom wall.

From evidence, I would say the most likely cause of dampness results from insufficient ventilation causing condensation.

Excess vapour caused by insufficient ventilation

The bathroom extractor ran at 4.5 l/s with an overrun of ~1 min. Building regs require 15 l/s for 30 mins.

We extended the timer to 30 minutes (see arrow to adjust timer) and cleaned up the fan to increase the extraction rate to 10.6 l/s which is now better than most UK bathroom fans. The kitchen has manually activated, wall mounted bathroom fan with a speed of 19 l/s. As a wall mounted fan it should be 60 l/s. However, an externally ducted hood would be better.

Excess vapour causing: Mould

There is mould growing in the bedroom corner behind bookshelves.

Mould grows where relative humidity exceeds 85%RH for 6+ hours. Excessive humidity results from insufficient ventilation, poor air circulation and a cold surface. There are many risk factors;

  1. A vapour source (local from penetrating damp and general from the poorly ventilated bathroom and kitchen).
  2. A cold external wall, relative humidity is a function of both vapour pressure and temperature.
  3. 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.

Hygroscopic salts

There are damp patches on the rear wall. They look like hygroscopic salts. Stains are from penetrating damp.
There are damp patches on the rear wall. They look like hygroscopic salts. Stains are from penetrating damp.

The stains look like hygroscopic salts, with signs of penetrating damp. Calcium nitrate is the most common form of hygroscopic salt in residential properties. They are often found on chimney breasts and cause deliquescence, a form of condensation, at normal levels of relative humidity, above about 50%RH.

Another source of penetrating damp: a blocked hopper

Outside is a streak of hygroscopic salts like damp (blue arrow) & plant growth by a blocked hopper (red).

In my opinion, the blocked hopper has dampened the chimney releasing hygroscopic salts

The hopper was blocked, see water in left before I cleared leaves. Note cement (red arrow) needs to be removed.
The hopper was blocked, see water in left image, before I cleared leaves.
Note cement (red arrow) in right image, needs to be removed.

Another source of penetrating damp: an upward sloping pipe

Immediately outside and above, there is an overflow pipe from a disconnected cold-water tank.
Immediately outside and above, there is an overflow pipe from a disconnected cold-water tank.

The pipe points up away from the house, any rain collecting on the end of the pipe will move to the lowest point, which is on the inside skin of wall, just above the damp patch.

In the loft a pipe stretches from the tank out. It has been removed from the top of the tank, causing it to pitch up.
A pipe stretches from the tank. It has been removed from the top of the tank, causing it to pitch up.

The pipe has no function. I pulled it out of the wall. You naturally asked if the hole could cause ingress. I don’t think so, but there is a small risk. The issue was the pipe concentrated rain in one small spot. Whereas the opening will only marginally change the natural pattern of absorption of rain and evaporation. It will provide additional ventilation in the loft.

Other matters

– none of which were damp at the time of the survey

There is a large damp stain on the bedroom ceiling to the right of the central gutter. The ceiling was dry.
There is a large damp stain on the bedroom ceiling to the right of the central gutter. The ceiling was dry.

I was unable to access the ceiling from immediately above, in the loft above.

The sarking material elsewhere was dry, but showing signs of staining.
The sarking material elsewhere was dry, but showing signs of staining.

It is difficult to be sure of the cause of the stain on the ceiling or sarking, if they are related or whether ingress will return, but it seems unlikely. The stain could have occurred before or during the roof replacement. I understand that no vents were installed into the new roof, until you pointed this out to the roofers, who returned to install vents. It is possible that there was some interstitial condensation before vents were installed, that is within a wall or void, in this case below the roof tiles. And that the ventilation has solved the problem.

There is a sealant around this window frame that appears to have a gap. It was not causing internal dampness.
There is sealant around this window frame that appears to have a gap. It was not causing internal dampness.
There is a slight render crack around the window frame. It was not causing internal dampness.
There is a slight render crack around the window frame. It was not causing internal dampness.

Data loggers

data logger
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.
  2. The difference between the solid green line and dotted line is calculated considering the temperature difference between the datalogger and affected wall during the survey. This is approximate and illustrative. Each -1°C increases relative humidity by ~ 5%RH.
  3. The relative humidity of the bathroom is shown as a dashed line, see later.
  4. The thick, transparent pink line is the outside air temperature.
  5. The solid and dashed red lines are for the affected wall and bathroom temperatures.They are relatively even and similar to the average outside temperature and suggestheat loss (poor insulation) is not the primary cause of mould and dampness on the wall.
  6. The solid and dashed blue lines are for dew point, which amongst other things is a measure of quantity of vapour in the air, see below. It shows when and how muchvapour is being generated. It suggests vapour is not coming from the bathroom.
  7. The thick, transparent light blue line is the external dew point. In a well-ventilated property, you would expect internal dew points to align with external dew points,especially when people open windows in summer, which is the case here.
  8. The grey line in the graph is called the DP diff. or dew point differential below 0°C meansvapour is coming from the bathroom. The line shows that vapour is mainly being generated in the bedroom. There times of high bathroom dew points, but these don’t appear to result in increased bedroom dew points. See daily condensation & mould risk @ mouldpoint.co.uk, and surface %RH calculator.

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 penetrating damp.

Following my recommendations to drive down relative humidity and reduce the risk of condensation and mould forming. Keep monitoring the bedroom.

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