102P: Hygroscopic salt – calcium chloride – first floor

A RICS surveyor misdiagnosed hygroscopic salts as a roofing or guttering defect. Here is a first floor rear one bed flat in a Victorian house, with a chimney removed – almost certainly because of a previously misdiagnosed chimney issue.

Root causes

The root cause was hygroscopic salts causing condensation at normal levels of humidity.

Surveyor Tips

  1. The second most common hygroscopic salt in properties, after calcium nitrate, is calcium chloride, which doesn’t test positive for nitrates.
  2. Use experience and common sense to identify it.
  3. You will be amazed how easy it is to find very old photos of the same property 10 or so years ago, to compare a wall to.
  4. Hygroscopic salts is a sub-category of vapour as they effectively condense vapour and are in dynamic equilibrium, so don’t add water over the medium term. They are benign, can’t cause damage or mould, but unsightly and often mistake for something very costly to repair or replace.
  5. Beware misdiagnosing chimney defects and recommending costly remediation, such as chimney removal, flaunching, cowls, re-roofing as more often than not, colourless dampness on chimney breasts is caused by hygroscopic salts.
  6. This month 5 (~20%) of damp surveys were of properties with hygroscopic salts on or near chimney breasts, 2 were in basements, 2 in top floors flats, 1 house – it is often greater in summertime, when the risk of standard condensation is less.


The main concern was dampness evident internally on a chimney breast in the reception.

The damp patch is colourless. If the dampness was caused by rainwater penetrating through from the outside, then there would be discolouration like that of a teabag stain.

The damp meter reading showed surface moisture.

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. Readings below 20WME are considered dry. See surveyor.tips/dampmeter. Walls measured were largely dry on the surface except where mentioned in this report.

I used the meter in radio-frequency mode to profile the dampness in the wall.

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 dampness stops abruptly on the external wall.

This profile of dampness is not consistent with rainwater penetrating from a leaking gutter or roof.

Dampness stretches across the chimney breast to the partition wall, separating the reception from the kitchen.

I tested for dampness on the other side of this chimney breast in the kitchen but found no evidence of dampness there.

The initial thoughts were that the dampness must come from simple condensation.

However, I measured the temperature of the chimney first and found it to be warmer than the external wall.

I inspected the external brickwork under the gutter. There were no signs of leaking from the gutter.

When gutters leak, they leave tell-tale green, brown or white marks on the external side of walls. There was no discolouration of the wall.

Unlike the right-hand neighbouring property, the chimney stack has been removed with the roof laid over.

It is common for hygroscopic salts to be misdiagnosed as roofing all guttering problem, and thousands of pounds is spent needlessly on removing chimneys and reroofing.

An image from the internet dated June 2013, shows slight dampness on the chimney breasts.

Zoom in to see visible dampness, consistent with the chimney breast in its current state – despite it being, summer 8 years ago.

Having established that the likely cause was hygroscopic salts, I tested for the most common, calcium nitrate.

Calcium nitrate is the most common hygroscopic salt found in properties. I took a sample of the surface salts and tested for nitrates, but the test came back negative. This leads me to believe that the hygroscopic salt is calcium chloride, which causes deliquescence (a form of condensation) at about 25%RH. The simplest solution is to cover the salts with an oil-based primer such as made by Zinsser, see you later in this report.


The root cause of dampness is from hygroscopic salts. They would cause dampness whether or not the ventilation was good. However, you should improve the flat’s ventilation.

There is no externally ducted extractor fan in the bathroom.

Ventilation is most effective when air is extracted close to the vapour source; bathroom, kitchen, drying clothes and occupied rooms. Modern showering habits require modern ventilation systems.

There is no externally ducted extractor fan in the kitchen.

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.

Other matters

Some of the timber is affected by Phellinus contiguus or window rot.

Window rot is very 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. In addition to internal timber, external timbers have early signs of window rot.

There are some rust marks underneath the bedroom lintel.

This is more decorative spoiling, but could ultimately result in significant rust damage. Ideally sand down and repaint the lintel with gloss paint.

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