103C: hygroscopic salts penetrating damp through render

This Victorian end of terrace house had been damp proofed with impermeable slurry up the chimney breasts. Despite been made using minimal water, to keep pore sizes to a minimum, there is still moisture applied to the wall. Hygroscpic salst will dissolve in water becoming drawn to the new surface. It’s expensive and not as effective as an oil based primer, such as Zinsser stain block.

Surveyor Tips:

  • Properties with multiple sources of damp can be complex.
  • The simple rules always apply:
    • Discolouration is likely to be cause y penetrating damp.
    • A damp patch on or near a chimney breast is likely to result from hygroscopic salts.
    • Colourless damp by metal, door or window openings is likely to result from condensation.
    • Rising damp is rare and the risk can be assessed before undertaking a survey.

Root cause

Penetrating damp, vapour (condensation) and hygroscopic salts (although technically they are not a source, but a sub-division of condensation).


There is dampness around the front reception chimney breast.
There is dampness around the front reception chimney breast.

I understand you had a damp proofer replaster the chimney breast, yet the damp returned. This type of colourless damp patch is typical of hygroscopic salts. They are benign and not a source of moisture per se, but cause deliquescence, a form of condensation at normal levels of humility, like silica gel. See more details about hygroscopic salt later in this report.

Looking through a thermal imaging camera, we see cold areas that don’t correspond to damp patches.

Hygroscopic salts are in dynamic equilibrium. That is evaporation is matched by condensation, so there is no heat loss from drying unlike a source of water such as rain. A damp brick reduces insulation, so there can be some heat loss when the other side is cold.

You left a datalogger next to a damp patch in the reception.

I don’t believe I have received the data back. See https://surveyor.tips/datalogger-set-up/ for an example spreadsheet with formular and example graphs.

We can see the effects of hygroscopic salts on the top of the back addition.

There is no way of seeing the external side of the front reception wall. This image illustrates the location, prevalence and pattern of hygroscopic salts. The dark colour is from hygroscopic salts causing moisture in bricks, increasing adhesion to soot, see later.

By contrast the damp patch at the rear of the reception is discoloured like a tea bag stain.

The stain is a sign that water has flowed through brick or timber. There is a risk of rot in the first floor timber. I recommend that you lift the carpet in the rear bedroom and measure humidity in the void using a hygrometer probe, see later. In the unlikely event that the ceiling void is humid, open up under the floorboard. The risk of rot will recede once penetrating damp is stopped.

There is a different thermal imaging pattern – blue it’s about 5°C colder then yellow.

There is a horizontal crack in the render above the flashing.

As a general rule horizontal cracks cause more ingress the vertical cracks. Furthermore, a crack at the bottom of a wall will allow more ingress (from accumulation of rain on a long wall) than at the top. So this is most likely be the cause of dampness on the ground floor.

Looking on the outside we see a vertical crack, and a crack at the top of the lead flashing.

There has been a past attempt at filling the crack with cement. It is my opinion that an external acrylic-based caulk provides more flexibility, especially important on this the sunny side of the house and so is better than cement. But it should be filled and checked regularly.

The damp on the first-floor rear bedroom is even more even extensive.

The root cause is the same as on the ground floor, that is penetrating damp from render cracks.

Again the thermal image provides a visual illustration of where moisture is at its greatest.

Joists are probably supported by an RSJ (steel beam). However there is still a risk of rot from above.

Each and every crack on the outside of this property is likely to be contributing to internal dampness.

Rain is drawn in and runs down behind the render until absorbed by bricks. You should rake out and fill cracks; larger ones with a cement-based mortar, smaller ones with acrylic caulk.

Very fine cracks cause water to be drawn in through a similar action to that of capillaries.

Raking out involves cutting about 1 – 2mm either side of a crack, so that sufficient filler placed inside to can stop ingress. Once filled, repaint the wall with a masonry paint.

There is discolouration on the wallpaper in the loft.

This looks like penetrating damp.

I measured with a damp meter and found that the wall was perfectly dry.

This suggests that the top of the wall only becomes damp when it is exceptionally wet weather, probably with a strong wind.

The infra-red image identifies some potential cold spots.

It is possible that condensation is forming on the top floor, typically these would result in colourless watermarks. However mould can also leave wallpaper stains. 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. The secondary cause of high relative humidity is low temperature relative to the source of humidity.

Consider installing a cowl on the second chimney pot, such as ones from Wicks.

I also note that there is a crack in the flaunching ideally this should be filled, but I doubt it is the cause of any internal dampness.

Ideally the rear chimney breast should also have a cowl to stop rainwater flowing down it.

You told me about a hole in the gutter – seen here just up from bottom centre of this image.

I suggest it would be wiser to simply put a little black sealant in the hole on a dry day, rather then risk replacing the gutter and damaging the flimsy supporting clips. There was no internal damp resulting from this hole.

The kitchen extractor fan does not vent out.

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.
The first-floor bathroom extractor fan does now work,  and I understand it is not being used.

Consider installing a continuous flow extractor fan to the bathroom, see recommendations.

There is a gap in the sealant to the base of the shower.

The gap doesn’t appear to be causing internal dampness, but you should cut old sealant out and reseal it.

Some water is dribbling from underneath in the bathroom shower panel.

It may be necessary to adjust the panel, or seal up the gap with silicon on the inside.

You sent me an image of water damage under the Velux in the kitchen.

It is difficult to tell from the image whether this dampness is caused by penetrating damp, or from interstitial condensation. Condensation can form within a section of building, such as under metal, which loses heat rapidly.

The ceiling is still damp.

A damp patch takes about a month per centimetre to dry out.

It is good that your builder sealed up around the lead. 

It would also be wise to monitor and minimise humidity in the kitchen, especially when it’s cold outside.

There was mould growing around the front bedroom windows.

This is a sign of excess vapour, insufficient ventilation, insufficient airflow perhaps from closed shutters and curtains, and heating imbalance, perhaps resulting from long periods of time with the radiators off.

There is a crack to the windowsill. 

Although there appear to be no internal dampness, all cracks should be filled and flaking paint should be removed and repainted.

There were normal levels of relative humidity in the subfloor void.

The high temperature suggests the floor is being heated, and therefore there is probably and on insulated pipe in the subfloor void. You should monitor the subfloor humidity see you recommendations. You the same probe to investigate humidity in the ceiling void(s), see above.

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