103S: hygroscopic salts two floors up

Victorian house converted into flats – one bed with hygroscopic salts on a second floor chimney breast.

Surveyor Tips:

  • Look everywhere and speak to your buyers about their concerns,
  • I might have missed these unusually placed damp patch had I not understood the buyers concerns.

Root cause

Hygroscopic salts and condensation.


Damp wall

You were concerned that damp on the bedroom chimney breast was caused by a leaking roof.

Somebody has tried to block the damp on the chimney breast by painting with a stain block. Although in principle this is the right approach, the result is poor. It would have been better without wood chip paper. The stain is caused by hygroscopic salt, see later in this report.

Damp wall

A continuation of the same damp stain is visible on the other side of the chimney breast in the bathroom.

Hygroscopic salts re not a source of water, but do cause a condensation like damp patch at normal levels of relative humidity.

Damp wall

With a damp meter I determine that the discolouration was indeed damp, not just poor paint work.

I tested a sample of internal and external wall surfaces 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.

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Just outside the damp looking area the chimney breast is completely dry.

The damp patch on the chimney is isolated and distinctively caused by hygroscopic salts. These salts are released when wood or similar is burnt. The most common hygroscopic salt is calcium nitrate which causes condensation or deliquescence at about 50% RH, i.e. typical levels of relative humidity. The best way to treat hygroscopic salts is to cover them in an oil-based primer, see recommendations.

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The damp patch in the bathroom is similarly from hygroscopic salts.

Penetrating damp, such as from rainwater seeping through a roof or the bricks in a chimney stack, brings with it colour, like a teabag stain. I find hygroscopic salts in roughly 20% – 30% of the damp surveys I conduct of Victorian properties.

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I believe it is a chimney breast and that the chimney stack has been removed above.

Looking at Google maps and at the similarly designed neighbouring property, there appears to have been a chimney stack removed from above your bedroom/bathroom. There is therefore an opportunity to remove the chimney to floor level and enlarge the bedroom. I have removed the chimney breast of property at a cost of about £500 but could be more. You should check with a structural engineer, flat above, builder and the freeholder. The flat below may wish to do the same and share costs.

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I had a look on the outside and could see no signs of the chimney stack.

I also had a look at the guttering which seem to be OK, as were the tiles. (This is an observation, not a roofing survey).

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The exterior looks like it needs cracks filling and repainting.

Typically freehold properties split the cost of repair and redecoration to common parts evenly between flats. Ideally the work should be done about every seven or so years. You should ask your solicitor for details of when it was last done, the costs and if there are any plans to update, remediate or otherwise improve the building.

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The sign of damp I thought was concerning you most, was the extensive mould throughout 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 surveyor.tips/mould.

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Mould is often found on the walls suffering from condensation.

The cupboard walls were dry at the time of the survey.

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A bag was left out with mould across it.

It is clear that the former occupant struggled with mould, condenation, ventilation, and heating.

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Using a thermal imaging camera were blue is about 5° C colder than yellow, we see heat loss in the cupboard.

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

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The only form of mechanical ventilation in the flat comes from a fan ducted through the reception.

I tested the extractor rate it is running at about 20% of the building regulations requirements so you should replace it. There is a gas heating system. These open flame systems which generate vapour as a biproduct of combustion. 

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The kitchen is internal and open to the reception.

This will render the extractor fan on the wall, insufficient, even if it was powerful enough.

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The bathroom has no ventilation only a trickle vent that was closed.

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.
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The corner of the toilet has two external walls at a right angle, and so will lose heat rapidly.

The surface of the wall was damp.

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There is mould growing around the toilet.

The best solution is improved ventilation but also consider insulation, such as Sempatap thermal solid wall insulation, a form of insulated lining paper, check product details.

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There are damp stains underneath the windowsills.

Typically penetrating damp is more discoloured. So it’s difficult to know how much water is coming from the outside, how much is mould and how much is condensation.

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Using a metal detector I was able to determine that there is metal in the wall.

Metal loses heat rapidly increasing the risk of condensation.

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The damp meter was high underneath both windowsills.

Damp metres don’t determine the cause of dampness, but given the property is vacant and therefore unlikely to suffer from condensation from currently generated vapour, this suggests that there is some rain water penetrating.

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I looked under a plant pot to see if there are any cracks.

There appeared to be no cracks to the windowsills.

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There is a slight crack around the double glazing. 

Cracks around windowsills need to be regularly checked and updated with and external sealant. This is probably a shared cost. However, given that it’s relatively easy to reach from your windowsill, it’s probably worth getting a builder to do this for you when redecorating.

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The junction between the render and windowsills is cracked and could be the root cause of penetrating damp.

You should fill large render cracks with a cement-based mortar, and small cracks with an external acrylic caulk.

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There are lots of cracks throughout the property this should be a shared cost.

Other matters

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There is a very strange boiler and insolation in the shared stairway on the floor above.

You should ask your solicitor to find out from there freeholder via the vendor, whose boiler is, why it is there and whether there is a concern, such as fire safety considerations.

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I note the ground floor has been damp proofer despite the property being on a hill.

This suggests past mis- diagnosis of rising damp. Make sure that the freeholder does not go back down the route of treating for non-existent rising damp.

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