102R: cavity membrane lower ground floor

The owner of a vacant property was concern about unusual damp spots, despite an extensive damp proofing treatment with a structural waterproofing – cavity membrane system.

Root causes

The root causes were penetrating damp rain, condensation and interstitial condensation.

Surveyor tips

  1. This was a good example where I only found one of the root causes by further investigating after I had given keys back. If there is something that is troubling you, or doesn’t quite add up, be prepared to return.
  2. This property had been empty for a while, with cavity membrane on virtually every wall. However, condensation could still form along the steel joist (“RSJ”) and normally absorbent surfaces where the membrane had not been applied.
  3. To some degree this demonstrates the weakness or vulnerability of damp proofing compared to addressing root causes.
  4. I have seen examples where the whole lower ground floor has “tanked”, and dampness formed on the ridge at the top of the damp proofing between floors.


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The primary concern is dampness to the reception.

The property has been damp proofed, with a membrane system and dry lining. The wall has a hollow sound when tapped, demonstrating that the surface is not in contact with a brick wall, therefore dampness on such walls does not come through the brickwork. The front has a semi-circular damp patch. The point of ingress is above or below the windowsill.

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Looking immediately outside we see that the window is partially open to rain.

The shape and location of the damp patch is consistent with rainwater flowing in. It is not consistent with rising damp. It could also be consistent with condensation if there was a thermal bridge or if the membrane was “sweating” in the centre of the damp patch. I could not detect a thermal bridge and normally sweat flows down. For the above reasons I believe the issue is a simple case of rainwater coming through the open window.

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Looking immediately outside we see that the window is partially open to rain.

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.

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All the other cavity walls, where dry on the surface.

A hole has been drilled into one of the walls, possibly to establish the type of damp proof membrane.

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The membrane can be seen behind the boiler in the under stair storage area.

The walls behind the damp proof membrane are damp. I see hundreds of under stair cupboards. There is always some dampness, whether penetrating or condensation. These units were built for storage of coal, not for storage of perishable items.  

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Many of the walls in the storage area at the base of the wall have a marginally high damp meter reading.

The problem with using a damp proof membrane, or indeed the damp proofer’s impermeable slurry is it reduces the natural buffering effect of normal plaster, through absorption of vapour. This result in an increased risk of condensation in untreated areas.

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A good example is the condensation that was visible own the radiator in letting outlets.
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Looking below the bathroom radiator you can see to drip marks, and condensation along the line of grout.

Other matters

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There is visible dampness to the top of the door, to the under stair storage area about 2.5M off the ground.

This form of dampness could not possibly be from groundwater, the source of water and rising damp, which has only ever been recorded as rising a few courses of bricks up a wall.

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I can’t see any gaps between tiles above, but it is possible. Therefore there is a small risk of penetrating damp.

However, penetrating damp is normally accompanied by a teabag like stain, as colour leaches through brick. The damp on the internal wall did not have a tea bag like stain.

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There is a steel column and joist “RSJ”, supporting the entrance wall above the kitchen.

RSJs are prone to heat loss as heat is drawn out along the metal when it is cold outside. This can result in interstitial condensation, that is condensation within the building material.

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There was a high damp meter reading under RSJ.

This damp meter reading is not consistent with rising damp, penetrating damp or a leak but is consistent with condensation.

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I used a damp meter and radio-frequency mode – it is low at the top of 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. 

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The damp meter reading is considerably higher further down the wall.

This supports my proposition that the source of moisture it’s unlikely to be rainwater from the step above. The profile is consistent with condensation.

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The issue is; vapour in or evaporated rainwater entering the property and insufficient ventilation and heat.

There is no effective heating system. With your permission I tried to restart the boiler, but I could not get it working. I am not expert in this field.

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It’s not an issue for a vacant property, but there was a no kitchen or effective bathroom extractor fan.

I improved the bathroom fan speed from 0 to 3.6 l/s It should be replaced. Also the flexible ducting was blocked stopping air flow, it should be replaced with ridged ducting. 
I recommend installing a continuous flow fan such as and Elta Mori. 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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There is what looks like a damp stain above the kitchen worktop.
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However the damp meter reading was low.

I can only assume that this was an old accidental water spillage or similar.

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Similarly there is a damp patch on a chimney breast, it was dry at the time of the survey.

Old buildings are prone to hygroscopic salts on or near chimney breasts, originating from the historic burning of coal. They are not a source of moisture nor a sign of rising damp but can cause condensation at normal levels of relative humidity, especially in summertime. In the unlikely event that these appear, paint two coats of a solvent based stain block like Zinsser Cover stain, with a 300mm overlap. Then repaint with emulsion for minimal disruption or cost.

Other matters

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The external drain to the front of the building was partially blocked.

I removed the plastic and leaves, to reduce the blockage. There is no sign that this is causing internal dampness.

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There is a crack to the windowsill.

Cracks should be raked out and filled with an external acrylic based caulk or cement render. This maybe a freeholder responsibility, with cost shared. However, from a practical perspective it may be better to rake out and fill cracks without concerning the freeholder or other leaseholders. 

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There are a few render cracks, which can cause ingress.

I don’t think that these cracks are the root causes of the internal dampness, but I can’t completely exclude them.

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Water seems to be falling from the gutter onto the windowsill of the flat above.

Again it didn’t seem to be causing internal dampness. You should check and film the rainwater goods every six months during a rainstorm.

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The windowsill of the property above does not have a good trip guard.

Rainwater could possibly roll back onto the wall, which is fine so long as there isn’t a crack in the render. It is easy to use an angle grinder cut a drip guard into the windowsill. Typically a general builder with charge £300 for a day’s work, which should cover all the other matters.

Rising damp risk assessment

Elevation is: 51M above sea level.                 The flood risk is: no risk.

Sub-soil rocks are: Rocks with essentially no groundwater.
Signs of groundwater: there are no signs of groundwater.

Therefore the risk of rising damp is a remote possibility, see a good explanation by Dr Robyn Pender of Historic England  https://youtu.be/Jo8oF9ubvtI

Despite the low risk of rising damp, there has been an attempt at treatment against rising damp. Rising damp treatment is normally a sign of insufficient ventilation. The walls have a damp proof membrane system, with dry lined plasterboard. This can cause problems with absorption imbalance, resulting in increased condensation on other walls. There was evidence of surface condensation in areas  not treated. The solution is through humidity control such as ventilation and heat balance.

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