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
- 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.
- 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.
- To some degree this demonstrates the weakness or vulnerability of damp proofing compared to addressing root causes.
- 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.
Illustrations
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.
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.
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.
A hole has been drilled into one of the walls, possibly to establish the type of damp proof membrane.
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.
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.
Other matters
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.
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.
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.
This damp meter reading is not consistent with rising damp, penetrating damp or a leak but is consistent with condensation.
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.
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.
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.
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:
- Bathroom 15 l/s with a 30-minute overrun.
- Kitchen 30 l/s adjacent to hob; or 60 l/s elsewhere in kitchen.
I can only assume that this was an old accidental water spillage or similar.
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
I removed the plastic and leaves, to reduce the blockage. There is no sign that this is causing internal dampness.
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.
I don’t think that these cracks are the root causes of the internal dampness, but I can’t completely exclude them.
Again it didn’t seem to be causing internal dampness. You should check and film the rainwater goods every six months during a rainstorm.
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.