- Here are two neighbouring ground floor flats in a Victorian terrace.
- Both have been damp proofed.
- Both have dampness rising damp like symptoms at the base of the walls.
- Both have recently been diagnosed with rising damp.
At least £10,000 saving by identifying the root cause of damp, instead of a second round of damp proofing from London’s “leading damp proofing” Plc, which is typically only a short term solution.
Rising damp like symptoms are not a sign of rising damp.There has to be ground-water below to cause rising damp. The following also cause rising damp like symptoms:
- penetrating damp at the base of a wall,
- a leak at the base of a wall,
- hygroscopic salts possibly coming from a chimney breast,
- mains-water or central heating leak into the sub-floor void,
- rainwater penetrating into the sub-floor void,
Always test the ventilation, here the fix took about 5 minutes each to fix extractor fans.
Vapour causing condensation made worse by damp proofing treatment.
The building has been damp proofed against rising damp, the horizontal covered drill holes across the centre of this image, is a sign of chemical damp proofing, hear what Dr Robyn Pender of Historic England says about it.Despite the low risk of rising damp, there has been two attempts at chemical treatment against rising damp. Rising damp treatment is normally a sign of insufficient ventilation. The walls have been drilled into by about 200mm and injected with damp proofing chemicals. In theory the chemicals are absorbed into the brick pores reducing the bricks ability to absorb moisture. Walls are replaced with damp proofers slurry. This can cause problems with
Sockets are prone to dampness as a section of wall is cut out, a metal box installed and covered in absorbent plaster, all of which exacerbate the effects of condensation and absorption, see surveyor.tips/sockets. This form of condensation is known as interstitial condensation, forming within the building material.
I tested the surface on the inside at the base of walls with a Protimeter damp meter in conductance mode. These meters measure electrical conductance of salts in water, a proxy for damp. Below 20WME is considered dry. See surveyor.tips/dampmeter.
There are calcium sulphate salts on the surface. Calcium sulphate is a key ingredient in cement and other building materials. If diluted in water salts tend to move to the surface. These can be removed with sandpaper and decorated.
I measured the other side of the wall in the same location and did not find a source of water other than vapour. Their side was not damp on the surface but was deep inside. It may have been damp-proofed blocking water inside. It is unlikely and would not be a source of water.
It is likely that the temperature was lower with the wall as the metal in electrical wires causes heat to be lost to the subfloor void, with a similar temperature to outside.
Every single area of dampness had metal embedded within the wall behind it.
Certainly, the extensive metal either side of the doorway suggests that there is a steel column supporting a steel joist (“RSJ”).
Using a radio frequency damp meter which uses radio waves, the high reading at the base of the wall suggests that there is dampness blocked behind damp proofers slurry.
Rising damp results from groundwater, that is the water under the water table. Over a short distance such as the length of a building, gravity ensures that groundwater is essentially flat. Therefore rising damp will have a more or less horizontal profile, like a flood. It affects all walls in the property evenly, unless damp proofer’s slurry is obscuring the rise.
By contrast condensation forms on the coldest places first, which is often near ground level. Moisture is absorbed in all direction, inside and down through gravity. Damp bricks lose heat more rapidly than dry bricks, exacerbating risk of damp at the base of a wall.
If there was groundwater then there would be high subfloor relative humidity unless there was considerably more subfloor ventilation, which itself would reduce the risk of rising damp.
The green stain tells me that the property has previously suffered from rot, possibly Dry rot. This suggests that there was in the past a leak in the subfloor void.
A damp proofing membrane (DPM, made of plastic) has been loosely left up against the wall. This could cause any rainwater absorbed into the concrete being forced onto the wall. Ideally this DPM should be tucked in underneath the damp proof course “DPC”.
I recommend solving the root cause of excess humidity first. In, addition the plaster has been allowed to touch the solid floor. Plaster is considerably more absorbent than brick and can draw up any moisture forming on the solid floor. Consider cutting the plaster back at the base by 10 – 20mm.
While it is easy to disprove rising damp, it can be harder to explain why condensation is taking place, especially when a property is unoccupied in summer. In summertime external air can hold a considerable amount of moisture. Yet at night the temperature often drops below the dew point. Most walls can cope by evaporating and condensing. Problems start when changing plaster (on either side of a wall). Slurry is less absorbent than normal plaster, increasing the burden on remaining walls. By contrast bonding, a plaster used around sockets and finishing plaster, is more absorbent than the original plaster. This imbalance of absorption forces condensation to condense and evaporate from a small section of wall. This in turn increases the local movement of salts, which often blister behind paint.
The temperature of your wall was below the dew point resulting in condensation, especially evident during the cold snap in February, but also other times of year.
On the face of it you have normal levels of relative humidity, averaging around 75%RH. But what is missing from this data is the temperature difference between the datalogger and the external wall. Taking the difference in the two above laser thermometer readings of 2.5°C, I have recalculated the surface relative humidity applying the August-Roche-Magnus formula: 100*𝑒(𝑐𝑏(𝑇𝐷−𝑇)(𝑐+𝑇)(𝑐+𝑇𝐷)) [c d are constants, TD = dew point and T=temperature). See seperate Excel file.
Groundwater risk assessment
- Rising damp is the “upwards flow of moisture through a permeable wall structure, the moisture being derived from groundwater”.
- Groundwater is subterranean water at or below the water table.
- Groundwater behaves different to rainwater in soil, as it is at or below the water-table therefore there is no gravitational force, it can’t disburse. By contrast rainwater can and should be drained away from a wall.
Flooding from surface water
Floodwater can be mistaken for groundwater. But even the risk of floodwater is very low.
Chemical damp proofing for misdiagnosed rising damp
Chemical damp proofing often causes major problems if rising damp is misdiagnosed, as the original absorbent plaster is normally replaced with impermeable (non-absorbent) damp proofer’s slurry, which hides rather than stops the underlying root cause of dampness.
Example from another property
If, as is commonly the case, condensation is the root cause, excess humidity can become trapped at the interface between the original permeable plaster and impermeable slurry. This damage results from two damp proofing treatments from the same damp proofer you were considering. They said the work was not covered under their guarantee, instead quoted an additional £16,000 to repair.
Example from second property
Unfortunately conflict of interests and lack of evidence results in overstated rising damp diagnosis. Contractors can’t be blamed, treatment for rising damp may be their only profit.
I recommend overrunning the extractor fans until relative humidity comes down and the walls dry out, which can take 1 month per 1mm, i.e. about 9 months.
Note the small crack in the brick it’s on likely to cause too many problems but should all other cracks to masonry paint or render.