The single most important method for determining the root cause of damp is comparing a damp stain’s profile to standard profiles. Yet damp profiles are not taught at RICS or the PCA and poorly understood by damp proofing contractors.
Rising damp is rare. The key distinguishing feature is the horizontal line that drop by entrances. Note it is similar to sub-floor vapour and condensation, but unlike condensation and sub-floor vapour it contains nitrates from groundwater, which inhibit mould. If there’s mould the issue is excess vapour or humidity, not rising damp.
The DPC can be breached either because the ground has been built up, in which case it should be lowered, or because plaster has breach it in the sub-floor void, again easy to rectify. Slates or bitumen used as the damp proofing material rarely fail. Even if the DPC did fail, virtually no water would be absorbed through the gap.
Condensation is often misdiagnosed for rising damp, because it’s often at the base of a wall. Vapour condenses on the coldest surfaces, which is often the floor. Opening around cold entrances or windows are more susceptible, as are cold corner and ceilings. In worst case dribbles form on the coldest surfaces.
Mould grows on cold surfaces, such as by windows, electrical sockets and poorly uninsulated eaves and where there is poor air circulation, such as behind cupboards, curtains and clutter.
Mould and condensation
It is common to find mould and condensation in the same environment, both are caused by high humidity, which is as a result of insufficient ventilation made worse by heat or thermal imbalance and poor air circulation.
Penetrating damp comes from rain. It can be anywhere on a wall or ceiling. However a typical source is either raised ground levels or damaged rainwater goods such as a rupture or blocked gulley. The damp patch is centred close to the source of moisture. Ceiling level damp patches are typically stained. The risk is timber forming rot.
Sub-floor vapour, is condensation under timber flooring. It’s a secondary cause, often resulting from penetrating damp. Vapour condenses above the damp proof course causing water to be drawn upwards. It is normally misdiagnosed as rising damp. The risk is water forming on timber, causing rot.
The sub-floor void is built assuming that some damp will penetrate the sub-floor brickwork. Sub-floor ventilation, high walls or a damp proof course (“DPC”) will restrict the amount of absorption into the habitable space. However, water in the sub-floor void can result in rot or woodworm. The solution is to either ensure there is sufficient airflow from front of the house to the rear or monitor relative humidity in the sub-floor void, looking at the timber below the floor annually or when relative humidity in the sub-floor is consistently above 90%RH.
Often a leak or penetrating damp from rainwater is found in combination with condensation. The simplest issues should be resolved first and the dampness monitored before considering more damaging or costly remediation.
Hygroscopic salts are surprisingly common. Sometimes they represent 30 – 40% of damp surveys. The salts normally come from the historic burning of fossil fuels and are released when chimneys are removed, or become saturated in water such as after a leak, interstitial condensation or replastering.
Interstitial condensation is condensation that forms within the wall or substrate. Condensation form whenever vapour meets matter below the dew point. Dew point is the temperature when the relative humidity reaches 100%RH, or capacity. The dew point may not be on the surface, but within the wall. This is called the dew line.
Interstitial condensation – RSJ
Interstitial condensation forms on cold metal such as a roll steal joist (RSJ), holding up a roof extension or column.
Leaks are typically one-off events that happen quickly. A slow leak can be very difficult to identify. The easiest test to establish if there is a mains water leak is using a mains water pressure meter and or looking at the external meter readings over time. Wastewater leaks can be found using blue dyes. Sub-floor leaks are often misdiagnosed as rising damp, consequently the leak can continue and could result in rot to the sub-floor timber.
Many apparent dap issues are dry. However, we aware that a leaking bath that hasn’t been used for a while, could cause the leak to return.
Some damp patches are marginally damp. They may have been successfully repaired, or it may be that the dampness only appears occasionally, such as rain water from cracked render that only results in penetrating damp after heavy rainfall.