103B: Rental property badly damp proofed, PIV unit condensation

This is a one bed ground floor in a large Victorian house damp proofed a few times. It clear that water is draining under the floor. However, that was not the primary finding, which was that the PIV unit was not providing sufficient ventilation as it was short circuiting out of the nearby bathroom..

Surveyor Tips:

  • I’m no fan of Positive Input Ventilation (“PIV”) as I see too many that don’t function, typically because of short circuiting issues, that is where air is being push in and coming out of the path of least resistance, such as a nearby vent, without limited benefit to the remainder of the property.
  • I have said to John Bradley head of the Property Care Association that I will try a PIV in a house that I am updating. I suspect that it will fine, because I will take great care to ensure there will be no short circuits. However, I will be mixing ventilation types, with purge and continuous flow and probably won’t buy a factory made system as I doubt there will provide me with sufficient control to ensure sufficient ventilation in the property as a whole, without too much heat loss – see cost of running a continuous flow extractor fan.
  • Continue flow fan are considerably better, as well as cheaper to buy, install, maintain and run. Although a short circuit is less likely to result in vapour, you still need to ensure that all sources of vapour are properly vented.
  • Cook and vapour from drying clothes tend to be the issues that are forgotten about, as they were in this case.

Root cause

Condensation from insufficient, inappropriately position ventilation – namely a short circuiting PIV unit – as is so often the case.


Lines of injection hole covers shows at least two attempts at damp-proofing treatment against rising damp.

It is rare that rising damp is correctly diagnosed. The problem with misdiagnosis is damp continues and can cause rot. I could not see any rot. However, there is a good chance that there will be rot, especially if the root cause is not stopped.

I measured the subfloor humidity which is high. This is a sign that water is flowing into the subfloor.

There is a real risk of rot, such as Dry rot.

Looking around the property I see a gap between the gulley outside the flat’s reception and the drain.

It is likely that water is seeping between the two and into the cellar or sub-floor void. This can cause high sub-floor humidity, which in turn can result in water condensation on the damp proof course, and from there can be absorbed upwards, in a rising damp type profile.

The rear drain by the bathroom who is blocked.

I tried unblocking the drain during the survey but it may need a plumber with suitable equipment to complete the work.

Inside there is dampness along the exterior reception wall.

There were dribble marks at the top of the wall. I also saw images of condensation provided by the tenant. There were no signs of penetrating damp or leak from above, which will tend to leave distinction brown stains, like tea bag stains.

At the base of the wall that were the same damp marks and surface salts.

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. 

The dampness continues in a triangular shape on the internal wall.

This profile is consistent with condensation, but not rising damp.

Using a damp meter and radio-frequency mode I was able to determine dampness at the base of the wall.

Condensation forms on the coldest part of the wall which tends to be the base of the wall. Gravity pulls moisture down. A wet brick has about half the installation of a dry brick. In combination these factors increase the risk of condensation forming at the base of the wall.

There was that nice for me at the top of the bedroom.

Distinctive condensation dribble marks can be seen in the corner of the room. This happens when a wall loses heat rapidly. There is a small amount of mould in the top corner. 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.

The dribble marks were dry at the time of the survey.

These dribble marks are a distinctive characteristic of condensation. Water from either a leak or penetrating damp would have an accompanying brown stain.

Rust and calcium sulphate salts can be seen on the lintel above the bedroom window.

Rust results from a metal coming into contact with water.

Using a metal detector we could determine that that was metal supporting the joist. 

This metal is probably used to reinforce the lintel or a steel joist (“RSJ”).

Heat loss can be illustrated with a thermal imaging camera where blue is about 5°C warmer then yellow.

We can see heat loss to the corner and top of the room, on a cold night the temperature difference will be much greater.

Looking outside we can see some damage to the brickwork.

I doubt that water is penetrating through because there are no tell-tale signs inside. However, it would be wise to have all external masonery cracks filled up and painted.

There is an unheated conservatory next to the bedroom, this reduces airflow and ventilation from the bedroom.

Condensation was forming on the conservatory window during the survey.

Mould is growing at the top of the conservatory, despite a small vent.

The secondary cause of high relative humidity is low temperature relative to the source of humidity, see surveyor.tips/humidity. Relative humidity is a measure of how much vapour is in the air compared to air’s capacity to hold vapour. It is a function of vapour pressure (quality of vapour) and temperature. As temperature rises, air can hold more vapour.

A passive vent, PIV unit has been installed into the central corridor.

Air is being blown in from the outside in the hope of diluting humid internal air. The issue with PIV units is that they don’t target the source of humidity. According to PCA guidance, PIV units should be installed along with bathroom and kitchen extractor fans meeting building regulation see surveyor.tips/vent_regs.

I find targeting vapour production; kitchen, drying clothes and bathroom with balanced heat and insulation are better and cheaper than PIV units. I recommend continous flow out from bathrooms. I suspect the bathroom is short circuiting airflow, minimising the PIV’s benefit.


I placed a datalogger by the cold wall in the bedroom and asked the tenant to send back readings.

We can see that temperature is the primary issue with this property. None of the walls have radiators near them. The tenants are keeping the property quite cold. With two or three short bursts of heat every day. Human relative humidity is a function off vapour and temperature. Ideally there should be a balance of heat and installation across the property.

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