A picture containing wall, indoor, white Description automatically generated

104E: First floor rental penetrating damp or condensation

This first floor Victorian property appears to have penetrating damp, but it may just be condensation.

Surveyor Tips:

  • Ask tenants to send data back from dataloggers.

Root cause

Condensation from high occupancy during Covid.


A picture containing wall, indoor, dirty
Description automatically generated

The primary concern is dampness on the front wall behind the curtains

Here we can see salts bubbling up underneath the paint and finishing coat of plaster. These are calcium sulphate salts. 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. Depending on the outcome it may be necessary to use lining paper, a thick anti-condensation paint or a skim coat of plaster and paint. Although penetrating damp is possible, it is normally accompanied by a brown tea bag like stain, thereby making it less likely than condensation.

A picture containing wall, indoor, white, dirty
Description automatically generated

Mould is growing on the front wall. This tells me that there is excess humidity and insufficient use of ventilation.

A picture containing ground, outdoor, cement
Description automatically generated

Looking immediately outside the damp patch we can see possible routes for water, such as past the timber.

Timber should be protected such as with gloss paint, otherwise it is at risk of rot. Window rot is very common in old properties, it is a slow growing white rot. None of the timbers are structurally significant. The windows can be sanded, filled and painted for protection.

A picture containing brick, stone
Description automatically generated

No major issues are seen under the windowsill. But, there is no groove under the sill to stop water backtracking.

The detailing supporting the windowsill is almost certainly acting as a thermal fin increasing the risk of heat loss and condensation inside the flat.

A picture containing indoor, kitchen, wall, person
Description automatically generated

I understand the tenants removed a lot of mould from under the windowsill which is good.

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 secondary cause of high relative humidity is low temperature relative to the source of humidity, see surveyor.tips/humidity.

A picture containing wall, indoor, white
Description automatically generated

There is mould growing behind the cupboard by the external wall in the bedroom.

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. Conversely as temperature drops, air holds less vapour until it meets the dew point at 100%RH, when dew or condensation forms.

Reduced air circulation behind the cupboard accounts for the location of the mould.

A picture containing wall, indoor
Description automatically generated

The bathroom extractor fan was switched off at the isolator, suggesting tenants are annoyed by the sound.

I measured the speed of the bathroom extractor fan. It is running at 14.4 L/s which is sufficient. However the fan’s overrun is one minute after lights are switched off. 90% of humidity remains in a bathroom after a shower is stopped. Therefore the vent should run for the full 30 minutes. With challenging properties it is worth considering installing a continuous flow extractor fan. As a landlord myself, I find fans become noise and dirty after about 5 years unless maintained. Noise is a major concern with ensuite bathrooms. The Elta continous flow provides 5l/s quiet background ventilation with a humidity boost function.

I noted trickle vents were closed. I open them during the survey and asked the tenants to keep them open.

Occupiers are often unaware of a trickle vent’s purpose.

Monitoring relative humidity

I encourage landlords with properties at risk of damp or mould to remotely monitor the relative humidity and temperature, such a Govee WIFI datalogger surveyor.tips/datalogger.

I left a datalogger with the tenants, who send data back.

The data sent back from the datalogger demonstrates that the property is currently being maintained without risk of mould and condensation.

I ask tenants to maintain a property as they normally would do, so as to have a baseline to compare improvements to. The tenants were very keen to learn and I suspect made all the necessary changes immediately. The main improvements discussed were;

  1. Keep the bathroom extractor running for 30 minutes such as by keeping the light on and door closed.
  2. Take a shower well before going to bed.
  3. Dry clothes in the bathroom until 90% dry, by keeping the lights on to keep the vent running and door closed.
  4. Keep all trickle vents open.
  5. Always use the kitchen extractor fan when cooking.
  6. Maintain a gap around furniture for air to circulate.
  7. Maintain some night-time heat – they don’t seem to be doing this. However, they are maintaining relatively low %RH, so it’s less of an issue.

Note the flat’s dew point is running in paralleled with London external dew point, this is a sign of an appropriately ventilated property.

Leave a Comment

Your email address will not be published.

This site uses Akismet to reduce spam. Learn how your comment data is processed.