During the summer renovation of an interwar terraced house’s ground floor bathroom, it was discovered that the wooden floor joists had become rotten or soft. These were subsequently replaced, and the bathroom was rebuilt from scratch. Despite these efforts, mould quickly began to appear on the non-tiled areas. Initially, it was thought that poor sealant around the bath, leading to water pooling on the floor and increasing humidity, was the cause. However, even after resealing the bath and employing dehumidifiers to reduce humidity levels, brown and green mould patches persistently reappeared above the floor tile line, upstand, and behind the toilet. The search for the root cause of this problem is ongoing, with various potential sources considered, yet a definitive source remains unidentified. The affected bathroom, being entirely internal except for an extractor fan venting to the outside, features mould on a party wall. Interestingly, during a 10-day absence from the house, a pasta-like mushroom, a few inches wide, emerged from one of the damp patches on the wall.
In conclusion, from the data loggers
- Looking at the final graph, dew points outside the bathroom, we can see the vapour in the house is exceptionally high. In London all properties are subject to similar external conditions. Ventilation works by targeting humid air from a vapour source, such as the bathroom, and replacing it with external air containing vapour at external dew point. A well ventilated property should therefore contain air at, or at least close to the external dew point.
- Cold water typically comes into a property at about 8°C in winter. Your dew points are far in excess of 8°C, risking condensation on cold water pipes, as we saw under the bath. It is possible that the root cause of your damp is entirely condensation and that the mains water pressure drop is due to some benign, unrelated reason with no effect on the dampness below the bath.
- I recommend that owners aim to bring their internal dew points down to the external temperature. There are many reasons for recommending this. The most important is that most properties have an internal surface that is poorly insulated, meaning that condensationcan take place if the dew point is much higher than the external temperature.
Evidence & Illustrations
To keep the property dry
|Fix mainswater leak
|The pressure loss on the mains when the stopcock was turned off indicates a leak, which is probably in the zone under the bath. It is possible that there is an innocent, unrelated cause of this pressure loss, such as a dripping shower head, but I did not find any. You will require a plumber or leak detection company. Some of these companies are a fairly mixed bag, a few with a background as unsuccessful damp proofers. One of the few that has done a good job for a client is De-Yany Leak Detection Specialists, 17 Pevensey Avenue N11 2RB Email: LALeakdetection@gmail.com, 07886 702094. I have no connection.
|Insulate cold water pipes
|Cover the cold water pipes, especially in the bathroom with pipe insulation.
|Use a dehumidifier
|Maintain relative humidity, especially at night when it’s cold outside, with a dehumidifier such as PureMate 20L/Day Portable & Compact Dehumidifier with 6.5 Litre (£179.99 Refrigerant), Duux Bora Smart Dehumidifier (DXDH02UK £300 Refrigerant) or Meaco DD8L Zambezi (£260 Desiccant – best for colder environments) – Which! Best buys. There are plenty of alternatives available. I choose a dehumidifier based on daily and tank capacity, low noise, whether it has a pipe out, and ease of programmable functions over other considerations such as weight and aesthetics. The technology is old and easy to manufacture. So if these brands are not available, you are sure to find another alternative. Evaluate based on daily capacity (20L minimum) and whether it has a continuous pipe out (use this to drip into a sink when away in winter).
|Use fan to expedite drying
|Direct a fan onto the damp patch to speed up the drying process.
|Target vapour from bathroom
|TIP: Keep the bathroom door closed with the fan running or window open until the humidity from the shower has subsided – typically at least 30 mins after the last shower.
|Target vapour from cooking with an extractor fan
|Always use the kitchen extractor fan when ever cooking, and cook with tops on pots and pans.
|Target vapour from clothes drying
|Use a tumble dryer (like a heat pump), dry in a vented room with the door closed (possibly a bathroom with the extractor running or window open), or use a powerful dehumidifier. Don’t dry clothes in an unventilated room.
|Monitor vapour with data loggers
|Monitor relative humidity and temperature against the wall or ceiling to observe the effects of improvements, such as ORIA Wireless Thermometer Hygrometer (20m (2 Packs)) – already in place. In addition monitor improvements on the damp wall a pencil line and damp meter such as dr.wood moisture meter, logging improvements each month. Monitor relative humidity and temperature against the wall or ceiling to observe the effects of improvements, such as ORIA Wireless Thermometer Hygrometer (20m (2 Packs)) – already in place. In addition monitor improvements on the damp wall with a pencil line and damp meter such as dr.wood moisture meter, logging improvements each month.
|I would buy a Dr. Meter Wood Moisture Meter 2 in 1 Pin & Pinless Multifunctional Water Detector £23.99 to log and monitor reduction to dampness in walls.
|Remedial actions – estimate of costs