In a top-floor mansion block flat, tenants have reported persistent damp patches and black mould along the outer walls for the last two years, with complaints intensifying during the winter months. The nature of these issues raises questions about their origin, whether they stem from condensation, a fault in the external render, or a leak in the roof. The flat’s elevated position complicates the diagnosis, as it could be exposed to various elements that lower floors might avoid, making it challenging to pinpoint the exact cause of the dampness and mould growth.
In conclusion, from the data loggers
- The surface relative humidity is consistently at or near 100%RH for long periods of time.
- Relative humidity drops down when the property appears to be vacant and or windows are open.
- The above illustrates the needs for mechanical ventilation in the bathroom and kitchen, and dehumifiers, such as for drying clothes internally.
Evidence & Illustrations
To keep the property dry
|Fill the gap around the window
|The gap around the window is likely to be a freeholder responsibility. It appears to be a defect, not properly filled when the window was installed, so hopefully will be covered by the installer. The time and cost is hard to estimate, as it comes down to safety more than time and material cost. Make sure the whole window is sealed and check for other defective windows in the block.
|Target vapour from bathroom
|Ask the tenant to keep the bathroom door closed with the window open until the vapour from the shower has dissipated – typically at least 30 mins after the last shower. I would however, also provide a fast bathroom extractor fan with 30 minute overrun.
|Help tenants to remove vapour from cooking
|Ask tenants to cook with tops on pots and pans to reduce the production of vapour from cooking by 80%. I would however, provide a kitchen extractor fan which is ducted out, and then insist that the tenant uses it whenever cooking.
|Use a dehumidifier
|Ask tenants to reduce vapour levels 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).
|Help tenants dry clothes
|I would provide tenants with a means of drying clothes internally, which could be a powerful dehumidifier or tumble dryer (like a heat pump). Alternatively ask them to dry in a vented room with the door closed (possibly a bathroom with the extractor running or window open). Ask that they never dry clothes inside, in an unventilated room. Consider offering to pay for the cost of running a dehumidifier or heat pump dryer, in the region of £1 – £2 per day, plus it is energy efficient and environmentally friendly, as all energy is electrical, which is converted to heat plus a 50% bonus.
|Consider sanding down mould where dry, and removing it with either bleach or anti-mould foam. I personally use HG Mould Remover Foam Spray, but imagine there is no major difference between brands. Don’t use these products on shoes, follow manufacturer’s directions.
|Maintain airflow close to cold walls
|Ask the tenant to keep objects away from cold walls where condensation or mould has built up, such as the shelf where the shoes are becoming mouldy. Consider insulating cold internal surfaces, for example, with floor insulation from B&Q or Wallrock Thermal Liner (ensure to use Wallrock Thermal Liner Adhesive).
|Repair render cracks
|Check and repair any cracks in the exterior walls to prevent water ingress. Address external render cracks by raking out, that is slightly enlarging and removing loose material and filling with an acrylic based caulk for small gaps and cement based mortar for larger gaps over 3mm. This should be a freeholder cost, it is hard to estimate, but maybe £1,000 per flat, with painting – consider adding EWI, external wall insulation – the cracks should still be filled under the EWI – make sure to check fire safety certificates for compliance with all regulations, avoiding any Grenfell Tower cladding errors.
|Consider insulating around the chimney breasts, with insulation, such as Wallrock Thermal Liner (ensure to use Wallrock Thermal Liner Adhesive). Note insulation is never a replacement for ventilation and dehumidification. Ask the tenants to keep the trick vents open. Once the bathroom ventilation is in place seal up any passive vents.
|Mark improvements on damp walls with a pencil line or sticky note and use a moisture meter, such as the Dr. Meter Wood Moisture Meter 2 in 1 Pin & Pinless Multifunctional Water Detector £23.99 or similar damp meter, to log and monitor reduction to dampness in walls. By doing so, you will gain early confidence in these recommendations. Use devices like the ORIA Wireless Thermometer Hygrometer to monitor relative humidity and temperature near walls or ceilings, look to keep the dew point at or below the external temperature. The dew point is shown on the app (if not use https://mouldpoint.co.uk to calculate the dew point from humidity and temperature readings).
|Remedial actions – estimate of costs