Brief: To address damp and mould issues at Jonny Murton-Lavelle’s one-bedroom rental flat in Sheffield city centre.

The Problem

Jonny’s letting agent was surprised and concerned to find such a severe decline at his flat during her recent property inspection. Along with mould on some walls and ceilings, there were condensation-soaked curtains and standing water on windowsills that probably weren’t helped by the trickle vents being closed and the tenant not wiping away any condensation pooling on the sills.

“This hadn’t been a problem for any previous tenant in the last 15 years. I’m aware that all my agent’s tenants get a leaflet outlining how to prevent condensation and mould, but the message doesn’t always get through. It’s possibly a generational thing, a resistance to being told what to do, or an attitude of ‘it’s not my problem, it’s the landlord’s property’.”

says Jonny.

While keen to maintain a good relationship with his tenant, as an energy professional with an insight into the problem, Jonny realised a thorough investigation would help find a solution.


During a visit, his tenant admitted that she’d previously rented older buildings which, unlike this modern flat, weren’t as airtight: an obvious – if partial – explanation for the increase in condensation. Jonny realised hard data was needed to confirm his suspicion that her behaviour was not the only contributing factor; it could also be a problem with the bathroom extractor fan and cooker hood or she might also not have been heating the flat to an adequate temperature.

Inspection & Monitoring

As he’d feared, a flow rate test on both the bathroom extractor fan and cooker hood confirmed they were not performing to Building Regulations for the time they were installed and hence not up to current standards, raising questions about the efficacy of checks carried out by Building Control bodies.

The manufacturer’s data sheet for the bathroom fan gave a flow rate of 21l/s which would have been compliant with Building Regulations, but the small print stated that this was based on a through wall installation. With a 5m duct run, where the duct was probably a flexible tube (rather than rigid or semi-rigid), this would significantly reduce the flow rate.

“I needed some real-world data over a period of time to get a full picture. Were the high humidity levels just down to the poor performing extractor fans or were there other contributing factors?”

Jonny explains.

To find the answer, he used several sensors from to monitor the humidity and temperature in each room along with a CO2 sensor in the living room.

These sensors provided a live stream so he could dial in to see the data without disturbing the tenant. Hermione Crease, CEO at Purrmetrix, explains that they are very unobtrusive.

“Our portable sensors and software help when doing retrofit, repair work and to monitor whether projects are delivering, measuring damp, humidity and CO2 readings as well as picking up data on energy efficiency,”

she says.

Clients get information within 30 minutes along with alerts about condensation risks or if CO2 levels are getting too high. They soon discover if there are patterns of behaviour and, if it’s more to do with a tenant’s lifestyle, there are problems in particular rooms. If problems arise due to weather patterns, there could be an issue with the building fabric.

“Once you’ve made the necessary changes to ventilation, we can then see if there is still a high risk or if the risk has lowered,”

Hermione adds.

“In Jonny’s case we collected data across the house and ran it through the calculations to discover how big a risk each area was for problem condensation.”

Within a week it became clear that the humidity in the bathroom was taking three to four hours to dissipate after a bath or shower. This confirmed that the intermittent extractor fan was not performing and would need to be replaced. Jonny also checked on the temperature of each room.

“The risk of condensation is increased by a combination of poor ventilation and low temperature,”

he says.

“A room can have adequate ventilation, but if the ambient temperature is low, there is a heightened risk of any moisture in the air condensing on walls, floors and ceiling. Conversely, if the room has a high temperature, but inadequate ventilation, there can also be a heightened risk of condensation due to the air being able to hold more moisture.”

The Purrmetrix data highlighted the dew point in each room where the sensors were positioned alongside the room temperature. It was clear that his tenant was being frugal with the heating.

“This was no surprise really, since the heating is provided by standard electric panel heaters and she was evidently concerned about the running costs,”

he adds.


Jonny replaced the intermittent extractor fan in the bathroom with a dMEV unit (continual extract), installed a new dMEV in the open plan kitchen/living room – both with new semi-rigid ducting with new fire barriers at the exit of the building – and replaced the cooker extractor with a new recirculatory cooker hood. All of which should improve ventilation.

However, during the installation, he found that the ducts were short lengths of plastic tube taped together with insulation tape instead of proper ventilation ducts. There were no fire barriers to the ducting where they exited the building either, raising another concern about the efficacy of Building Control bodies checking compliance with Building Regulations and safety standards, as well as the integrity of the contractors involved in the original build. A possible lack of fire breaks between the apartment block’s floors would have contravened Building Regulations at the time of construction.

Future plans

The next step is to tackle the more sensitive issue of raising the temperature, which is key to ensuring a healthy environment that is cost effective to run and doesn’t cause damage to the building fabric.

“Once I’ve built up a clear picture and have the data, I can then have an informed conversation with my tenant about her lifestyle. The data is already showing reduced humidity levels, but we are now entering the summer months and I will need to review the situation during the next heating season.”

Although it might seem like a lot of time and effort, Jonny is determined to nip any problems in the bud. After all, untreated condensation and an unhappy tenant – perhaps with related health problems – could result in legal action. Plus, while private landlords already have a moral, legal and contractual obligation to keep properties safe, they could face even tougher regulations under a Labour government which has promised to come down hard on those who don’t sort out damp and mould.

While the rent barely covers the mortgage payments on Jonny’s flat, which unfortunately has cladding and has devalued the property and restricts his ability to sell, the £4,000 spent on investigation and remedial work is money well spent, says Jonny.

“It’s definitely been worth the time, effort and up-front expense,”

he adds.

“The Purrmetrix sensors and monitoring have been instrumental in being able make good decisions and take targeted action.”

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