Number 23 retrofit renovation
Number 23 is an award-winning whole house retrofit and refurbishment, made possible through an Ecology Renovation Mortgage.
This three-bed 1960s riverside bungalow has been transformed from a hard to heat, oil-fired dwelling, into a healthy, fossil-fuel-free home. Winner of the Energy Efficiency Awards 2025 ‘Small Scale Project of the Year’ for the East Midlands region, this impressive naturally ventilated retrofit renovation also achieves an exceptionally high standard of indoor air quality.
Located on the north bank of the River Glen, the building once operated as a shipwright workshop, repairing fishing vessels in the off-season. It was previously extended in the 70s and 80s to the West and South, eventually becoming a residential dwelling.
Architecture and planning consultant, Dr Jerry Harrall and his wife Kay, originally bought the property as a holiday home several years ago but couldn’t resist the pull to make it their permanent residence.
Through an Ecology Renovation Mortgage, they have achieved an advanced building performance standard that offers a national retrofit benchmark that is low tech, cost effective and replicable.
“Ecology is a business you can talk to. You’re able to pick up the phone and talk to a human being. You’re not treated like a number and that’s becoming rarer and rarer, particularly in the finance sector.”
Dr Jerry Harrall, independent architecture, planning and development consultant.
Going fossil fuel free
Jerry is the founder and Technical Advisor to the Policy Liaison Group for Future Homes in Westminster, a cross-party group that brings together parliamentarians and construction leaders to drive the decarbonisation of our homes.
With a background specialising in environmentally responsible architecture, environmental performance, ‘true cost of ownership’ and reducing reliance on fossil fuels, Jerry developed a retrofit model called Greening the Box (GTB). It applies a set of passive solar design techniques to adapt buildings designed for fossil fuel heating systems into fossil-fuel-free buildings.
Having undertaken over 20 greening the box schemes over his career, Jerry has applied his experience and know-how to the 1960s bungalow.
Jerry explains: “It’s quite radical to go from a fossil fuelled, oil fired central heating system to a fossil-fuel-free building. We stripped out the oil boiler radiators, pipe work, thermostatic radiator valves, boiler flue, and bunded oil tank. Quite a messy, smelly job!”
Adopting a ‘true cost of ownership’ model, there are no gas or oil central heating systems, no ground or air source heat pumps, no MVRH (mechanical ventilation with heat recovery) and no mechanical ventilation.
In the absence of mechanical equipment for heating and ventilation, there is no maintenance, servicing nor future capital replacement costs.
The GTB retrofit includes renewables, a passive solar design strategy and energy storage facility, natural ventilation and significant fabric upgrade.
Super-insulated
In addition to rewiring and replumbing the entire building, all the walls and ceilings were plaster boarded, skimmed and insulated.
The outside of the external wall was upgraded with insulation and render, and all the windows were replaced with triple glazing.
Jerry explains, “There is between 220 millimetres to 1.2 metres of expanded polystyrene below the floor, so it’s what you could describe as a super-insulated building.
“The building’s primary heating has always been the wood burner at the far end of the property. As we’ve drawn closer to finishing the insulated fabric, we’ve noticed the frequency with which we use it has reduced significantly and the amount of fuel needed is far less.”
Naturally ventilated
Number 23 is naturally ventilated using passive stack vents, rather than mechanical ventilation. The duct goes from ceiling vents in the kitchen, ensuite and shower room, to the outside of the roof.
“It’s ever so simple,” Jerry explains, “the idea is to try and reduce the amount of energy you consume in your building. I don’t know about you, but I don’t really like paying high electricity bills.
“Mechanical ventilation is an encumbrance on the household. It uses electricity and increases the true cost of ownership of that building. With a passive ventilation regime, you’ve not got lots of mechanical extraction fans to replace, maintain, or service, and your electricity bills will be lower as a result. But guess what?
“There’s a good chance the air quality is going to be better because the building is naturally ventilated. Fresh air is drawn into the building simply by opening trickle vents or a window. You simply create a draught through the building, controlling the ventilation yourself.
“I follow the adage, ‘build tight, ventilate right’.”
Enhanced indoor air quality
Creating a healthy home was at the heart of this retrofit renovation.
Jerry explains: “We’re probably one of the healthiest homes in the country. All metrics are significantly lower than the World Health Organisation (WHO)’s recommended threshold for VOCs.
“The intention right from the start was to select materials with low volatile organic compounds (VOCs).
“We’ve used timber skirting and architraves (door and window frames) rather than MDF, solid oak floorboards rather than chipboard, and a low VOC paint, by Little Greene.”
A suite of built-in sensors measures Indoor Air Quality (IAQ), providing in-use building performance data on things like relative humidity, CO2 and HCHO (formaldehyde), internal and external air temperature, and barometric pressure.
“We’re scratching the surface of indoor air quality in our homes in the UK. Installing indoor air quality sensors into dwellings is a start, but we need better information to understand what the numbers mean.”
What are VOCs?
“VOCs, or volatile organic compounds” Jerry explains, “are chemicals used in the manufacturing process of everyday household appliances and can be found in many building materials.
“They give off potentially harmful gasses which can build up in your home. It’s very hard to get away from VOCs, but they can be reduced.”
In the UK, there is currently no comprehensive inventory of VOCs that might be found in construction materials, household furniture and appliances. Jerry is calling on the UK Government via the Policy Liaison Group for Future Homes to add a list of the potential VOCs that could be found in the home or office.
Jerry explains: “Once a list exists, the general public can begin to make informed decisions about what they’re prepared to see in their homes.”
Number 23 sets a new standard for in-use building performance data on indoor air quality, and shows us what’s possible for both new homes and retrofit renovations across the country.
Jerry and Kay are now planning an energy efficient self-build next door, and we can’t wait to see what else they will achieve.
Want to learn more about Ecology’s Renovation, Conversion or Self-Build Mortgages?
Ecology provides a range of residential mortgages that reward you for your home’s energy efficiency, either through our C Change Discount or as with our Eco Home Mortgage, our unique full term mortgage discount.
