Since we began our retrofit quest in 2008, we have worked on buildings of every shape, size and vintage. A particularly interesting example is Kilowatt House, a grade II listed 1930s Modernist gem in Bath. In the summer of 2025 work was completed on a whole-house refurbishment that has dramatically boosted its thermal performance and slashed running costs.
Britain never really swallowed Modernism, which makes pre-Second World War examples like Kilowatt House very rare. The house was designed in the 1930s by a 23-year-old architect, Mollie Taylor, in collaboration with the owner, acoustic engineer Anthony Greenhill.
Greenhill was an intriguing character, an electrical engineer by day, who became fascinated with electronics and sound in the 1920s. He filled his flat, and then his parents’ house in Bath with home-made equipment in a quest he described as “a search for bigger and better noises”. He eventually purchased a disused quarry at the top of Bathwick Hill and constructed a permanent home for his experiments, which he named Kilowatt House after the huge amount of electricity his equipment consumed.
The house itself was as unconventional as its owner. A cavernous studio occupied one end, so large it could be mistaken for a ballroom today. There was only one bedroom, for guests, and two further studios, in one of which Greenhill “intended to sleep on those occasions when there was time to go to bed”. The stairwell is curved to achieve certain acoustic properties, and a small semi-circular room on the roof was used as an echo chamber.
Greenhill died in 1958 and by the 1990s the house stood abandoned and decaying. It was purchased by a developer, who carried out a cosmetic refurbishment, but failed to address the issues with the building fabric, leading to damp, mould growth and rot. When our clients purchased it, they embraced the huge task of tackling the moisture and thermal problems to transform it into a comfortable, energy efficient home.
So, how do you improve the thermal performance of a quirky, listed building like this? The principles are relatively simple – more insulation, better windows, air-tight construction and adequate ventilation, but the way these measures are implemented can be fiendishly complicated, and depend on the specific characteristics of the building. Moisture is the biggest risk because adding insulation and sealing up gaps can trap water inside the building, causing long-term damage. Every building is unique, often consisting of multiple layers of work over time using different construction methods that require different approaches. Precise analysis is required to understand the existing fabric, and every move has to be modelled, measured, and tailored. To carry out the thermal modelling and mechanical heating system design we brought in building physicists Greengauge.
The first step was to open up the walls and study the construction. Because the house is listed, the exterior couldn’t be changed, so all insulation had to be fitted internally. Breathable wood fibre and insulating plaster were used, which allow water vapour to escape, and ultra-thin aerogel was added around the window reveals where space was tight. Greengauge determined how much insulation could be added to keep moisture in check.
Several lengthy listed building applications were required to gain approval for the various interventions. Consent was granted to replace the single glazed Crittall windows with a double glazed, thermally broken system with very similar dimensions. Solar panels have been added to the flat roofs and the house is heated with air-source heat pumps. There is also a discreet MVHR (Mechanical Ventilation with Heat Recovery) system that supplies constant fresh air throughout the year. Routes for the ductwork and other services had to be carefully worked out so as not to be visible. In the main studio ducts are cleverly concealed behind the high-level cornice.
The transformation of Kilowatt House is a remarkable achievement, and a labour of love for our clients. The story of the original house and the quotations come from Anthony Greenhill’s own highly entertaining account of the building of the house.
