How to give your listed home an eco-friendly makeover

Retrofitting a draughty Grade II listed Georgian townhouse so it becomes more comfortable, and as airtight as an efficient new-build, is no mean feat. Bob Prewett, co-founder of Prewett Bizley Architects, has done just that with a five-storey 1820s property in Bloomsbury, central London. The all-brick building – which had been converted into offices – was given a thorough makeover, transforming it into a highly energy-efficient family home, with its space-heating demand reduced by about 90%.

“There are three good reasons for doing an eco-retrofit,” says Prewett, whose firm is a one-stop shop for homeowners considering this kind of renovation. The first is comfort and health: “In most homes, air quality isn’t great. Historic houses tend to be insulation-free and are overventilated to the point of being draughty.” Second, there are economic benefits. “The choice at the moment is to spend increasingly vast amounts of money heating your home or to run it cold and tolerate long periods of discomfort.” Third, you’ll be using less energy. “The majority of household carbon emissions come from heating, so any retrofit measures that address this make a significant contribution.”

Improving insulation and air tightness, eradicating “thermal bridges” – weak points in the building that allow heat to pass through – and installing mechanical ventilation/heat recovery and air-source heat pumps were the main energy-efficiency components of this retrofit, which cost £500-£1,000 per square metre, Prewett estimates.

With a listed building, or one in a conservation area, there are limits to what you can do to its external appearance. “External wall insulation on the front is banned, even if you think it a good idea,” Prewett explains. “On the rear, though, where many Georgian townhouses have had rooms added in later centuries, external insulation might be possible.” In the Bloomsbury townhouse, external wall insulation was specified for the rear and, inside, a variety of breathable insulation materials, from wood fibre to aerogel, were used. Deployed by Nasa in space missions, aerogel is the lightest solid material known, and can be applied so thinly that original cornices and mouldings remain in place.

As for windows, “new glazing technology allows very thin units”, says Prewett. “Evacuated glass [also known as vacuum glass] looks like single glazing, but performs in line with triple glazing. We’ve done multi-pane sash windows in it. In conservation areas, this is becoming an easy win. Several listed buildings have received this treatment.”

Innovations of this kind are easing the way for eco-retrofits of heritage properties where renovation work is considered on a case-by-case basis. For decades, the standard response from conservation officers to most suggestions was “no”. Now, according to Prewett, things are beginning to change. “There’s a realisation that change has to happen [to tackle the climate emergency]. In the next few years we’ll see more of a shift. It’s odd in an era dominated by climate change that we’re not adapting buildings to mitigate this.”

Conservation officers and planners often delay eco-retrofit projects by months while they decide whether adaptations are appropriate – it took 18 months for new windows to be approved on the Bloomsbury townhouse. “Clients are spending large amounts of money on pushing forward newer technologies,” Prewett says.

A thorough eco-retrofit will take at least three months, but the cost and disruption will be worth it in the end. “You’ll enjoy great comfort,” says Prewett, who has overseen an eco-retrofit on his own home. “Our house is a much more enjoyable place to be. Even on freezing cold days I can get up and the house is warm. There’s no running across cold landings at night, either.

“It’s a much healthier environment – with good air quality and fresh, filtered air with low carbon-dioxide concentrations. And I don’t think about energy bills – they are probably a third or a quarter of what they used to be.” If all that weren’t reason enough to embark on a retrofit, there’s another upside. Homeowners can sit back and enjoy their “nice halo”, Prewett says.

Prewett Bizley Architects

Pictures from top: the interior of the Bloomsbury house retrofitted sustainably by Prewett Bizley