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How hard is it to retrofit a house to zero carbon levels?

16 January 2020

It's one thing to design and construct new, exemplary zero-carbon buildings, but quite another to tackle the huge carbon footprint of the existing built environment.

Four out of every five homes that British people will be living in in 2050 have already been built, points out a report released by the Institution of Engineering and Technology (IET) and Nottingham Trent University in October 2019. To meet the government’s zero carbon targets, over 26 million deep retrofits will be needed between now and 2050, the report says.

That would mean, the report concludes, a rate of around 1.5 homes every minute. This is a huge challenge, especially when it is considered how much of the UK’s housing stock consists of draughty, energy inefficient Victorian and Edwardian houses.

Engineer Kit Knowles is pushing the boundaries of retrofit projects further than most and, even more ingeniously, doing so while maintaining a ‘passive design’ perspective. He believes in using only natural materials and retaining as much of the original structure of a building as possible.

His deep retrofit of two semi-detached Victorian townhouses on Zetland Road, Chorlton, Manchester is, he believes, the first petrochemical-free Passivhaus refit.

The Zetland Passive House project is proof that Victorian housing can not only be made zero-carbon but can actually generate energy. Not only does the project meet the Passivhaus retrofit standard Enerphit, it exceeds it, reaching Enerphit Plus levels. This standard requires buildings to generate at least 60kWh of renewable energy per year per square metre of floor area.

Knowles was the developer and lead design consultant and approached it with the intention of retaining as much of the original building as possible. Almost all the original bricks have been retained, along with the joists and rafters from the 100 year old roof.

He believes architects that really do want to bring carbon emissions down should be sitting down with domestic clients to explore how they can achieve what they are looking for through remodelling of the existing building.

In his own projects, Knowles says he has tweaked the ‘Fabric First’ approach to energy-saving design into a kind of ‘Fabric Second’. While the building fabric remains of vital importance, Knowles’ first priority is exploring how much can be achieved through passive design measures and how much extra performance can be gained from remodelling.

Upgrading the building envelope comes second, followed lastly by implementation of renewables or efficient energy generation.

Technologies put to the test included hybrid heat recovery with passive stack ventilation, the first UK use of an ‘aerogel’ external wall insulation, hybrid solar PV-thermal panels and solar-powered rainwater harvesting.

Though he comes from a family of architects, Knowles' background is chemical engineering and a passion for integration engineering. Now he runs Ecospheric Developments, a consultancy and supplier for ecological construction.

Zetland Road's five-bed semis are claimed to be the greenest retrofits in the UK. Photo © Rick McCullagh

On his mission to eliminate petrochemical products completely from the two houses, Knowles found a Swedish wood product, Organowood, for external cladding that is preserved by dissolving inert silica into the wood fibres, mimicking the process of fossilisation.

The Zetland Project was very much a testing ground. “It was a laboratory, so there were many overspends,” he concedes. However, he says he would approach bringing a typical 2/3-bed semi-detached house up to Passive House standards, or close, with an estimated budget of around £100,000.

Where budgets do not allow for a Passive House retrofit, he advocates the ‘pressure neutrality’ principle which, simply put, seeks to eliminate monodirectional air flow. Even in a relatively leaky building envelope, Knowles says the best bits of Passive House can be achieved in many traditional homes for around £40,000 if the approach includes tackling air movement, typically the drawing-in of cold, moist air from air bricks and floor voids by chimneys and flues.

Kwowles is technical lead for Greater Manchester’s Retrofit Accelerator programme, which has ambitions to upgrade 61,000 homes a year through retrofit-led regeneration. He would like to see this ‘pressure neutrality’ approach rolled out across Manchester as part of it.

One policy approach he would like to see implemented would be for planners to get tougher on domestic extensions, coordinating them with efforts to improve energy efficiency. He would like to see one of the mayoral metropolitan authorities lead the way. They could, for example, only allow the standard glazed-box rear extension home improvement where compensatory measures are taken to upgrade the rest of the house.

“60% of domestic home improvement projects follow the same template of highly-glazed rear extensions. The add-on will typically increase the heating bill by 30%, so a lot of money is going into raising energy consumption," points out Knowles.

He would love to see more homeowners starting to think about energy use as a quality-of-life home improvement. “Domestic clients can achieve everything on the estate agent’s ‘Golden Checklist’ of desirable features through remodelling,” he suggests.

Thanks to Kit Knowles, Director, Ecospheric Developments.

Text by Neal Morris. This is a Professional Feature edited by the RIBA Practice team. Send us your feedback and ideas.

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