The ‘carbon-positive’ semi where energy bills pay for themselves

On a street of nondescript semis in Cambridge stands one house that is an exception. Look quickly and it does not seem to differ from its neighbours. Nor does it seem to differ much from the millions of semi-detached houses all over Britain. But it does. This 1930s semi, almost a century old, has been made “carbon positive”.

Long home to a local military family, the house was bought and upgraded by the Cambridge Building Society to showcase how a typical leaky old semi could be transformed into a home that exported more energy than it used.

It took six months to find the right house, which was “very, very untouched”, says Duncan Turner, a project manager at the lender. The wallpaper was peeling, some kitchen cabinets were broken and an abandoned sink stood in the overgrown back garden.

A “deep retrofit” gutted the house top to bottom, insulated it on every side and added a heat pump, solar panels and a battery. Two extensions — to the side and rear — plus a garden office increased the floor space 30 per cent from 74 to 96 square metres.

Above and below: the home was fitted with a new kitchen as part of the renovation

ALAN BENNETT

Cost savings from an energy-efficient home design

Despite the increase in size, energy bills will plummet. Instead of costing £1,250 a year to run, the property will make a projected profit of £159 a year from the electricity it generates.

Its energy performance certificate (EPC) now gives it more than full marks. Originally it was rated D (out of bands where G is worst and A is best) with a numerical score of 62 (on a scale of 1 to 100). Now it gets A and scores off the scale at 104.

• 9 eco-friendly home improvements that boosted our house price

This has made the house a national rarity. Only 0.3 per cent of homes in England achieve A, according to new analysis of government EPC data by the insulation specialist SuperFOIL. More than half of homes (57 per cent) fall below C — the target rating for private rented homes by 2030.

The aim was to show the “art of the possible”, says Carole Charter, the chief commercial officer at the Cambridge Building Society. “Very few people are going to want to do a deep retrofit on the scale that we’ve done. But I would like to think that [homeowners] get some idea of the options that are available to them. We really have put everything we possibly could into [this semi].”

Because it is a demonstration project for the public to visit, the lender would not disclose a total project cost, but did share what individual measures would cost owners of similar homes:

• Full insulation (£30,000) Internal and external, inside cavity walls of the extension, in the loft and under the floor.
• Airtightness measures (£9,800) Making a well-insulated building airtight is like putting on a windbreaker over a fleece — if you don’t, you’ll still lose heat. A liquid membrane called Passive Purple, applied to the walls, floors and ceilings reduced air leakage to Passivhaus levels, the pinnacle of low-energy homes. Leakage fell drastically, from 14.4 to 0.5 cubic metres of air per hour per square metre when pressure tested at 50 pascals.
• Air-source heat pump (£12,000 to £15,000) Replaces the original gas boiler for low-carbon heating. The Daikin heat pump stands in the back garden, while its brains and hot water cylinder are in what used to be the coal store. “That’s a really nice circle,” Charter says.
• Underfloor heating (£6,600) A wet system on both floors is powered by the heat pump.
• Solar panels (£12,200) Ten panels were installed on the east-west facing roof of the house and two panels on the new garden office. A Huawei Luna 5kW battery stores the electricity generated from the sun for use at night.

The rear extension and solar panels

• Mechanical ventilation and heat recovery (MVHR, £3,500) A machine in the side extension pumps out stale hot air and uses the heat from it to warm fresh incoming air. Ventilation is crucial in well-insulated homes, or you’ll end up with condensation, damp and mould.
• Triple glazing (£20,000) replaced the poorly fitted old windows.
• Sedum roof (£900) Adds insulation and biodiversity to the side extension’s flat roof.
• Rainwater harvesting (£3,500) Water shortages are a challenge in Cambridge. A 3,000-litre underground tank (rainwaterharvesting.co.uk) supplies the toilets, washing machine and garden watering. It requires double pipework and “not a small hole that you need to dig in the garden”, Turner says. As they were already doing groundworks for the extension, it was an easy “extra hour’s work with the digger”.

The home’s rainwater harvester and heat pump

The project team and neighbours unveiling the renovated home

ALAN BENNETT

Turner advises homeowners to work with a mechanical and electrical engineer or architect who knows how to integrate all the systems. “It’s not a case of oh, stick some insulation on it, it’ll all be fine. We’ve all seen news stories on how that’s worked with houses.”

Almost all of the 23,000 homes (98 per cent) that were insulated under a flagship scheme must be fixed to stop them growing damp and mouldy, the public spending watchdog found last month.

• Read more expert advice on property, interiors and home improvement

Charter adds: “A lot of people are not going to think, well, I’m going to retrofit my property. But what they will do is think about extending their current property or putting in a new kitchen or bathroom. We would encourage people to think that’s the time to think about some of the retrofit efficiency you could put in, because you’ve already got the disruption.”

Book a tour of the upgraded Cambridge semi at cambridgebs.co.uk