Here’s how a prefab Passive House outperforms even eco houses in the UK

A new factory-fabricated home has scored twice the Passivhaus Standard on airtightness and achieved an EU Energy Performance Certificate (EPC) score of 103 out of 100, using the UK Government’s Standard Assessment Procedure (SAP) for energy efficiency.

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Developer Etopia uses a special building method coupled with green technology that results in homes that can produce more energy than they use – they are negative carbon.

Six homes were tested, which together would effectively prevent an average of 0.525 tonnes of CO2 being emitted into the atmosphere each year by feeding surplus energy back into the grid.

In terms of airtightness, they performed 10 times better than British building regulations require.

Homes that achieve a score of 100 are given an A rating under the EPC testing regime and the average UK home achieves a D rating and emits six tonnes of CO2 each year.

They are the first eco homes pioneered by MMC housebuilding specialist Etopia. The six houses were constructed at the company’s development headquarters in Corby in Northamptonshire.

Building for 2050

The homes have been installed at Etopia’s 47-unit development in The Avenue at Priors Hall Park. They form part of the Building for 2050  research project created by the UK government’s Department for Business, Energy and Industrial Strategy, a research project to help housebuilders meet the challenge of delivering low cost, low carbon housing. It is examining the drivers, attitudes, barriers and challenges relating to low cost, low carbon housing.

Other developments in this project include:

Marmalade Lane, Cambridge, a custom built co-housing community of 42 homes plus first-class shared facilities. The homes are designed with a fabric-first approach and also manufactured off-site, with a closed timber panels supplied by Swedish builder Trivselhus, combined with air source heat pumps to supply heating and hot water.

Leyton, in Waltham Forest, London, is home to 50 new affordable and private flats and houses, part of the London plan for local homes. Although of traditional brick construction, their distinguishing feature is a large-scale communal air source heat pump which feeds an ambient temperature heat network to all of the buildings that also have individual heat pumps, solar photovoltaic panels, and are expected to reduce carbon emissions by 57 per cent from a normal house.

Neath, South Wales, has 16 social rent homes designed to generate around 60 per cent of the energy consumed using solar PVs, transpired solar collectors (a form of passive heating), air source heat pumps and batteries.

All developments include mechanical ventilation with heat recovery.

Test results

The airtightness tests were carried out under the official Air Tightness Testing & Measurement Association procedure which is used to estimate how well homes conserve energy by measuring the amount of air changes per hour in a property whose doors and windows are closed.

Many so-called eco-homes actually fail to perform according to designers’ expectations once real people start living in them.

The Passivhaus Standard requires homes to achieve 0.6 air changes per hour at 50 pascals (pa) or less; the Etopia home can achieve up to 0.3 air changes per hour at 50pa.

The Chartered Institution of Building Services Engineers (CIBSE) considers three air changes per hour at 50pa to be best practice — a level of performance that Etopia’s system beats by a factor of 10.

“We have designed these homes to smash through the eco ambitions set by national and international bodies”

When residents have moved in, the buildings will be subject to post-occupancy evaluation to see if they work as well as predicted in practice, as many so-called eco-homes actually fail to perform according to designers’ expectations once real people start living in them.

Joseph Daniels, Etopia’s chief executive officer commented: “We have designed these homes to smash through the eco ambitions set by national and international bodies. The early evidence is that the team and the build system have excelled themselves and these Etopia homes are setting new benchmarks for our competitors to meet.”

Construction

The pre-fabrication method uses a lightweight, structurally insulated panel system that is manufactured off site, and various energy usage and production technologies, a combination of heating and ventilation systems, with solar energy generation and energy storage. Render and brick slips form the exterior.

Price-wise, they are said to compete with traditional construction. Marketing officer Sarah Robinson said, “Our sales price are equal to conventional new build sales price per square metre in the area”.

Etopia reckons it can deliver up to 2000 high quality, energy efficient homes a year that can be designed to any specification. It claims to be the world’s first “global ready”, any weather, BDA Agrément approved, stock and store panelling system.

Technically, the walls achieve a 0.13 U-value and a structural loading strength of 825kn/80 tonnes. The homes are said to be able to handle winds exceeding 472 kph and be structurally fire resistance, and can be made flood proof. With an unpredictable climate becoming increasingly certain, all of these features may be necessary.

As the heaviest panel only weighs 97 kilograms, cranes are not required for construction.

Daniels said, “Our system was designed to combine energy, construction and intelligence to provide a truly scalable, green and economically viable building system. Our homes at Corby embody the huge potential of offsite construction and we have proven that modern housing can help tackle climate change while providing the homes of the future that will end the housing crisis.”

David Thorpe is the author of Passive Solar Architecture Pocket Reference, and ‘One Planet’ Cities: Sustaining Humanity within Planetary Limits. He also runs the online course, a Post-Graduate Certificate in One Planet Governance. He is based in the UK.