Net Zero Energy: Homes designed for the future

Image provided by Assael Architecture Limited (Assael Copyright)

With rising energy bills a concern for any tenant, the ability to generate more income through living in a zero-energy home can offset any worries about investing on a smart design. Financing such a project is the trickiest part but it has now been proven that the benefits and returns provide a viable and green future. A zero-energy building is a building with zero net energy consumption, meaning the total amount of energy used by the building on an annual basis is roughly equal to the amount of renewable energy created on the site.
The vision of developers, Green Construct Ltd, for their Meadowside development in Liphook, is to create nine unique net zero-carbon homes adjacent to the River Wey. The main pillars of the design are the reduction of the energy consumption/passive design and the minimisation of the consumption of water and carbon emissions regulated, unregulated and embodied.
Passive design includes building fabric specification near to the PassivHaus standard. The dwellings will be constructed with structural cross laminated timber (CLT) insulated panels. The advantages of the CLT structure is that it provides optimal-continuous insulation, optimal thermal bridging, prefabrication and low embodied carbon within the construction materials. The envelope will be super-insulated (average thermal envelope U-value close to 0.12 W/m2K). Special attention is given to eliminate thermal bridges. All junctions will be designed up to the enhanced thermal bridging standard saving up to 10% on the heat demand.
Advanced window technology will be specified including triple-pane insulated glazing, with a good solar heat-gain coefficient, low-emissivity coatings, sealed gas filled inter-pane voids, thermal break window frames and ‘warm edge’ insulating glass spacers. The U-value targeted for the windows is 0.8 W/m2K.
To reduce further the heating demand the envelope is required to be extremely airtight compared to conventional construction. The target required to meet is 1 m3/m2h at 50Pa based on the building’s volume. The houses are designed so that most of the air exchange will be achieved through a Mechanical Ventilation with Heat Recovery (MVHR) unit which is controlled ventilation through a heat-exchanger, to minimise heat losses. Additionally, the performance of the MVHR unit is maximised on airtight buildings <1.5 m3/m2h at 50Pa.

With a more airtight, better insulated envelope, Greengage were keen to understand how the buildings perform in summertime and whether the measures described increased the risk of overheating.
In simulations the MVHR units were equipped with summer bypass, meaning intake air physically by-passed the heat exchanger maintaining high ventilation levels to the home, providing night time ventilation. Coupled with phase-changing lining products or thermally massive interior elements, the night time ventilation cools down the internal thermal mass while reducing the overheating risk on the hot summer days. Fully openable windows will also be specified to provide full house purge ventilation in the dwellings in the event of a really hot summer spell, to minimise further the risk of overheating.
All spaces will have adequate natural daylighting that will attempt to maximise the solar heat gains (heating the thermal mass) in the winter and minimise electricity use from artificial lighting. All lighting fixtures will use low energy lighting and low thermal output LED lamps, reducing further the risk of summer overheating outside the daylight hours.
Passive design measures achieve a 25% carbon reduction compared to a Part L 2013 compliant scheme (See Figure 1).
In addition to the passive measures outlined, the project has benefitted from the introduction of individual high efficiency air source heat pump (ASHP) technology to provide low carbon, medium cost heating and hot water to future residents. The project’s design works to ensure that the dwellings are affordable to run as their energy efficiency is maximised. The domestic hot water (DHW) storage tank was chosen carefully to minimise heat losses from the heating system and simultaneously minimise heat gains inside the thermal envelope.
Energy efficiency measures associated with the building systems achieve a 60% carbon reduction compared to a Part L 2013 compliant scheme.
The next step of the energy design was to introduce renewable energy to off-set the remaining 40% emissions on the site (See Figure 2). Photovoltaic panels of an average of 6 kWpeak power was specified on each dwelling. The panels will be installed on the unshaded south-facing sloped roof of each dwelling at a 35-degree angle.
The dwellings will have some unique energy features which have been designed to work together, for example,

  • The ability of the ASHP system to use energy provided by the PV system behind the meter.
  • The ability of the dwellings to manage energy supply/demand with hot water storage tanks and thermally massive materials. The ASHPs will be programmed to use cheap renewable energy from the PV panels at times of low demand to effectively use the hot water tanks as heat batteries.

With the above technique dwellings will consume most of the free electricity produced on-site with fewer losses or the necessity to export to the grid.
Green Construct’s innovative project has demonstrated that with a Net Zero Energy building vision, designers can innovate and be creative in how they respond to the challenges posed. The result is that the developer team receives a superlative project and future occupants enjoy a wonderful space.


This article is written by Yanis Savvopoulos Senior Consultant Energy and Carbon Management at Greengage Environment Ltd.

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