Energy efficiency: sustainable construction

Speaking in Code

17 August 2011

Sustainable developer ZeroC was asked to carry out a retrospective upgrade of a development-in-progress to achieve much improved levels of energy efficiency and sustainability. Craig Bates and Robert Letts outline the process of getting an existing plan from unassessed to CSH levels 4–6

As a sustainable developer, ZeroC tries to think beyond current regulations (at the time of writing, Code 3 for energy) and aim for Codes 4, 5 and 6. But sustainability is meaningless without consideration of the commercial context in which development takes place. Expensive renewable technologies cannot be applied where the market for a house type in a particular location does not allow the extra cost to be reflected in the sale price. Experience has taught us that different sectors of the market in different geographical locations have a very different attitude to sustainable construction.

First and foremost, purchasers want homes that meet their personal needs in terms of design, accommodation and location. In most cases, an informed purchaser will also appreciate the benefits of energy efficiency, and will pay a small premium to have them. In other cases, buyers are less concerned, and need persuading of the longer term benefits in the form of reduced utility bills. In some instances, renewable technologies may even put off some purchasers, because they are perceived as too complicated in use, and a risk in terms of future maintenance.

The Code 6 home will generate approximately 4.5kW of PV, producing a very useful income of about £1,000 a year from the FIT scheme

In all cases, we believe excellent design and marketing are vital components of a successful sustainable development, as is aftercare in the form of good warranties from suppliers and installers. With energy prices set to rise by 25% this year, we have to take every opportunity to persuade purchasers that they can live in homes that are inexpensive to run and easy to maintain, while reducing their impact upon the environment. Post occupation, it is also important that purchasers are equipped to use the technologies properly.

The project

ZeroC was approached by South Somerset District Council (SSDC) in January last year and asked if we could upgrade one of our developments retrospectively (originally appraised by us in 2007, and with an existing planning permission from 2009) to incorporate much higher levels of energy efficiency and sustainability, funded by a government grant. The grant was important to help SSDC foster exemplary and replicable levels of sustainability on future housing development in the region.

The site was situated on the south side of Yeovil, adjoining Nine Springs Country Park, and consisted of brownfield land and a listed building, a Victorian glove factory. SSDC had sold the site to ZeroC in 2009 with a detailed development brief to build 37 homes and apartments and convert the glove factory to commercial space as part of its ‘Urban Village’ concept. SSDC was also involved in an EcoTown initiative, and wanted to explore the potential for higher levels of sustainability for their new housing and gather information and get feedback from the developer during construction and post occupation.

SSDC wanted to explore the potential for higher levels of sustainability for their new housing and gather information during construction and post occupation

What extra construction costs would be involved? Could the architecture remain traditional? How affordable could the homes be? The existing design for the new homes at the glove factory had predated the Code for Sustainable Homes assessment becoming part of SSDC’s planning requirements. The scheme already provided good levels of insulation and a 10% renewable energy provision from solar thermal arrays, in keeping with ZeroC principles.

The complication was that we were already past planning approval, and the contractors, Acheson Construction, were by now on site with ground works. As is often the case with grants, there was a pot of money available, but not much time to spend it. As an experienced sustainable developer, ZeroC already had many of the necessary skills to understand and respond rapidly to the brief. We were also able to employ our in-house sustainability and renewable energy consultancy, Ecofirst Consult, to produce a robust technical document within the short timescale required. Three funding options of £800k, £600k and £470k were presented to DCLG, based on the following measures:

  • a biomass (wood pellet) district heating scheme
  • solar PV arrays on the roofs of the houses
  • greywater recycling systems for the houses and apartments
  • improved sound insulation, provision of recycling facilities and security
  • increased biodiversity through extensive ecological planting
  • increased proportions of BRE Green Guide A+-rated materials
  • education and outreach features, such as an education centre in the building housing the biomass boiler.

The following three scenarios were offered, with option A providing 37 Code homes across the whole site, option B 22 Code homes and option C 13 Code-assessed units.

Achievable Code levels set against budget
Code Levels v Budget  Number of Dwellings 



Code 6

Code 5

Code 4
















Achievable Code levels set against budget

The bid submission was delivered as a joint proposal with SSDC and we soon learnt that we had won the lowest figure. With the build already under way by the time the grant was secured, we needed to undertake a detailed design review. The two Code 4 and 10 Code 5 homes would have an enhanced specification, but essentially retain their original layout and external appearance. The Code 6 house would involve a completely new construction method.


An important element of the grant requirement was to provide a long-term education facility to promote sustainable design for future developments in the region. Permanent information panels have been installed on site, accessible to the public, with energy data logged via a website link, and a live data display screen. This shows the amount of carbon saved and energy produced by the renewable technologies employed on the Code houses. The data can be used by schools and colleges for educational projects and may be of interest to others in the building industry.

A number of portable information panels have also been produced explaining why buildings have to change to reduce their impact on the environment and how this can be done. The boards are on display on site during the build and sales period and can be downloaded from the website as an educational aid. Once the site is fully occupied, the boards will be used by the council to educate their own members, such as the planning team, and as a basis for future development briefs.

The design challenge

The original development already had some commercially viable sustainable elements and the design team decided the existing scheme (apart from the Code 6 dwelling) could be improved to meet the grant requirements without a new planning application. The original design included good insulation standards with wall U values of 0.25 W/m2 K, solar hot water systems in a number of units, passive design and a combination of natural light and low-energy lighting, but without any Code assessment. The challenge for ZeroC was to find the least disruptive and most cost-effective ways of improving the existing specifications and designs to meet the requirements of Code levels 4, 5 and 6.


It was agreed that the Code 6 house needed a new planning consent and would be built later. Consent was granted for a redesign using insulated concrete formwork (ICF), which would deliver very high levels of insulation and airtightness while retaining the architectural style desired across the whole scheme. The roof was redesigned with an extended asymmetric pitch to allow for a large 4.5kW array of PV to generate enough electricity to run all the appliances and lighting in the home.

Difficulties arose with various aspects of the Code assessment for the 13 homes that were to be upgraded. Bike and bin stores needed careful thought, as did ecology and landscape design. The energy section proved to be the most complex to determine. Having our own in-house standard assessment procedure (SAP) and Code for Sustainable Homes (CSH) assessors proved invaluable for rapid decision-making.

Building fabric

As construction had started, a departure from timber-frame construction was not practical. In fact, that building method turned out to be an advantage, as it could accommodate an increase in the thickness of the insulation board without the wall depths increasing by more than 50mm.

Using SAP, it was determined that the Code 4 and 5 units would require U values of 0.23 W/m2 K for walls, 0.19 W/m2 K for floors, 0.15 W/m2 K for the roof, and 0.16 W/m2 K for windows. An airtightness level of 5 was required, far better than building regulations. The real complication was the number of different wall types that had to be assessed to determine the most efficient and economic solution for each.

Figure 1: The team decided that the original design and timber-frame construction could be improved (for all but the Code 6 dwelling) to meet the requirements of Code level 5 (Image courtesy of ZeroC)

Space heating and domestic hot water

Once insulation levels had been increased to reduce heat loss and improve sound transference between party walls, the next priority was to find the most economical and carbon-saving heating system. After various scenarios had been explored and assessed, the best solution was deemed to be a communal biomass wood-pellet boiler, which provided carbon savings of approximately 70% compared to the standard 2006 baseline using gas.

The bespoke boiler system designed by Woodpecker Energy ensures all 13 units pass the energy category of the CSH with flying colours. An additional complication was that the educational aspect of the grant funding called for the installation of individual heat meters to monitor the heat use of all the dwellings and supply the information to the live data panel. One of the retail units had to be made smaller to accommodate the boiler and the permanent display of information.

Renewable energy

To meet Code 5, the ‘non-regulated’ electricity demand for a dwelling (essentially all the lighting, pumps and fans) must be met by a renewable technology. A wind turbine on the adjoining hill had been seriously considered at an early stage, but the time required for wind-speed monitoring and obtaining planning consent was far too long for this project and the idea was discarded.

The tried-and-tested solution of PV panels was adopted, with the units sized to meet Code level 5 requirements (approximately 1.7kW each). As most readers will know, this low-maintenance micro-technology has the advantage of producing a significant income for the homeowner, in terms of excess electricity bought back by the utility company and payments available through the government’s feed-in tariff (FIT) scheme. The Code 6 home will generate approximately 4.5kW of PV, producing a very useful income of about £1,000 a year from the FIT scheme.

Figure 2: Solar PV panels were part of the original plan, but were increased to meet the level 5 requirement of 1.7kW per unit (Image courtesy of ZeroC)

Water conservation

Another important aspect of achieving the Code is saving water. Using rainwater to help supply toilets and washing machines was one option, but the site turned out to be unsuitable for the installation of large underground tanks. Instead, a greywater recycling system was installed by a company called Ecoplay. This collects and filters water from the baths and showers to supply the toilets. Taps and shower flows were restricted within acceptable limits and rainwater butts were provided for external use. Thus, water use was reduced by the 80-105 litres per person per day required for the various Code levels, with the potential benefit to occupants of significant savings on water bills.

The Code 6 house

Work on this detached house will begin as soon as construction plans have been finalised. The architectural style was kept as close to the original approved design as possible to make a new consent easier, but the roof line had to change on the southern side to accommodate the large array of photovoltaic panels required to supply 4.5kW of electricity.

Since the building has to be incredibly airtight, a mechanical ventilation system is essential, but the extra energy required to run it is mitigated by heat recovery, so there is a net gain in energy saving. It also has the advantage of finely filtering the air changes, which is useful for allergy and hay fever sufferers.

With ICF construction, the EPS (polystyrene) formwork surrounding the concrete stays in place as permanent building insulation. The forms are in fact interlocking modular units, somewhat like Lego bricks, which are dry-stacked (without mortar) and then filled with concrete. Usually, reinforcing steel is added before concrete placement to give the concrete flexural strength, as in bridges and high-rise buildings made of concrete. Like other concrete formwork, the forms are filled with concrete in 1-4ft ‘lifts’ to manage the concrete pressure and reduce the risk of blowouts. After the concrete has cured, the forms are left in place permanently and an external finish such as render, brick or stone can be applied.

Cost summary

The Glove Factory (as it is now called) development has been a valuable experience for ZeroC. It was unusual, in that the 13 Code homes had to be retrospectively planned, so costs were higher than they would have been if we had started from scratch. The average additional cost was approximately £16,500 for each of the two Code 4 homes (biomass, insulation, and water saving), £26,000 per unit for the 10 dwellings to Code 5 (biomass, PV, insulation, and enhanced water saving), and £45,000 to build the Code 6 home with ICF construction, plus ventilation system, biomass, 4.5kW PV and a greater water-saving target. The redesign, CSH and SAP pre-assessment consultancy costs totalled £7,000.

Exceptional site-wide costs included the loss of retail space to the plant room/exhibit space, which consumed £50,000. The building redesign necessary to accommodate the plant room (including a reinforced floor) and the ecology landscaping came to a further £45,000, while the educational aspects cost £30,000, including the hard-wire data logging linked to a website.

The additional ZeroC project management costs and the actual CSH assessment will be covered by our master model and are excluded from this analysis.

In this challenging market, ZeroC is offering the 13 Code-assessed homes at very realistic prices, without adding a premium for the sustainability features. We believe the cost of delivering Code homes is falling as the technology supply industry becomes increasingly competitive and more contractors get used to the Code standards. Government incentives such as FIT and the forthcoming Renewable Heat Incentive will add further value to any sustainable home, and for us, help drive sales.

Craig Bates is Associate Director Special Projects, ZeroC and Robert Letts is Managing Director of Ecofirst Consult

Further information

Full-text ebooks available at to RICS members:

  • A handbook of sustainable building design and engineering by Dejan Mumovic and Mat Santamouris, Earthscan, 2009, and
  • Evaluating sustainable development in the built environment 2nd ed. by Peter S Brandon and Patrizia Lombardi Wiley-Blackwell, 2010

Related competencies include: M009 and T041

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