Case study Post-occupancy evaluation
Clay Fields specification
Location: Elmswell, Suffolk Scheme: 13 x 2-bedroom and 9 x 3-bedroom houses, plus 4 one-bedroom flats, each built to BRE EcoHomes Excellent standard, completed September 2008
Client: Orwell Housing Association Sustainability consultant: Buro Happold Architect: Riches Hawley Mikhail
Features
Biomass district heating network. Whole-house mechanical ventilation with heat recovery. High airtightness: at 3.47m3/sq m at 50Pa, is 65% improvement on regulatory minimum
Massing: homes grouped in threes to reduce area of exposed facade, two and three-storey buildings are positioned to maximise winter solar gain
Rainwater harvesting: each dwelling fitted with a 50 to 60 litre tank that supplies filtered, unmetered grey water for toilet flushing and garden irrigation
Optimised solar orientation: main portions of glazing face south to enable useful solar gain in winter
Clay Fields’ biomass store and boiler room
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the national average and 23% lower than the design target (see details in the box on page 40). Gill says there is no single explanation for why the design target was exceeded by such a large margin. The walls were made using hempcrete, a mixture of hemp and lime chosen for its excellent thermal properties. This was sprayed onto timber frames, and Gill believes this method enabled the team to achieve a good seal. In addition, he points out that energy modelling
tools, including SAP that was used for Clay Fields, can lack accuracy: ‘There have been calls for better modelling tools and there can be big variations between one type and another. I think this shows that they need to be informed by actual performance data, so this is an argument for greater use of POEs.’ However, one thing the Clay Fields POE does show
is the huge differences in energy use depending on occupier behaviour, something that, in Gill’s view, policymakers will have to consider when deciding the final form of levels 5 and 6 of the Code for Sustainable Homes. The lowest energy user in Clay Fields, in terms of
heating and hot water, consumed just 46 kWh/yr per sq m, while the highest consumed 144.9 kWh/yr per sq m – both are substantially better than the national average of 191.5 kWh/year. The homes are grouped in threes to minimise heat loss through the exterior walls and there is a 20% t0 5% difference in heat demand between mid-terrace and end-of-terrace properties, Gill says. However, the study has shown that the highest energy users are not necessarily those living in the end- of-terrace properties, as might be expected. Gill insists energy use could be reduced further
38 CIBSE Journal September 2010
Improved insulation: walls are 0.25W/sq m.K, glazing 1.9W/sq m.K, roof 0.12W/sq m.K and ground 0.25W/sq m.K. Low-energy lighting and low-flow fixtures throughout the homes
through greater engagement with the occupiers, stressing that the highest users often appear very interested in reducing their consumption. One of the biggest consumers had never lived in a centrally heated home before and only had experience of storage heaters. Initially, this occupier set this thermostat at 27C, although this was later reduced. But many occupiers complained that the thermostat
controls, which allow seven-day programming with days broken into five different time segments, were too complicated. ‘They said the heating controls were confusing and on some occasions would stop people from making the changes they wanted,’ explains Gill. Other researchers within Buro Happold are now studying the most effective forms of thermostat design. In addition, Gill notes that most occupiers at Clay
Fields keep their homes fairly warm – the average internal temperature being 21.9C, with thermostats set at 21C to 22C. ‘This tells us that the houses are controllable, but they are demanding a fair bit of heat in there, to be honest,’ Gill admits. The Clay Fields estate is linked to a district heating
system that runs on a mixture of gas and biomass that is controlled by the landlord, Orwell Housing Association. However, the study shows a relatively disappointing proportion of biomass being used – just 42% over the monitoring period, although Gill says the
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