Specialised Performance Actual Performance of Green Building (the specification trap) - 09

For those who would prefer a very highly efficient house in a more traditional mode, volume house builder, Miller Homes, has constructed an allegedly zero-carbon home as part of an ordinary commercial development, and the house is available for sale. It is one of 5 built to different CSH levels as a pilot project by the company in Basingstoke. A spokesman said it had been a huge challenge, technically and financially. The properties will be monitored using smart metering as well as sensors to monitor humidity for a period of 12 months after completion. This will give a better picture of the reality of living in an airtight, “zero carbon” home, as well as being able to gauge the most and least effective of the new technologies employed. When the results are clear, Miller says it intends to build more. Hopefully these prototypes will be more successful than the Stewart Milne Group's Sigma home at the BRE Innovation Park. Research conducted over a year, and four periods of evaluation when the home was occupied by a real life family, has resulted in the developer going back to the drawing board. The evaluation showed a need to concentrate on primarily low energy (highly insulated) homes rather than using bolt on microgeneration technology and aiming for theoretically zero carbon structures by producing the necessary power onsite. The company found their add-ons, such as wind turbines, photovoltaic panels and solar thermal, did not consistently deliver the required performance levels.

One of the major problems with the original structure was the under performance of the building envelope. Although built to a higher specification than a normal house, it was found to be 40% less efficient than calculated in design.

Meanwhile, in a similar vein, some serious flaws in the energy calculations used for the Code for Sustainable Homes have been revealed after research by Jim Parker on the current Denby Dale Passivhaus project in West Yorkshire. Parker has concluded that 'a Passivhaus dwelling's energy savings are not realistically represented by its Code for Sustainable Homes ratings'. The building would only meet CSH level 3 criteria for Ene 1: Dwelling Emission Rate, the mandatory aspect of the Code's Energy Category, despite its being projected to be one of the most energy efficient buildings in the UK.

All buildings meeting the strict Passivhaus standards must have space heating requirements of less than 15kWh/m2/year, and use up to 90% less energy to heat than standard UK homes, often requiring minimal or no heating.

In addition, airtightness for Passivhaus buildings, such as that at Denby Dale, is required to be no more than 0.6 air changes at 50 Pascals. The report points out that many buildings receiving higher CSH ratings (up to level 6) actually perform worse than the Denby Dale Passivhaus in terms of space heating requirements and airtightness, but gain points in other areas, and sometimes through the use of inefficient and expensive bolt-on renewable technologies. It's a pity this research wasn't available to the builders of the prototype homes mentioned above.