All the units on the site have been issued with their design stage Interim certificates under the Code for Sustainable Homes (May 2009) and are on course to get their final certificates once complete.
The Code 4 pass mark of 68 percent has been comfortably exceeded with dwellings scoring between 72 and 77 percent. Although not outstanding in itself these results are particularly pleasing given that there are tensions between the Code and Passivhaus that make some credits harder to achieve.
These areas include:
• Secured by design (Man 4) – although Passivhaus windows and doors (Internorm Edition are being used at Wimbish) have excellent locking mechanisms they do not have the SBD approvals that is often required by police Architectural Liaison Officers.
• Global Warming Potential of insulation (Pol 1) – nearly all insulation used on the site has a GWP of less than 5 but the German Passivhaus loft hatch supplier was unable to provide any evidence of this. Consequently the credit could not be achieved.
• Responsible resourcing of materials (Mat 2 and 3) – many of the materials were sourced from suppliers where the necessary certification could not be guaranteed and so the Code strategy set early in the project was to not pursue these credits. It is likely that a number of credits could have been achieved if necessary. More ‘Code compliant’ products could have been used but these often do not have the certification required for Passivhaus.
• Energy and Carbon (Ene1) – a gas heated Passivhaus should achieve a good score in this area but there are a number reasons why this is not the case. Firstly there is a well recognised problem in the Code with flats and mid terraces whereby the small amount of external surfaces makes it hard to show significant carbon reductions through insulation measures. The flats only narrowly achieved the necessary 44% reduction in carbon emissions (block average). PV could have been added if really necessary for Code compliance but this would not have helped Passivhaus performance since it focuses on rewarding demand reduction. Thirdly the non-standard (for the UK) heating strategy of a thermal store supplying a wet heating coil in the MVHR supply air duct cannot easily be entered in the SAP calculation making it hard for the likely carbon reductions to be recognised in the calculations.
These issues will all be resolved in time as the supply chain recognises the importance of both sets of certification. More products and UK suppliers in particular will all help to push down the cost of achieving Passivhaus in the UK.
Nick Jones, Associate Director, Inbuilt
