HLM Architects designed the competition winning scheme for Plots 11/ 12 within the masterplan and are working with O'Brien Properties and Buro HappoldEngineers to deliver the units ahead of the event. The Scottish Passive House Centre ( SPHC) also played a key role in the project, supporting the design team through the detail design stages with energy efficiency consultancy and Passive House Planning Package ( PHPP) Calculations. The SPHC will also carry out the air pressure testing to confirm the buildings as some of the most air- tight in the UK and will supply vital Passive House components for the construction of the units including the MVHR System and Triple Glazed Passive House Certified Windows. Passive Journey Our Passive House journey began in early 2007 when presented with the opportunity to look at the Highland Housing Fair competition ( as it was then known). The competition, organised by the RIAS and initiated by The Highland Council and the Highland Housing Alliance, set out to look for innovative, low energy housing which would look to raise the bar for Scottish and UK housing. The competition gave our practice the opportunity to continue our exploration of a low energy approach to our buildings and, in the context of previous work we had done on the Inverurie sustainable housing competition and a number of larger built projects, we embarked upon an investigation of European approaches to low energy housing design. Eventually, we looked towards the model of the Passive House, which refers to the rigorous standard for energy efficiency in buildings pioneered in Germany in the 1990s. Translated to Scottish Housing, it results in ultra low- energy buildings, dispensing with conventional heating systems altogether. Passive House Design Taking the competition zone theme of solar design, our terrace exploits the climate and in particular solar energy to significantly reduce energy consumption as part of the Passive Housephilosophy. The main living spaces are orientated to the southern climate- inclusive side of the house using the elements and climate to reduce reliance on energy; the utility spaces to the north inhabit what appears to be a thick, heavily insulated north wall - excluding the cold climate and closing out the elements. Small openings are provided in the north wall creating the effect of a quiet demeanour to North Street while minimising overlooking. The rear of the house, however, is intended to be much more private and, in a sense, more dramatic and is therefore treated with a more modern approach. Large expanses of south facing glazing help to create an open feeling towards the garden and to the woodland and trees beyond. At night the glazed living spaces can glow beacon - like into the dark reaches of the garden. HLM's terrace is one of the first Passive House Schemes in the UK, and will achieve an 80% reduction in energy consumption ( heating, domestic hot water ( DHW) and electricity). A typical three bedroom new build in Scotland of similar floor area would consume approximately 15,000 kWh/ a for heating energy. The heating demand estimated for our Passive House is 1,430kWh/ a ( less than 1/ 10th of a conventional new build house). The maximum power output for the heating has also been calculated at 9W/ m2 ( or 963W for the whole house), just below the maximum Passive House threshold of 10 W/ m2. The household appliances and occupants of the house will produce enough heat to cover this demand - with one person producing 60- 100w heat. We also calculated that a hairdryer could heat the house on a cold day! This achievement is made possible by careful orientation, shape and a compact form taking full advantage of the climate and solar gains and using a closed panel timber frame system to achieve a Super Insulated Building Fabric with the U- Values for walls, roof and floor of 0.1 W/ m ² K and windows a U- Value of 0.71W/ m ² K. The following gives a more detailed look at how we created a building envelope to meet 20 ATJULY- AUGUST2010 PASSIVEHOUSE Left: proposed interior

ATJULY- AUGUST2010 21 the stringent requirements for passive house construction. Building Fabric Ground Floor Slab We used ground bearing concrete slabs with 200mm rigid insulation below and 30mm perimeter insulation to slab edge. For the sub- structure, we used a 275mm aerated concrete block to help significantly with our u- values and thermal bridging. A common concrete block wall would not have provided us with sufficiently low thermal conductivity or indeed a suitable fixing base for the wide prefabricated timber panels which need to be anchored down to resist the significant wind loads experienced on such an exposed site. We did consider importing a unique German foundation system called ' Isoquick' which uses modular foam blocks to create a foam ' tub' into which the concrete slab can then be poured. It was ideal for Passive House construction, but we couldn't satisfy ourselves that it could deal with the stepped profile of our terrace or more importantly the significant wind loads it had to withstand. In the end we went for the best solution available in the wide format Aerated blocks. Of particular note is the finish of the concrete floor- it's important that slab or at least the area round the perimeter has a smooth finish preferably power floated to provide a suitable substrate and adequate adhesion for the air seal tape at the junction with the wall panels. Structural Envelope There were several constructionoptions available for the structural envelope. In the end, we opted for a closed panel timber frame system ( RTC PassiveWall) which local firm RTC Timber has created specifically for building to Passive House standard. We worked closely with Buro Happold and RTC to get the most out of the timber frame system. This included designing some Glulam elements to resolve some challenging structural issues like the corner cantilevers which will help form the distinctive architecture of the units. The panels are manufactured at RTC's Elgin factory using timber I beams ( 245mm deep for wall panels, 300mm for roof panels and floor cassettes) manufactured locally with 9mm sheathing boards on both sides of the panels to contain the 80% recycled content glasswool insulated core. The construction sequence for the frame is similar to that of a standard timber frame. Wall plates are fixed in position over the substructure block work. RTC has a rebate wall plate system which helps to correctly position the wall panels and improves air tightness. The ground floor panels are then lifted into place followed by the Glulam beam elements, the load bearing spine wall and the 300mm deep first floor cassettes. The first floor wall panels are then erected on top of the first floor cassettes, followed by the loft floor Glulam beams and loft floor panels, which were similar in design and layout to the first. Finally the roof panels are lifted and secured in place. PASSIVEHOUSE Above: typical base detail