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

For our three units the whole timber frame was erected within 9 days. An air seal tape is applied to the internal corner junction of the ground floor wall panelswith the floor slab. The tape is also applied to the subsequent joints between each panel/ element. Internally, a thermal vapour barrier is fitted to the panels on top of which 50mm deep timber studs are fixed to provide a service cavity and support the plasterboard finish. It worth noting at this point some of the implications of our consultation with the Scottish Passive House Centre ( SPHC) in terms of wall construction. Our design includes a clear two- storey height ground floor living room with roof lights above. Because of our compact layout, it was proving difficult to achieve Passive House standards and we were advised to remove one or both of the features. We felt that both were fundamental to our design and had to stay. After muchdiscussion, the agreed solution was to provide additional insulation to the walls in the form of 40mm rigid boards within the service cavity ( 50mm insulation was added to the roof panels), allowing us to retain both features without impacting on performance. External Finishes Wall Cladding Externally, a thermal breather membrane is fixed to the panels and the majority of the walls are clad with locally sourced larch timber cladding from FSC sources. The open jointed boards are fixed over insect mesh to 50 x 50mm battens and 50 x 50mm counter battens. The battens create an adequate depth behind the cladding to conceal the PPC pressed aluminium gutters and downpipes, without the airtight envelope as well as the extract and intake vents for the MVHR system( additional vent slots are routered out of the larch boards). The only visible section of gutter is the 30mm down turned edge designed to restrain the top cladding board. The horizontal boards are broken into panels which are defined by the vertical larch boards at the jambs of the window and door openings. One common problem we experienced with the cladding was the required provision of cavity barriers, which conflicts with the requirement for ventilation behind the timber cladding. The Highland council was unwilling to accept intumescent or flexible type barriers which would have assisted in the drainage, ventilation and drying of the cladding boards and instead stipulated that all cavity barriers were to be of a solid type even though the boards are open jointed. Our external walls will achieve a U- Value of 0.1 W/ m ² K and help us achieve air tightness 10 times better than current building regulations. Roof Cladding Externally, a breathable membrane is fitted over the panels beneath 50 x 20mm counter battens. For the roof finish, we originally intended to use a recycled aluminium profiled sheet but struggled with sourcing something of the right appearance and quality. Instead, we chose Marley profiled fibre cement roof sheeting as we felt it fitted with the rural 22ATJULY- AUGUST2010 This page and opposite: detail at double height glazed opening Because of our compact layout, it was proving difficult to achieve Passive House standards