Design Case Study
The team had to deliver the building in very tight time frame as the Welsh assembly funding had to be spent by the end of April 2008, which gave the team eight months from design to completion.
The office has a floor area of 400m2. It is used for council services and for the delivery of a wide range of training courses for local people including computing, so has three large rooms full of computers.
A design and build route was used to deliver the project and contracted under the New Engineering form of Contract (NEC).
The funding from the Welsh Assembly came with a requirment for the building to be BREEAM Excellent; to achieve this the building was designed around the Passivhaus principles of high insulation and air tightness, solar gain and efficient heat recovery systems to ensure low levels of energy use.
This was the first project in the UK that had both BREEAM and Passivhaus standards applied simultaneously, which meant that the team had to be receptive to modern methods of construction.
The timber frame and roof panels were fabricated off-site, whilst the foundations and internal masonry structure was constructed onsite. Imported materials were kept to a minimum in order to achieve the sustainable targets, which meant the timber frame was locally sourced. 85% of the existing building that had been demolished was also utilised on site and in the new building rather than sending it to land fill.
The office has concrete flooring and internal walls which are clad with a panellised timber system. The panels are filled with 200mm Warmcel insulation and clad with 40mm-thick fibreboard covered with timber and lime render. Impressive levels of airtightness were achieved: O.37m3/m2/hr@50Pa, which is 27 times better than the UK regulatory limit, and less than half of the Passivhaus requirement.
The team faced a number of challenges to ensure the internal temperature of the building was kept at around 20 degrees celeius, whilst keeping to the Passivhaus energy limits. This was made more difficult as the council wanted to include a server room and could not use ultra low energy PCs due to a prior arrangement with a computer manufacturer. In order to conteract the energy consumption and heat production the team added brises-soleil to the front of the building.
They also examined the ventilation plant to assess where changes could be made and it was decided that the three computer rooms would be ventilated independently of the main system supplying the public areas.
It was decided that heat could be purged from the building at night using the mechanical ventilation system, and the building has concrete floors and blockwork internal walls to add thermal mass. As with all Passivhaus designs, a mechanical ventilation system with heat recovery was fitted to retain heat in the winter. At the end of winter the heat exchangers are removed so fresh air can flow into the building without being warmed by the extracted air.
After a year of monitoring it was found the changes made were effective and the building was using less energy then expected (80kWh/m2/yr rather than the anticipated 144kWh/m2/yr). The building also has one of the lowest air leakage results in the UK (Q50 = 0.37m3).
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