46 BUILD 122 February/March 2011

RENovATIoN

RetRoFittinG coMMeRciAL BUiLdinGsRequests for tours and information have inundated Beca’s new Wellington office in the refurbished Aorangi house – proof that green retrofits of commercial buildings are a hot topic.By Alan Barbour, Technical Director, Beca Buildings

Built in 1970, Aorangi House was a relatively unremarkable example of a commercial office building of its time and had been unoccupied since 2005.

In 2008, owners Prime Property Group commissioned Studio Pacific Architects and Beca to design a complete upgrade of the building. They used an integrated green building approach and several major design interventions to give the building a new lease of life as a highly functional office.

Beca structural and building services engineers also compared notes with their colleagues in Singapore who were working on South-east Asia’s first zero-energy retrofit at the time for Singapore’s Building and Construction Authority.

The team made extensive use of 3D computer simulation analysis at the design stage. This provided valuable data for determining the right balance between project costs and key performance attributes, such as energy efficiency, occupant comfort, use of daylight, natural ventilation and glare control.

Full insulation reduces heating and air-con needsWell tested energy-saving improvements such as thermal insulation and double glazing were top of the refurbishment list. The aim was to add value by significantly reducing heating and air-conditioning needs.

The building was stripped of its existing windows, over-clad with 100 mm of high-density polystyrene insulation on the external façade and replastered with a high-quality proprietary system to provide a waterproof and well insulated building thermal envelope.

It was a New Zealand first for a commercial office building to be externally insulated and reclad to a full 12-storey height as a green retrofit.

Working with the sunThe sun has the largest impact on a building’s thermal load. To minimise these effects, Aorangi House features external solar shading. The solar shades are positioned to allow useful solar gains in winter while limiting heat gains and cooling requirements in summer. The fixed elements were designed to suit the differing solar impacts on each face, without the need to change their position.

Double glazing with a high visible light transmittance was used, providing good levels of daylight.

Internally, unlined walls and exposed concrete act as heat sinks to reduce temperature fluctuations. Inside the retrofitted Aorangi House.

Aorangi House in Wellington, one of the first retrofitted commercial buildings to receive a five-star Green Star rating.

BUILD 122 February/March 2011 47

Natural ventilation provides a comfortable environmentThe refurbished Aorangi House uses natural ventilation to take advantage of the Wellington climate. The desk layout on the office floors provides occupants with excellent views and natural light. People have control over the perimeter opening windows, which are manually operated to suit individual preferences.

A roof-mounted weather station constantly measures wind speed and direction, rain and outside air temperature. Although the windows are intended to be user-operated, temperature and air quality sensors continuously monitor conditions on each floor. These sensors and weather station automatically control selected windows as required to achieve the target conditions.

A night flushing feature passively pre-cools the exposed building structure ready for the day ahead. If the target temperature cannot be achieved (or if wind speeds are too high to have windows open), an exhaust fan provides assisted ventilation.

A variable refrigerant flow (VRF) comfort cooling system is installed to serve high tenant-load areas and provide a level of future-proofing. This only operates for the open plan office spaces to manage comfort in extreme conditions.

Although you’re unlikely to please all of the people all of the time, feedback from Beca’s staff indicates that the retrofitted passively designed system is providing a more comfortable working environment than previously experienced in air-conditioned offices. Staff report feeling healthier and more energised, particularly during the afternoon.

Measurements gathered to date indicate that the thermal performance of the building is in line with design predictions.

One-size green option doesn’t fit allKey to the Aorangi House retrofit was the building owner’s commitment to investing in long-term outcomes. While most pragmatic commercial retrofit projects can expect a positive financial return over the long term, high upfront capital costs can present barriers to green building investment in this sector. Beca uses a project-specific framework to assist owners to evaluate appropriate options to retrofit their properties.

Other considerations include the building’s suitability for different retrofit options. Not all commercial buildings would lend themselves to the approach adopted for Aorangi House, where the relatively narrow floor plates aligned well with a passive design approach. However, the roof to floor-area ratio of Aorangi House meant rainwater collection and reuse was not a favourable option.

Five-star accommodationAorangi House was one of the first retrofitted commercial buildings to be awarded a five-star Green Star rating by the New Zealand Green Building Council. Mike Kerr, Beca’s Wellington Regional Manager, believes the benefits for the business have been numerous, including savings in energy costs, reduced water usage and increased staff satisfaction with the new workplace environment.