The Sydney Opera House has been officially recognised for its sustainable credentials. It now finds itself - in good company, beside New York's Empire State Building - on a very short list of World Heritage sites from around the world that have achieved green certification.

The Opera House has had an environmental sustainability plan in place since 2009. During this time its team realised how hard it was to measure the plan's progress, and looked for ways to have it benchmarked.

Cue the Green Building Council of Australia (GBCA), which launched a pilot scheme with the venue in order to assess its practices holistically. After looking at everything, from cleaning methods to power and transport options to and from the site, the Opera House was awarded a 4 Star Green Star - Performance rating out of a possible six.

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There are around 900 other buildings in Australia with a green-star rating of one to six, but of that number, 800 were purpose-built to be green. The Opera House will work towards a five star, but to achieve a six, “you really need to start with a blank sheet of paper,” says building director, Greg McTaggart. Although the Opera House's heritage status limits what interventions can be made on the building (harnessing solar and wind power are not options, for instance), “We are lucky that we're starting from a pretty strong base,” says McTaggart.

Danish architect Jørn Utzon's original, nature-inspired design was in many ways sustainable ahead of its time. “A lot of good and interesting engineering went into the building at the design phases, which for the day was quite innovative,” says manager of building strategy and planning, Bob Moffat.

“They used the seawater as a cooling medium for the air-conditioning system,” Moffat continues, “and looked at using the heat recovery from the chillers to reheat the building. They actually developed a chilled ceiling in the drama theatre. It's very similar to a chilled beam, which is now modern technology, but in its day was quite advanced.”

These systems weren't originally implemented with the environment in mind, but rather their sustainable implications are likely a happy byproduct of an unwavering architectural vision. McTaggart suggests, for instance, that this non-traditional way of managing heat rejection is likely to have been developed to ensure those now-iconic “shells” weren't blighted by vents and chimneys.

“My favourite thing about the original design is the care that went into the selection of the materials,” says McTaggart. That includes robust, long-lasting materials such as the concrete - the predominance of which over-glazing keeps internal temperatures fairly stable - and bronze elements on windows and door trims. “It makes our lives so much easier now in terms of maintenance and longevity.”

And how about having a roof that is essentially self-cleaning? Because of the glazing on the shells' tiles, “We don't have to put people up there with harsh chemicals to do the cleaning. Pretty much every time we get a good shower of rain, it'll wash them off,” explains McTaggart.

The most interesting sustainable solutions are often the simplest.

Steve Tsouklas has worked at the Opera House since its construction phase more than 40 years ago. “A simple idea for cleaning the bronze and concrete came from him”, Moffat says. “He has a Greek background and his process for cleaning the bronze is based on olive oil - naturally it's got to be Greek!” Similarly, “The logic he uses when he looks at cleaning concrete is based on bicarbonate of soda.

“These old techniques are great solutions for a lot of the problems we currently have. You don't need to have harsh chemicals: use some elbow grease and simple, natural products and you make a world of difference.”