Setting energy goals for the design

Our design process followed the True Green approach used by eCubed Building Workshop, where Jo is an associate.

True Green is a results-driven framework focused on building performance and spans all stages of a building's lifecycle.

It begins with an integrated design process and also includes a focus on materials selection based on environmental impact.

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The design process produces plans and details based on goals established at the beginning of the project. For most houses the goals are around liveable areas and the aesthetics of the building. We had those too, but in addition we set the goal of Zero Energy.

Zero Energy is just one possible energy goal. Homeowners or homebuilders could require a certain percentage of their energy to come from solar (as is being done more and more in the UK) or they could stipulate a Homestar rating they want the house to achieve.

It's important to remember the building code establishes minimum standards of building performance. Unless design goals are set that exceed those standards, you may end up with a house that performs to that minimum code.

An integrated design approach

To achieve an energy performance target requires input from multiple perspectives. In the case of our house that input was from A Studio Architects, a thermal engineer (Jo) and solar engineer (Shay). More often than not engineers like us are brought in after the building envelope has been specified by the architects to meet the liveable space requirements of the client. We're then forced to work within the constraints of that envelope to try and make the building perform. The approach we took is called ‘Integrated Design’. Rather than working sequentially with, first, the architects, and then the engineers, all parties work in parallel right from the beginning, integrating building performance into the design.

Performance vs. liveable area?

New houses are usually designed with a focus on liveable area, and the goal is often to maximise it within the building planes allowable within the district plan. However a larger house will not perform as well as a smaller one as bigger spaces require more heating. With an energy target there may be a degree of balancing building size vs. energy performance.

We shaved 8m² off the building during design which saved enough money to pay for both the solar hot water and PV systems.

But does a smaller house necessarily mean less liveable area? Houses being built in Auckland these days are 50% bigger than they were a generation ago¹. Do we actually need - or even use - that extra space? It really depends on the needs of the building inhabitants. In our case we specified what we wanted in terms of functional living area and designed the smallest building envelope that could provide that. This is a different approach to expanding to the edges of allowable building planes simply to make the house as big as possible without thinking about how it will be used.

Another benefit of designing a smaller, more efficient building envelope means it frees up money to spend on the systems that provide energy. In our case, we shaved 8m² off the building during design (~4% of the size) and that saved enough money to pay for both the solar hot water and PV systems.

Building vs. operating costs

A 200m² house could have a cost-to-operate per square metre of $400.

A key figure people track when designing and constructing a new home is the cost to build per square metre, which dictates how much homeowners will need to borrow and pay back over their 25-year mortgage term. But there's another cost that is rarely thought about - the cost to operate. If electricity prices continue to rise as they have in the past decade or so, over 25 years a house the size of ours could expect to pay $80,000 for electricity². In a 200m² house this equates to $400 a square metre. It can make sense to spend extra money on the design & build up front to minimise or avoid that operating cost, even before you consider the environmental and health benefits.