Tim Porter

Prefabrication and evidence-based design promise to boost the sustainability of housing in NZ, according to Tim Porter, board director of PreFab NZ and project director – major projects for Holmes Solutions.

Presenting at the recent NZ Green Building Council Sustainable Housing Summit, he said there were also gains to be made from improving construction sector efficiency and “designing smarter” structures, transport and assembly practices.

His firm specialises in product testing and development, and up to 70 per cent of its work comes from offshore, including from Australia, the US and Europe.

The products are generally for high-risk industries, where there is a real risk of harm to people if the product does not function correctly. Construction is one of those sectors.

Porter says he has also observed an increasing requirement for on-site testing of thermal performance and acoustic performance of buildings.

He says this is particularly true in the UK, and while it is not currently the case generally in NZ, it is expected to happen soon.

Porter thinks Australia and NZ will see more design-based rating systems for performance come into the picture.

“Evidence-based validation [of products and design performance] will make sure contractors and builders are building to the required quality needed to meet targets,” he says.

In terms of prefab in NZ, Porter says the size of the market is “a little bit vague” based on the available data.

In terms of the domestic construction sector, already the vast majority of frames and trusses used are prefabricated.

“But when you look at more value-based applications like panels, pods and three dimensional volumes, it is a smaller but growing market.”

Porter says he would prefer to see the construction sector move into more valuable prefabrication technologies such as panellisation, bathroom pods and prefabricated plant rooms. This, he says, will increase the quality of final outcomes.

Research by BRANZ has shown that 87 per cent of new homes in NZ have defects, he says. By contrast the manufacturing industry uses approaches like Lean and Six Sigma to achieve a 99.9996 per cent defect-free result.

“The difference between the two [methods] is obvious.

“We will get better quality from a manufacturing-led approach.”

Another aspect of prefab that has social sustainability implications is the improved working environment.

People are inclined to do a better job in a comfortable ambient environment, Porter says. They can focus better, maintain higher levels of safety and also the materials themselves do not suffer quality degradation from being exposed to the elements.

“Something I’ve seen, which is a social benefit, is people who otherwise think construction would not be for them gaining employment in a factory build environment,” Porter says.

This includes working women with children, who can take a factory job because it is a fixed point of work, something that is much more compatible with having childcare arrangements.

The setting also encourages camaraderie and team building, Porter says, as people have regular workmates. Also older, experienced workers that no longer want to work on a cold, exposed building site can bring that depth of knowledge and expertise into the factory.

“That is an excellent benefit some practitioners say they experience.”

In the US, prefabrication is providing jobs for veterans and people that are mobility impaired, he says. It allows them to still have a job in construction, be part of making things, and work effectively regardless of physical constraints.

Porter says the prefab industry is benefiting from engineered timbers that deliver improved tolerance and dimensional stability. These include high-end LVL systems and also CLT, which is a strong competitor for pre-cast concrete.

The engineered timbers stack up even better when other research around their ability to control CO2 and humidity is factored in, as they lead to more comfortable ambient surroundings.

“[Engineered timber] is the poster child for apartments, hotels and other sleeping environments,” Porter says.

XLam CLT was used as the primary structural material for a recently completed backpackers hostel in Christchurch. The two-storey building had a site manager claiming to be the “happiest in the country” because the project was free of some of the hassles associated with traditional construction, Porter says.

The combination of CLT and prefabrication meant the defects issue was minimised, the right tolerances were engineered and the construction crew could “get it right the first time”.

“The benefits of that approach are so tangible and obvious,” Porter says.

The use of timber as a first choice combined with an offsite construction approach “sends a big signal” to the industry, he believes.

“People seem stuck on the old way, thinking that construction is safe from disruption. The truth is traditional construction is just as vulnerable as any other industry.”

Evidence-based design

The use of evidence-based design is another element that can also improve the efficiency of the structural system, he says.

One of the issues with the building codes, standards and legislation in NZ, Australia and Europe is the layers of conservatism embedded into the system.

When considering structural performance each standards committee has to make allowances for the worst-case scenario, Porter says. However the hierarchical approach means that the conservatism that is present at each level amplifies – from building legislation down to the standards for individual elements such as materials or fasteners.

“When we test systems designed to meet the prescriptive standards we generally find they are much stronger than they need to be,” Porter says.

That means there is much more material and capacity going into the building than is necessary.

“So, from a materials efficiency point of view we are not building very sustainably,” Porter says.

By contrast, if there is increased certainty in how something will be built and how it will be assembled, the legislation allows you to develop your design capacities from testing. By using physical evidence you reduce uncertainty which allows you to optimise the structural system without compromising safety.

For prefab, where there is standardisation of components, connections and systems, it particularly makes sense to invest in testing and evidence based design, Porter says.

“It makes sense to the bottom line and it makes sense to the planet too, saving that material.”

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  1. It is great to see Tim looking at the evidence base of NZ housing as currently being delivered. It is an issue close to our hearts in Australia, with the recent National Energy Efficiency Building Project finding a significant gap between houses as energy modelled, and what is being delivered on the ground. Details of this study can be seen at: http://www.sa.gov.au/topics/water-energy-and-environment/energy/government-energy-efficiency-initiatives/national-energy-efficient-building-project
    Our manufactured building components are delivered to site in a flat-pack format and fit together to get a house to lock-up (weathertight and secure) in a matter of weeks. A Melbourne University study, presented at the 2014 World Sustainable Building Conference in Barcelona, showed that Habitech’s building fabrics outperforms the predictions of energy rating software. We’ve launched our product in New Zealand through LiteGreen projects and we’re looking forward to proving the thermal integrity of our building envelopes.