FITTING TOGETHER: Pre-cast panels are craned in and dropped into place making the complex jigsaw work. Photos by Wayne Martin.

PREFABRICATED housing has moved up a notch at a housing development. Typically prefab housing involves lower cost timber-frame construction, but a new lightweight precast concrete building system is providing upmarket aesthetics coupled with time and cost advantages.

Work on the 67-house Lily Garden development in Flat Bush has started with the delivery from a Papakura factory of pre-painted panels with windows already installed. Wilco Precast has spent the past six years developing the lightweight concrete system with architects and engineers.

Crushed rock is traditionally used as the aggregate in concrete. Research undertaken by the Cement & Concrete Association of New Zealand has shown that concrete has an inherent capacity – because of its mass – to absorb and store thermal energy.

After absorbing and storing energy, it releases it when the internal home temperature drops below that of the concrete. The buffering effect means heat fluctuations from heaters and the sun are less noticeable.

However, developers of the Litecrete system have honed in on pumice, a natural inorganic product which, because it floats in water, provides its own built-in insulation.

Volcanic eruptions spew vaporised rock into the atmosphere and air cells are trapped. This gives the pumice its ability to float in water and its natural insulation value.

Pumice can be found from Taupo to Tauranga. The deposits resulted from volcanic eruptions thousands of years ago and the supply is expected to last for at least another 200 years, says Philip Archer, Litecrete’s marketing manager. At the moment, pumice is mainly used in the horticultural market.

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FINISHED JOB: Another wall is almost there, then the house is ready to be sealed and fully closed in.

Incorporating inorganic pumice into a concrete precast structure means there’s nothing in the panel that will rot, an important feature in the wake of the leaky-homes debacle. “We have completed numerous one-off houses,” says Philip. “But Lily Garden is the first major housing development and when we have volume the end price can be reduced.”

Construction is claimed to take 40 per cent less time than a traditional timber-framed house because once the panels are placed, the only remaining job is to seal the joints. It takes about one-and-a-half days to set the panels up on-site.

“The walls are virtually finished when they are installed and plasterboard is used on the inside to be painted,” explains Philip. “Aesthetically, no one would know on the inside that it’s a concrete house. The result is the same as timber-framed.”

Houses can be closed in within seven days because the roof is built on the site. “We crane it on when the walls are finished. Even the flooring is pre-built.”

As well as being rot-resistant, the product is being marketed for its fire-resistance properties (it can withstand four hours of fire), good acoustic values (less noise between rooms and between floor levels), built-in insulation properties provided by the pumice, and it provides a low-humidity environment – good for people with respiratory problems.

The lightweight panels are cured for two weeks to give them optimum strength to hold fast on trucks.

However, they are half the weight of normal concrete and are built to last with a “100-year minimum life cycle”.

MOVE IN: An artist's impression of one of the final products. Photo supplied.