82 | NZGBC: Stellaria NZ Clay block system durable, efficient Virginia Wright The Porotherm clay block construction system avoids the contraction and expansion that accompanies seasonal changes from winter to summer in the timber systems that are prevalent in New Zealand. BUILDING In the 25 or so years that Cambridge based Chris and Robyn Macpherson of MacPherson Architecture Ltd (MAL) have been designing houses they’ve had plenty of handson experience with how different building systems cope with New Zealand’s unique challenges. Whether it was a new build using a concrete, rammed earth, mud block, straw bale, or timber system; or a reclad required by a leaky home, they found themselves increasingly frustrated by the quality and/or the performance of the different systems. Robyn puts New Zealand’s continued focus on timber systems partly down to seismicity concerns, but MAL has its own concerns about the timber systems they now avoid. By 2014 those concerns led them to set up the company Stellaria NZ Ltd. to import the Porotherm clay block construction system which has been used in Europe for over 150 years. “We were really looking for performance and durability. “For New Zealand homes the builder will provide a 10-year Master Build guarantee, but now many housing companies put into their contracts that they’re not responsible for how the client heats and cools their home,” says Robyn. “As a design office, we are required to tick the 50-year durability box but this is barely reflected in the actual lifetime of materials in most NZ homes and we believe it should be!” Which is why MAL adopted the Porotherm clay block construction system which avoids the contraction and expansion that accompanies seasonal changes from winter to summer in the timber systems that are prevalent in New Zealand. Its thermal qualities give it an energy rating similar to that of a passive housing system, without being airtight, and it uses a quarter of the concrete and steel used in a traditional concrete block build. “From that point of view it’s cost-effective, and it has a lot less waste on site,” says Robyn. The Macphersons keep sustainability and health firmly in their sights when designing homes to which end they recommend the foundation pod system which uses recycled plastic and keeps both polystyrene and timber framing out of their builds. “Our walling system is a single leaf clay block wall which we usually plaster both sides. You can put an ordinary cladding system on it if you want to but we use specialised plaster systems on the outside that use mineral paints, and we put lime plaster inside, because the walls are breathing, and scientifically designed to move moisture and air across the clay blocks. It acts as a thermal sink which is why the energy requirement is so low,” says Robyn. Painting the exterior plaster with breathable or mineral paint is part of meeting New Zealand’s E2 building requirements: meant to avoid issues around weather-tightness, leaking, and rotting; whereas in Europe many Porotherm buildings are left raw, trusting the system which is now used in up to 80% of commercial and residential construction. In New Zealand the Porotherm system is in use from Mangawhai to Warkworth to Queenstown. One client clad their house in Japanese Larch, others have used brick, but regardless of the cladding used the Macphersons have no qualms in guaranteeing the thermal and energy quality, the durability, and the sustainability of their system which easily meets proposed new thermal H1 standards. Architectural builders servicing the Matakana Coast www.bigwoodbuilders.co.nz DESIGN | BUILD | PROJECT MANAGEMENT LANDSCAPING | DECKS & POOLS
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