House Carbon Calculator

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A careful choice of materials when building a house can make the difference of nearly 50 tonnes in CO2 emissions, according to a new carbon calculator for houses.

Fifty tonnes of CO2 is almost the same as that emitted by an average car over its entire lifetime. It’s the same as the CO2 emitted from flying nearly half a million miles - that’s around the world 12 times.

The NZ Wood website has just launched its carbon calculator for working out the CO2 emissions or savings that can be attributed to the building materials used for a new home.

Programme manager for NZ Wood, Geoff Henley, says the calculator is a simple but dramatic demonstration of the amount of difference construction materials can make to the environment.

“The calculator shows that an average house can make a significant contribution to reducing CO2 simply on the basis of the materials it’s built from alone.

“At one end of the spectrum, you can actually be removing CO2 from the atmosphere and locking it up for hundreds of years, if not permanently. At the other, your choice of materials could have pumped out tens of tonnes of new CO2 in their manufacture.”

The calculations are based on what is known as “embodied CO2″. This represents the amount of CO2 either emitted or absorbed by the building materials in their production.

In the case of wood, for example, Pinus radiata has absorbed a net 1.7 tonnes of CO2 for every tonne of wood used in the house. That’s over and above all the energy used (and subsequent CO2 released) in its growing, harvesting and processing - right up until it leaves the sawmill door.

This compares with a material such as aluminium, which has released over nine tonnes of CO2 into the atmosphere for every tonne of final product. Steel releases 1.2 tonnes of CO2 for every tonne of final product. Concrete releases around 160 kilos of CO2 per tonne of final product.

Geoff Henley says the calculator was modelled on a standard, single-storey house design.

“While the number of design options are fairly limited, it still serves as an approximation for the contribution of CO2 the construction materials make to the environment, whether it be a small sleep-out built in the back yard or a 40-room mansion.”

In terms of building materials alone, the choices go way beyond carbon-neutrality, he says.

“If you simply use a wooden frame and a wooden floor, you’re already well on the side of the angels. If you choose a weatherboard cladding, over say a brick cladding, and then use wooden window frames instead of aluminium, you’re making a huge difference, and end up with a carbon footprint of minus 20-25 tonnes.

“That’s 20-25 tonnes of CO2 removed from the atmosphere.

“Of all the construction materials, wood is the only one that removes CO2 from the air rather than adds to it.

All New Zealand-grown wood is grown sustainably - usually from a forest plantation. This means new trees will be grown to replace those harvested - removing still further CO2 while the original timber is providing generations of inhabitants with a warm and natural home environment.

The carbon calculator on the NZ Wood site (www.nzwood.co.nz) complements one developed for non-residential buildings - including multi-storey buildings that can be built using wood instead of the traditional concrete and steel.

The calculator was developed with the help of researchers and engineers at Victoria and Canterbury universities and quantity surveyors Davis Langdon.

It will be joined next year by a more complex carbon calculator designed as a sophisticated tool for building industry professionals such as architects and specifiers, Geoff Henley says.