House carbon calculator

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

Fifty tonnes of CO₂ is almost the same as that emitted by an average car over its entire lifetime.

The NZ Wood website has 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.

Geoff Henley, program manager for NZ Wood, said 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," Henley said.

“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 kg of CO₂ per tonne of final product.

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,” Henley said.

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

“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.

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

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 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, according to Henley.